The large-scale atmospheric circulation strongly influences Earth's climate, both locally and globally, via its transport of energy, moisture, and momentum. While our ability to simulate the global circulation is improving, large model biases and uncertainties in climate change projections persist. Our theoretical understanding of how atmospheric circulations respond to climate changes is also limited, particularly on regional scales and in the presence of zonal asymmetries. Advancing our knowledge of the underlying dynamics is therefore crucial for reliable climate projections and for correctly interpreting palaeoclimate records.

The objective of this session is to advance our mechanistic understanding of atmospheric circulation changes and to analyse their impacts at global and regional scales, specifically on precipitation in past, present, and future climates. We encourage theoretical, observational and modelling contributions on tropical (ITCZ, monsoons, Hadley & Walker circulations, MJO) and extratropical circulations (jet streams, storm tracks, blocking).

Co-organized by AS1
Convener: Michael ByrneECSECS | Co-conveners: Thomas Birner, Nicholas LutskoECSECS, Max Popp, Talia TamarinECSECS
| Attendance Thu, 07 May, 08:30–12:30 (CEST)

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Chat time: Thursday, 7 May 2020, 08:30–10:15

D3430 |
Ori Adam and Hilla Gerstman

The tropical zonal-mean precipitation distribution can vary between single and double peaks, which are associated with intertropical convergence zones (ITCZs). Here, the meridional modality and the sensitivity to hemispherically-asymmetric heating of tropical precipitation is studied in an idealized GCM with parameterized wind-driven ocean energy transport (OET). In the idealized model, transitions from unimodal to bimodal distributions are driven by equatorial ocean upwelling and cooling which inhibits equatorial precipitation. For sufficiently strong cooling, the circulation bifurcates to anti-Hadley circulation (AHC) in the deep tropics, with a descending branch near the equator and off-equatorial double ITCZs. The intensity of the AHC is limited by a negative feedback: the AHC drives westerly surface winds which balance the easterly stress (and hence equatorial upwelling) required for its maintenance. The modality of the precipitation affects the response to asymmetric heating: For weak ocean stratification, OET damps shifts of the tropical precipitation centroid but amplifies shifts of precipitation peaks. For strong ocean stratification, which leads to double ITCZs, asymmetric heating leads to relative intensification of the ITCZ in the warming hemisphere, but the positions of the double ITCZs are insensitive to changes in the asymmetric heating and ocean stratification. The dynamic feedbacks of the coupled system damp the slope of the atmospheric energy transport (AET) near the equator. This justifies a cubic root relation between the cross-equatorial AET and the position of the ITCZ, which captures migrations of the ITCZ significantly better than the commonly-used linear relation.

How to cite: Adam, O. and Gerstman, H.: Dynamic and energetic constraints on the modality and position of the intertropical convergence zone in an aquaplanet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4903, https://doi.org/10.5194/egusphere-egu2020-4903, 2020.

D3431 |
Frederik Wolf, Aiko Voigt, and Reik V. Donner

The intertropical convergence zone (ITCZ) is an important component of the tropical rain belt. Climate models still struggle to represent the ITCZ and differ substantially in its simulated response to climate change. Here, we study to what extent complex network theory, which can effectively extract spatio-temporal variability patterns from climate data, helps to understand the dynamics of the ITCZ and model differences therein. For this purpose, we study simulations with 14 global climate models in an idealized aquaplanet setup performed within the TRAC-MIP model intercomparison project.

We construct network representations based on the spatial correlation pattern of surface temperature and perform a detailed study of the zonal mean patterns of different topological and spatial network characteristics. This allows us to identify clusters of climate models which differ not only in their current climate state dynamics but also in their response to climate change. Specifically, we address possible mechanisms controlling the seasonal change of the location of the ITCZ, and we connect our results to previous work on ITCZ controls by cross-equatorial heat transport and tropical sea-surface temperature gradients.

How to cite: Wolf, F., Voigt, A., and Donner, R. V.: ITCZ dynamics as seen by complex network theory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11593, https://doi.org/10.5194/egusphere-egu2020-11593, 2020.

D3432 |
Angeline Pendergrass, Oliver Watt-Meyer, Michael Byrne, Penelope Maher, Mark Webb, Kathleen Schiro, and Hui Su

The intertropical convergence zone (ITCZ) is a band of intense rainfall near the equator that dominates tropical climate. Recent work has demonstrated that most climate models predict a narrowing of the ITCZ under global warming and this narrowing may act as a control on global precipitation (Byrne et al., 2018). Observations suggest that a narrowing of the ITCZ has already occurred (Wodzicki and Rapp, 2016). However, a firm theoretical understanding of what sets ITCZ width is still lacking and an understanding of how ITCZ width influences circulation and climate elsewhere is only beginning to be developed (e.g. Watt-Meyer and Frierson, 2019). Theoretical advances to date have been tested in an idealized gray-radiation model and across comprehensive coupled atmosphere-ocean models. We have begun an effort to systematically test theories of ITCZ width at an intermediate level of complexity: in aquaplanets with full radiation schemes but no seasonal cycle. Our approach is to use a slab ocean boundary condition to ensure energy conservation but at the same time constrain global mean surface temperatures to be similar across a small set of models. By imposing idealized q-flux profiles of heating in the deep tropics and cooling elsewhere, we vary ITCZ width. We also perform instantaneous CO2 quadrupling experiments to test the response to greenhouse gas forcing. Results from the protocol development and proof of concept phase of the effort, including simulations from three climate models, show changes in ITCZ width of up to 40% that are roughly linear in forcing. In these experiments, ITCZ width has substantial effects on global climate and circulation, including the strength of the ITCZ, global-mean temperature, and the Hadley cell. 

How to cite: Pendergrass, A., Watt-Meyer, O., Byrne, M., Maher, P., Webb, M., Schiro, K., and Su, H.: ITCZ-MIP: Understanding ITCZ width and its impacts on climate and circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20223, https://doi.org/10.5194/egusphere-egu2020-20223, 2020.

D3433 |
Bowen Zhao and Alexey Fedorov

Arguments based on atmospheric energetics and aqua-planet model simulations link the latitudinal position of the Intertropical Convergence Zone (ITCZ) to atmospheric cross-equatorial energy transport –- a greater southward transport corresponds to a more northerly position of the ITCZ. This idea is often invoked to explain an interhemispheric dipole pattern of precipitation anomalies in paleoclimates. In contrast, here we demonstrate that in the tropical Pacific the response of the fully coupled ocean-atmosphere system to a hemispherically asymmetric thermal  forcing, modulating this energy transport, involves an interplay between the ITCZ and its counterpart in the South Pacific - the Southern Pacific Convergence Zone (SPCZ). This interplay leads to interhemispheric seesaw changes in tropical precipitation, such that the latitudinal position of each rain band remains largely fixed, but their intensities follow a robust inverse relationship. The seesaw behavior is also evident in the past and future coupled climate simulations of the Climate Model Intercomparison Project Phase 5 (CMIP5). We also show that the tropical Pacific precipitation response to thermal forcing is qualitatively different between the aqua-planet (without ocean heat transport), slab-ocean (with climatological ocean heat transport represented by a ``Q-flux'') and fully-coupled model configurations. Specifically, the induced changes in the ITCZ latitudinal position successively decrease, while the seesaw precipitation intensity response becomes more prominent, from the aqua-planet to the slab-ocean to the fully-coupled configuration. Thus, the ITCZ/SPCZ seesaw can explain the paleoclimate precipitation dipole pattern without invoking a too strong climate forcing and is relevant to future projections of tropical precipitation.

How to cite: Zhao, B. and Fedorov, A.: The seesaw response of the Intertropical and South Pacific convergence zones to hemispherically asymmetric thermal forcing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11225, https://doi.org/10.5194/egusphere-egu2020-11225, 2020.

D3434 |
Panini Dasgupta, Roxy Mathew Koll, Michael J. McPhaden, Tamaki Suematsu, Chidong Zhang, and Daehyun Kim

The Madden–Julian Oscillation (MJO) is the most dominant mode of intraseasonal
variability in the tropics, characterized by an eastward propagating zonal circulation pattern
and rain bands. MJO is very crucial phenomenon due to its interactions with other
timescales of ocean-atmosphere like El Niño Southern Oscillation, tropical cyclones,
monsoons, and the extreme rainfall events all across the globe. MJO events travel almost
half of the globe along the tropical oceans, majorly over the Indo-Pacific Warm Pool
(IPWP) region. This IPWP region has been warming during the twentieth and early twenty-
first centuries in response to increased anthropogenic emissions of greenhouse gases and
is projected to warm further. However, the impact of the warming of the IPWP region on
the MJO life cycle is largely unknown. Here we show that rapid warming over the IPWP
region during 1981–2018 has significantly changed the MJO life cycle, with its residence
time decreasing over the Indian Ocean by 3–4 days, and increasing over the Indo-Pacific
Maritime Continent by 5–6 days. We find that these changes in the MJO life cycle are
associated with a twofold expansion of the Indo-Pacific warm pool. The warm pool has
been expanding on average by 2.3 × 105 km2 per year during 1900–2018 and at an
accelerated average rate of 4 × 105 km2 per year during 1981–2018. The accelerated
warm pool expansion has increased moisture in the lower and middle troposphere over
IPWP and thereby increased the gradient of lower-middle tropospheric moisture between
the Indian Ocean and western Pacific. This zonal gradient of moisture between the Indian Ocean
and west Pacific and the increased subsidence over the Indian ocean due to increased
convective duration of MJO over maritime continent are likely the reasons behind the
changing lifecycle of MJO.

How to cite: Dasgupta, P., Koll, R. M., McPhaden, M. J., Suematsu, T., Zhang, C., and Kim, D.: Indo-Pacific warm pool expansion modulates MJO lifecycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1039, https://doi.org/10.5194/egusphere-egu2020-1039, 2020.

D3435 |
Pablo Zurita-Gotor

This work is concerned with the large-scale structure of the upper-level divergence/precipitation field in the deep tropics. Once the fine ITCZ structure is filtered out, the coarse-grained eddy divergence field is found to tilt eastward moving away from its maximum near the equator in the summer hemisphere. This robust tilt (observed for both hemispheres and seasons) is also present in the classical Gill solution.

In this presentation we show that the sign of the tilt is intimately linked to the direction of the eddy momentum flux. The observed eastward tilt is such that the momentum flux is directed towards the wave source, suggesting that the observed tilt is determined by wave propagation.

We also discuss the determination of the tilt in the simple Gill model and its sensitivity to the meridional Hadley flow. We show that the increase in the cross-equatorial momentum flux when the Hadley cell strengthens is associated with an increased tilt of the divergence field in the downstream direction of the flow, supporting the conjecture that the tilt is associated with propagation. 

How to cite: Zurita-Gotor, P.: The large-scale tilt of the eddy divergence in the tropics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14862, https://doi.org/10.5194/egusphere-egu2020-14862, 2020.

D3436 |
Zixuan Jia, Massimo Bollasina, Chaofan Li, Ruth Doherty, and Oliver Wild

The East Asian winter monsoon (EAWM) is a prominent feature of the northern hemisphere atmospheric circulation during boreal winter, which has a large influence on weather and climate of the Asian-Pacific region. At interannual time scales, the strength of the EAWM is strongly influenced by the El Niño-Southern Oscillation (ENSO), while the ENSO-EAWM relationship displays pronounced interdecadal variations associated with changes in the ENSO teleconnection pathways to East Asia. Using future transient simulations from the Max Planck Institute-Grand Ensemble (MPI-GE), changes in the ENSO-EAWM relationship are examined at various global warming levels during the 21st-century. Results indicate that this relationship will enhance from present-day to +1.5°C, and then weaken until +3°C, strongly impacted by changes in anthropogenic forcing with internal variability playing a negligible role. The ENSO-EAWM relationship is strongly related to the background mean state of both the EAWM and ENSO under global warming. Both the climatological EAWM strength and the ENSO-related anomalies across the Asian-Pacific region contribute to changes in the ENSO-EAWM relationship. Furthermore, anthropogenic aerosols are also found to play a major role in influencing the ENSO-EAWM relationship under moderate warming (up to 1.5°C).

How to cite: Jia, Z., Bollasina, M., Li, C., Doherty, R., and Wild, O.: Changes in the relationship between the East Asian winter monsoon and ENSO under global warming , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16892, https://doi.org/10.5194/egusphere-egu2020-16892, 2020.

D3437 |
Dana Raiter, Eli Galanti, and Yohai Kaspi
The Hadley circulation (HC) is a key element of the climate system. It is traditionally defined as the zonally averaged meridional circulation in the tropics, therefore treated as a zonally symmetric phenomenon. However, differences in temperature between land and sea cause zonal asymmetries on Earth, dramatically affecting the circulation. The longitudinal dependence of the HC evokes questions about where and when the actual large scale tropical circulation occurs. In this study, we look into the connection between the longitudinally dependent HC and the actual large scale movement of air in the tropics using a coupled Eulerian and Lagrangian approach. Decomposing the velocity field, we identify the components affecting the actual circulation. In addition, we calculate trajectories of air parcels to analyze the actual movement. We propose an alternative definition for the circulation, that describes the actual path of air parcels in the tropics, as a tropical conveyor belt. The Indo-Pacific warm pool is the driver of the circulation, where air converges and ascends, then moves westward and poleward before entering the jet stream, moving eastward with it, eventually beginning its descent near the Americas. Furthermore, using an idealized moist GCM, we explore how tropical asymmetries affect the circulation and discuss the possible mechanisms controlling the tropical conveyor belt.

How to cite: Raiter, D., Galanti, E., and Kaspi, Y.: The tropical atmospheric conveyor belt: a Lagrangian perspective of the large-scale tropical circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4633, https://doi.org/10.5194/egusphere-egu2020-4633, 2020.

D3438 |
Hu Yang, Gerrit Lohmann, Xiaoxu Shi, and Evan J. Gowan

Abundance of evidence shows that the tropics are expanding in the past four decades. Despite many attempts to decipher its cause, the underlying dynamical mechanism driving tropical expansion is still not clear. Here, based on observations and multi-model simulations from the Coupled Model Intercomparison Project phase 5 (CMIP5), the variations and trends of tropical width are explored from a regional perspective. We find that the width of the tropics closely follows the meridional displacement of oceanic subtropical front. Under global warming, the subtropical ocean experiences more surface warming due to convergence of surface water. Such enhanced warming, superimposing onto the variation of Pacific Decadal Oscillation, leads to poleward advancing of subtropical front and drives the tropical expansion. Our results, supported by both observations and model simulations, imply that the observed expanding tropics may largely attributed to the anthropogenic global warming rather than the natural climate variability.

How to cite: Yang, H., Lohmann, G., Shi, X., and Gowan, E. J.: Tropical expansion driven by poleward advancing subtropical front, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5042, https://doi.org/10.5194/egusphere-egu2020-5042, 2020.

D3439 |
Rei Chemke and Lorenzo Polvani

The weakening of the Hadley cell and of the midlatitude eddy heat fluxes are two of the most robust responses of the atmospheric circulation to increasing concentrations of greenhouse gases.  These changes have important global climatic impacts, as the large-scale circulation acts to transfer heat and moisture from the tropics to polar regions.  Here, we examine Hadley cell and eddy heat flux trends in recent decades: contrasting model simulations with reanalyses, we uncover two important flaws -- one in the reanalyses and other in the model simulations -- that have, to date, gone largely unnoticed.

First, we find that while climate models simulate a weakening of the Hadley cell over the past four decades, most atmospheric reanalyses indicate a considerable strengthening.  Interestingly, that discrepancy does not stem from biases in climate models, but appears to be related to artifacts in the representation of latent heating in the reanalyses.  This suggests that when dealing with the divergent part of the large-scale circulation, reanalyses may be fundamentally unreliable for the calculation of trends, even for trends spanning several decades.

Second, we examine recent trends in eddy heat fluxes at midlatitudes, which are directly linked the equator-to-pole temperature gradient.  In the Northern Hemisphere models and reanalyses are in good agreement. In the Southern Hemisphere, however, models show a weakening while reanalyses indicate a robust strengthening.  In this case, the flaw is found to be with the climate models, which are unable to simulate the observed multidecadal cooling of the Southern Ocean at high-latitudes, and the accompanying increase in sea-ice.  While the biases in modeled Antarctic sea ice trends have been widely reported, our results demonstrates that such biases have important implications well beyond the high Southern latitudes, as they impact the equator-to-pole temperature and, as a consequence, the midlatitude atmospheric circulation.

How to cite: Chemke, R. and Polvani, L.: Recent atmospheric circulation trends: two major flaws in reanalyses and in climate models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5636, https://doi.org/10.5194/egusphere-egu2020-5636, 2020.

D3440 |
Or Hadas and Yohai Kaspi

The midlatitude storm tracks are one of the most prominent features of the extratropical climate. Much of our understanding of what controls the storm tracks comes from linear theory of baroclinic instability, which explains generally most of the observed response of storms to the general circulation. One example to where this approach is lacking is the Pacific midwinter minimum, a decrease in the eddy activity over the Pacific storm track during midwinter when baroclinicity is at its peak due to extremely strong zonal jets. A similar response was found recently for the Atlantic storm track, in correlation to periods of strong zonal jets. Following on these findings we study the effect of strong zonal jet streams on eddy activity in the midlatitudes. In order to isolate the effect of the jet strength we used several idealized GCM experiments with different jet strengths, and analyze the formed storm track from a Lagrangian perspective by using a storm tracking algorithm. In both the Eulerian analysis and analysis of the tracks a strong reduction of high level eddy activity is prominent, as well as a modest weakening of the low-level activity. The observed response is then further analyzed by studying the connection between the upper and lower wave and how it changes with jet-stream intensity. 


How to cite: Hadas, O. and Kaspi, Y.: The Effect of a Strong Zonal Jet Stream on the Temporal Evolution of Baroclinic Eddies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4648, https://doi.org/10.5194/egusphere-egu2020-4648, 2020.

D3441 |
Sandro Lubis and Pedram Hassanzadeh

Some types of extreme events in the extratropics are often associated with anomalous jet behaviors. A well-known example is the annular mode, wherein its variation e.g., the meandering in the north-south direction of the jet, disrupts the normal eastward migration of troughs and ridges. Since the seminal works of Lorenz and Hartmann, the annular mode has been mostly analyzed based on single EOF mode. However, a recent study showed that the first and second leading EOFs are strongly correlated at long lags and are manifestations of a single oscillatory decaying-mode. This means that the first and second leading EOF modes interact and exert feedbacks on each other. The purpose of this study is to develop an eddy-feedback model for the extratropical low-frequency variability that includes these cross-EOF feedbacks to better isolate the eddy momentum/heat flux changes with time- and/or zonal-mean flow. Our results show that, in the presence of the poleward-propagation regime, the first and second leading EOF modes interact and exert positive feedbacks at lags ~10 (~20) days about ~0.07 (~0.16) day-1 in the reanalysis (idealized GCM). This feedback is often ignored in the previous studies, and in fact, the magnitude is nearly double the feedback exerted by the single EOF mode. We found that this apparent positive eddy feedback is a result of the effect of jet pulsation (strengthening and weakening) in zonal flow variability (z2) on the eddy momentum flux due to the meandering in the north-south direction of the jet (m1). A finite-amplitude eddy-mean flow interaction diagnostic has been performed to demonstrate the dynamics governing the positive feedback in the propagating regime of the annular modes. It is shown that the poleward propagation is caused by an orchestrated combination of equatorward propagation of wave activity (baroclinic process), nonlinear wave breaking (barotropic processes), and radiative relaxation. The latter two processes follow the first one, and as such, the meridional propagation of Rossby wave activity (likely generated by an enhanced baroclinic wave source at a low level) is the central mechanism. Finally, our model calculations suggest the rule of thumb that the propagating annular modes (i.e., when EOF1 and EOF2 together represent quasi-periodic poleward propagation of zonal-mean flow anomalies) exist if the ratio of the fractional variance and decorrelation time-scale of EOF2 to that of EOF1 exceeds 0.5 or the two leading PCs showing maximum correlations at larger lags. These criteria can be used to assess the predictability of preferred modes of extratropical circulation in GCMs. The present study advances and potentially transforms the state of our understanding of the low-frequency variability of the extratropical circulation.

How to cite: Lubis, S. and Hassanzadeh, P.: An Eddy-Feedback Model for Propagating Annular Modes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5917, https://doi.org/10.5194/egusphere-egu2020-5917, 2020.

D3442 |
Mahshid Kaviani, Farhang Ahmadi-Givi, Ali. R. Mohebalhojeh, and Daniel Yazgi

Interaction between CO2 and atmospheric radiation plays a significant part in changing horizontal and vertical temperature distributions through which it can affect the mid-latitude atmospheric dynamics. The baroclinic instability, which is the source of large-scale eddy formation in mid-latitudes, depends on meridional and vertical eddy fluxes of heat. In addition, the eddy available potential energy, which comes from the mean available potential energy, relies on meridional temperature gradient and provides eddy growth by conversion to eddy kinetic energy. The aim of this study is to determine how the amount of CO2 concentration present in the atmosphere affects the baroclinic instability and formation of large-scale eddies in mid-latitudes. In contrast with what is common in climatological studies, the response of atmospheric flows to CO2 radiative effects has been investigated for a short period relevant for the duration of baroclinic instability. For this purpose, the RRTMG radiative parameterization scheme has been coupled with the DCASL dry dynamical core. The Jablonowski–Williamson test is used to carry out baroclinic instability simulations for five cases with the same initialization but different CO2 concentrations (0, 250, 500, 750 and 1000 ppm). The impacts of different CO2 concentrations on eddies growth, mean flow and eddy-mean flow interaction are discussed. Results show that increase in the concentration of CO2 decreases the meridional temperature gradient and thus reduces the eddy kinetic energy at lower atmospheric levels. Also, increase in CO2 concentration has a considerable impact on the growth rate, meridional and vertical eddy propagation and jet intensification. It is also interesting to note that the CO2 radiative impacts on baroclinic instability are saturated at 750 ppm.

How to cite: Kaviani, M., Ahmadi-Givi, F., Mohebalhojeh, Ali. R., and Yazgi, D.: A Quantitative Assessment of the Impact of Increase in CO2 Concentration on Baroclinic Instability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-682, https://doi.org/10.5194/egusphere-egu2020-682, 2020.

D3443 |
Pei-Chun Hsu and Huang-Hsiung Hsu

There is a growing concern that human-induced climate change has been affecting weather systems. However, robust observational evidences that confirm the links between global warming and synoptic phenomena at the global scale are lacking. Here we reveal robust covarying signals between poleward temperature gradient and baroclinic life cycle of synoptic (1-10 days) eddies under global warming. We note that the changes in temperature structure in Northern Hemisphere winter and summer in the past decades are different. In boreal winter, the tropospheric warming has been larger in tropical upper troposphere and around 30°N than for the midlatitude (30-60°N). This inhomogeneous warming resulted in the enhancement of poleward temperature gradient in the subtropical upper troposphere and in the lower midlatitude (30-45°N). We observed correlated increasing trends in the entire baroclinic life cycle of synoptic eddies — including eddy fluxes of heat and momentum, and zonal mean jet — associated with steepened poleward temperature gradients in these regions in the winter Northern Hemisphere over the past four decades. By contrast, in the summer Northern Hemisphere, the overall tropospheric warming over the mid- to high-latitude land areas has been accompanied by weakly reduced synoptic eddy activities and zonal mean flow. Our findings suggest that if greenhouse gas–induced warming continue to change the atmospheric thermal structure as projected in a warming climate, extratropical synoptic disturbances and large-scale circulations may change accordingly.

How to cite: Hsu, P.-C. and Hsu, H.-H.: Coherent changes in large-scale thermal structure and baroclinic life cycle of synoptic eddies in the Northern Hemisphere under global warming , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13290, https://doi.org/10.5194/egusphere-egu2020-13290, 2020.

D3444 |
Bradford Barrett and Gina Henderson

Atmospheric flow blocking can be defined as a quasi-stationary center of high pressure that deflects traveling cyclones from their usual storm tracks. Over Greenland, flow blocking can result in a large-scale reversal of the meridional geopotential height gradient. Blocking often produces a strong equatorward deflection of polar air on the eastern flank of the anticyclone, and for Europe, blocking over or near Greenland can lead to severe cold episodes in winter and severe droughts and heat waves in summer. Moreover, because blocking is associated with an amplification of the meridional flow structure, it can be connected to extremes in poleward moisture transport. When these temperature and moisture extremes occur over Greenland itself, they exert significant stresses on the surface ice sheet. For these reasons, it is important to examine atmospheric blocking, both historically and in future climates.

There have been many metrics created to identify and quantify atmospheric flow blocking. Here, we use one such metric, the Greenland Blocking Index (GBI), to examine atmospheric flow blocking over and around Greenland and link that blocking to moisture transport. Moisture transport was calculated at each grid point in the ERA-Interim reanalysis using the integrated vapor transport (IVT) method applied between 200 and 1000 hPa. The GBI was calculated daily for the period 1948-present by averaging the 500-hPa height field in the NCEP/NCAR reanalysis over 80°E-20°E and 60°N-80°N. An IVT index was calculated from 1980-present by averaging IVT at each grid point over a North Atlantic region encompassing Greenland (85°E-15°E and 55°N-80°N). Both GBI and IVT were also examined in the historical NCAR CESM run of the Climate Model Intercomparison Project 6 (CMIP6), but over a much longer time record (1850-2015). In both datasets, extreme instances of blocking and IVT were examined at the 90th, 95th, 97th, and 99th percentiles, for both summer (JJA) and winter (DJF) seasons. Blocking frequency was found to increase in the latter half of the period in both datasets and over both time records. Moreover, a time lag was found between the instances of extreme blocking events and above-average IVT: high moisture transport more frequently preceeded instances of extreme blocking than lagged after it (by an average of 3 days). Implications of these results for Greenland ice mass balance will be explored in the presentation. 

How to cite: Barrett, B. and Henderson, G.: The past and future of flow blocking around Greenland: connections between extremes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3025, https://doi.org/10.5194/egusphere-egu2020-3025, 2020.

D3445 |
Maria Madsen and Jonathan Martin

The deficiency in predictability at subseasonal-to-seasonal timescales, as compared to prediction at conventional weather prediction timescales, is significant. Intraseasonal variability of atmospheric features like the jet stream, occurring within this gap, lead to extreme weather events that present considerable hazards to society. As jets are an important feature at the interface of the large-scale general circulation and the life cycle of individual weather systems, there is strong incentive to more comprehensively understand their variability.

The wintertime Pacific jet manifests its intraseasonal variability in two predominant modes: a zonal extension or retraction and a meridional shift by as much as 20° of the jet exit region. These two leading modes are associated with basin-scale anomalies in the Pacific that directly impact weather in Hawaii and continental North America. Although recent work has demonstrated the impact intramodal changes of the Pacific jet have on large-scale structure, sensible weather phenomena, and forecast skill in and around the vast North Pacific Basin, the transitions between the leading modes have hardly been considered and, therefore, are poorly understood. Consequently, this work examines the nature and predictability of transitions between modes of wintertime Pacific jet variability as well as their associated synoptic environments.

We apply two distinct but complementary statistical analyses to 70 cold seasons (NDJFM 1948/49-2017/18) of daily 250-hPa zonal winds from the NCEP/NCAR Reanalysis to investigate such transitions. Empirical orthogonal analysis (EOF)/principal component (PC) analysis is used to depict the state of the daily Pacific jet as a point in a two dimensional phase space defined by the two leading modes.  Supporting this technique is a self-organizing maps (SOMs) analysis that identifies non-orthogonal, synoptically recurring patterns of the Pacific jet. Together, these analyses show that there are, in fact, preferred transitions between these leading modes of variability. Composite and individual case analyses of preferred transition evolutions provides new insight into the synoptic-scale environments that drive Pacific jet variability.

How to cite: Madsen, M. and Martin, J.: Intraseasonal Transitions of the Wintertime Pacific Jet Stream, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3762, https://doi.org/10.5194/egusphere-egu2020-3762, 2020.

D3446 |
Reinhard Schiemann, Panos Athanasiadis, David Barriopedro, Francisco Doblas-Reyes, Katja Lohmann, Malcolm J. Roberts, Dmitry Sein, Christopher D. Roberts, Laurent Terray, and Pier Luigi Vidale

Global Climate Models (GCMs) are known to suffer from biases in the simulation of atmospheric blocking, and this study provides an assessment of how blocking is represented by the latest generation of GCMs. It is evaluated (i) how historical CMIP6 (Climate Model Intercomparison Project Phase 6) simulations perform compared to CMIP5 simulations, and (ii) how horizontal model resolution affects the simulation of blocking in the CMIP6-HighResMIP (PRIMAVERA) model ensemble, which is designed to address this type of question. Two blocking indices are used to evaluate the simulated mean blocking frequency and blocking persistence for the Euro-Atlantic and Pacific regions in winter and summer against the corresponding estimates from atmospheric reanalysis data. There is robust evidence that CMIP6 models simulate blocking frequency and persistence better than CMIP5 models in the Atlantic and Pacific and in winter and summer. This improvement is sizeable so that, for example, winter blocking frequency in the median CMIP5 model in a large Euro-Atlantic domain is underestimated by 32 % using the absolute geopotential height (AGP) blocking index, whereas the same number is 19 % for the median CMIP6 model. As for the sensitivity of simulated blocking to resolution, it is found that the resolution increase, from typically 100 km to 20 km grid spacing, in the PRIMAVERA models, which are not re-tuned at the higher resolutions, benefits the mean blocking frequency in the Atlantic in winter and summer, and in the Pacific in summer. Simulated blocking persistence, however, is not seen to improve with resolution. Our results are consistent with previous studies suggesting that resolution is one of a number of interacting factors necessary for an adequate simulation of blocking in GCMs. The improvements reported in this study hold promise for further reductions in blocking biases as model development continues.

How to cite: Schiemann, R., Athanasiadis, P., Barriopedro, D., Doblas-Reyes, F., Lohmann, K., Roberts, M. J., Sein, D., Roberts, C. D., Terray, L., and Vidale, P. L.: The representation of Northern Hemisphere blocking in current global climate models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6804, https://doi.org/10.5194/egusphere-egu2020-6804, 2020.

D3447 |
Luise J. Fischer, Dominik Büeler, Christian M. Grams, Urs Beyerle, David N. Bresch, and Heini Wernli

We present findings from an analysis of weather regimes over the North Atlantic and Europe in present and future climate conditions. Weather regimes strongly influence the statistical distribution of surface weather variables. We use a recently developed, all-season North Atlantic - European weather regime classification with seven regimes. These regimes were originally identified in ERA-Interim reanalyses and, in this study, we investigate how they are represented in climate simulations using the CESM1 large ensemble for present-day and future (RCP8.5) climate conditions. With these regimes, the classification of the flow conditions in the considered region goes beyond the classical categorization according to the North Atlantic oscillation index; the weather regimes explicitly capture different flavors of strong zonal flows and the occurrence of blocking over Greenland, Scandinavia, and Central Europe, respectively. In ERA-Interim they explain 70% of the variability in geopotential height at 500 hPa year-round. Our analysis quantifies how well CESM1 represents the statistics of the weather regimes in present-day climate and how strongly their frequencies change in the future climate scenario. In addition, we identify statistical relationships between weather regimes and their resulting impacts on spatial patterns of surface variables such as precipitation. We compare those patterns and characteristics of the weather regimes identified in ERA-Interim to their characteristics in simulations of present and future climate conditions.

This analysis leads to insight into the representation of and changes in atmospheric circulation in one particular climate model, and, at the same time, it quantifies how well the climate model captures the observed link between surface weather and weather regimes. This approach contributes to improving our understanding of atmospheric circulation changes and their impact on a regional scale, and it may benefit the interpretation and communication of climate projections.

How to cite: Fischer, L. J., Büeler, D., Grams, C. M., Beyerle, U., Bresch, D. N., and Wernli, H.: North Atlantic – European weather regimes in a changing climate: present and future, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4589, https://doi.org/10.5194/egusphere-egu2020-4589, 2020.

D3448 |
Chaim Garfinkel, Ian White, Edwin Gerber, and Martin Jucker

A common model bias in comprehensive climate models used in climate assessements such as the Coupled Model Intercomparison Project is a double inter-tropical convergence, with excessive precipitation in the tropical eastern South Pacific. In addition, the current generation of climate models cannot adequately resolve the dynamics of the Agulhas Current, and in particular the relative fraction of the Current that leaks into the Atlantic as opposed to retroflecting back into the Indian Ocean. The intermodel spread in the magnitude of the double ITCZ bias is   significantly correlated with the strength and phasing of   SH stationary waves in the CMIP archive, with models with a smaller bias generally showing more realistic stationary waves. An intermediate complexity moist General Circulation Model is used to demonstrate the causality of this connection: by fluxing heat out of  the tropical South Pacific Ocean, we can capture the  responses seen in CMIP5 models.  Finally, the same intermediate complexity moist General Circulation Model is used to demonstrate that an overly diffuse Agulhas leads to an equatorward shift of the Southern Hemisphere jet by more than  3degrees, and indeed an overly equatorward Southern Hemisphere jet is a common model bias in most CMIP5 models.

How to cite: Garfinkel, C., White, I., Gerber, E., and Jucker, M.: The impact of biases in the tropical South Pacific and near the Agulhaus Current on the large-scale Southern Hemisphere circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5124, https://doi.org/10.5194/egusphere-egu2020-5124, 2020.

D3449 |
Tiffany A Shaw and Robert J Graham

Modern theories of the midlatitude storm tracks connect their intensity to surface baroclinicity (latitudinal surface temperature gradient). However, simulations show storm tracks were weaker during past cold, icy climates relative to the modern climate even though surface baroclinicity was stronger. We revisit this surface baroclinicity-intensity puzzle for Snowball Earth using simulations across the climate model hierarchy. Here we show the Moist Static Energy framework for storm track intensity solves the puzzle for Snowball Earth. It connects the weaker storm track to the increase of surface albedo, decrease of latent heat flux and decrease of latitudinal surface Moist Static Energy gradient. Weaker intensity can be predicted assuming a surface ice albedo and zero latent heat flux (large Bowen ratio) everywhere in Snowball Earth. The weaker storm track is also consistent with weaker Mean Available Potential Energy (weaker upper-tropospheric baroclinicity), however that cannot be predicted. Overall, the exotic Snowball Earth climate reveals storm track intensity follows the surface Moist Static Energy gradient and not surface baroclinicity. Our insights may help resolve the puzzle in other climates such as the Last Glacial Maximum.

How to cite: Shaw, T. A. and Graham, R. J.: Moist static energy solves storm track intensity puzzle for Snowball Earth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2462, https://doi.org/10.5194/egusphere-egu2020-2462, 2020.

D3450 |
Sebastian Scher and Gabriele Messori

Little is known on whether and how global warming may affect the atmosphere's predictability and thus our ability to produce accurate weather forecasts. Here, we combine a climate and an ensemble weather prediction model to show that, in a business-as-usual 21st century setting, global warming could significantly change the predictability of the atmosphere, defined here via the expected error of weather predictions. Predictability of synoptic weather situations could significantly increase, especially in the Northern Hemisphere. This can be explained by a decrease in the meridional temperature gradient, which seems to control the inter-annual variability of atmospheric predictability. Contrarily, summertime predictability of weekly rainfall sums might significantly decrease in most regions.

How to cite: Scher, S. and Messori, G.: How Global Warming Changes the Difficulty of Synoptic Weather Forecasting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20184, https://doi.org/10.5194/egusphere-egu2020-20184, 2020.

D3451 |
Matthew Priestley, Duncan Ackerley, Jennifer Catto, Kevin Hodges, Ruth McDonald, and Robert Lee

Extratropical cyclones are the leading driver of the day-to-day weather variability and wintertime losses for Europe. In the latest generation of coupled climate models, CMIP6, it is hoped that with improved modelling capabilities come improvements in the structure of the storm track and the associated cyclones. Using an objective cyclone identification and tracking algorithm the mean state of the storm tracks in the CMIP6 models is assessed as well as the representation of explosively deepening cyclones. Any developments and improvements since the previous generation of models in CMIP5 are discussed, with focus on the impact of model resolution on storm track representation. Furthermore, large-scale drivers of any biases are investigated, with particular focus on the role of atmosphere-ocean coupling via associated AMIP simulations and also the influence of large-scale dynamical and thermodynamical features.

How to cite: Priestley, M., Ackerley, D., Catto, J., Hodges, K., McDonald, R., and Lee, R.: Drivers of biases in the extratropical storm tracks in CMIP6, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9745, https://doi.org/10.5194/egusphere-egu2020-9745, 2020.

D3452 |
Isla Simpson, Fances Davenport, Abdullah Al Fahad, and Flavio Lehner

Accurate future projections of the climate system are hindered by a number of sources of uncertainty: forcing uncertainty, internal variability and model structural uncertainty. An ``Emergent constraint'' is a technique that has been devised to reduce projection uncertainties arising from the model structural component. It consists of a statistical relationship (across a model ensemble) between a model’s representation of some aspect of the present day climate and its future projected climate change. This relationship can then be used to imply the future projected change, given the observed value of that present-day aspect. However, in order for the emergent constraint to be considered robust it must: (a) be accompanied by a physical mechanism and (b) be robust to out-of-sample testing.


In prior Coupled Model Intercomparison Projects (CMIP), in particular CMIP5, a number of emergent constraints on the large scale atmospheric circulation were proposed, with implications for regional hydroclimate change. These include: (1) a relationship between a model’s climatological jet latitude and its future projected poleward shift in the Southern Hemisphere; (2) a relationship between a model’s future projected wintertime circulation and hydroclimate change over North America and its climatological representation of stationary waves in the North Pacific; and (3) a relationship between a model’s future projected precipitation change over California and its representation of the relationship between ENSO and California precipitation. Constraints (2) and (3) actually imply opposite constraints on California precipitation changes for the real world, which speaks to the need for a deeper understanding of these emergent constraints and a comprehensive assessment of their robustness.


While the CMIP6 archive does not represent a true ``out-of-sample’’ test of CMIP5 emergent constraints, it does provide us with a new dataset composed of new and/or more advanced models in which to assess their robustness. This presentation will review the proposed emergent constraints on the large-scale atmospheric circulation and assess whether or not they are robust across both the CMIP5 and CMIP6 ensembles. Their potential for constraining regional hydroclimate projections will also be discussed.


How to cite: Simpson, I., Davenport, F., Al Fahad, A., and Lehner, F.: Do CMIP5 emergent constraints on the large scale atmospheric circulation work to constrain CMIP6 projections?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4001, https://doi.org/10.5194/egusphere-egu2020-4001, 2020.

D3453 |
Camille Li, Fumiaki Ogawa, Martin King, Jerry Tjiputra, Bjørnar Jensen, and Klaus Johannsen

IPCC climate models (CMIP3/5) predict a poleward shift of the Southern Hemisphere (SH) jet stream under global warming, with a large spread across the models. Efforts to find emergent constraints for the future jet shift (response) have relied on the simulated present-day jet position (observable). However, this has been investigated primarily in a zonal-mean framework, which averages out important zonal asymmetries. In this study, we revisit the problem allowing for variations in the longitude, height and season of the response to gain a better physical understanding of the nature of the future jet shift in individual models. Results from a manual data analysis will help guide an exploration of the problem using a big data approach, in particular, the application of a genetic algorithm that finds optimal solutions based on iterative random selection within large sample data spaces.

How to cite: Li, C., Ogawa, F., King, M., Tjiputra, J., Jensen, B., and Johannsen, K.: Southern Hemisphere jet stream: emergent constraints on future shift in zonally varying framework, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10820, https://doi.org/10.5194/egusphere-egu2020-10820, 2020.

D3454 |
Natasha Senior, Manoj Joshi, Adrian Matthews, and Pranab Deb

Intensification of extreme precipitation and weather events are some of the projections under a 2°C average global temperature increase scenario. Rossby wave trains may be triggered by anomalous tropical precipitation through the interaction of the associated upper level divergent wind and the vorticity gradients of the subtropical jet streams. In this way, anomalous tropical precipitation can influence weather patterns in the Northern Hemisphere. Owing to the quasi-linearity of this teleconnection pattern, it may be studied statistically as a series of signal-response functions. Here the anomalous precipitation events are treated as input forcings and the resulting geopotential height anomalies are the output signals. Through calculating the response functions we are able to realistically capture the 250 hPa geopotential height response to a step-like change in precipitation over the Maritime Continent or the eastern Indian Ocean during the boreal winter. When examining these responses using the same forcing for a selection of CMIP5 models, we find that there is a large inter-model spread, owing to differences in the model basic state. Since these teleconnection patterns are not faithfully represented in climate models, this can obscure our ability to develop realistic projections of atmospheric circulation and extreme weather. We discuss the potential of the linear response theory method to provide improved projections for Northern Hemisphere climate variability.

How to cite: Senior, N., Joshi, M., Matthews, A., and Deb, P.: A new method for studying the extratropical response to tropical precipitation anomalies and its role in improving projections of Northern Hemisphere climate variability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11911, https://doi.org/10.5194/egusphere-egu2020-11911, 2020.

D3455 |
Kana Nagashima, Jason Addison, and Naomi Harada

   The North Pacific Ocean is the largest geographic feature in the Northern Hemisphere and its interactions with the overlying atmosphere drives critical components of the global climate system. The Aleutian Low (AL), the semi-permanent atmospheric low-pressure system centered near the Aleutian Islands, is dynamically linked to environmental change in the North Pacific and surrounding continental areas. However, the multi-centennial and longer time-scale history of the AL during the Holocene is poorly understood.

   In this study, AL variability since 7.5 ka was examined by applying principal component analysis (PCA) to published δ18O data of sedimentary calcite, peat, and speleothem deposits (n = 7) from western North America. Extracted Principal Component 1 (PC1) is characterized by multi-centennial to millennial-scale oscillations, with a spatial loading pattern that suggests PC1 reflects intensification and westward shifts of the AL during ca. 7.3–7.1, 6.3–5.2, 3.6–3.3, 2.9–2.7, 2.6–2.1, 1.8–1.2 and 0.5–0.3 ka. The timing of these shifts are coeval to periods characterized by large meanderings of the Westerly Jet (WJ) Stream over East Asia and solar activity minima, which together suggest that AL variability is related to declines in solar irradiance through its interactions with the WJ. In contrast, PC2 represents a dramatic change between the middle and late Holocene, and appears to reflect long-term intensified AL conditions related to orbitally-driven El Niño–Southern Oscillation intensification between the middle to late Holocene at ~4.5 ka. These findings are critically important for understanding background natural climate variability during the Holocene.

How to cite: Nagashima, K., Addison, J., and Harada, N.: A review of Aleutian Low variability for the last 7,500 years using pan-Pacific delta 18O records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13085, https://doi.org/10.5194/egusphere-egu2020-13085, 2020.

D3456 |
Thomas Wood, Amanda Maycock, Christine McKenna, Andreas Chrysanthou, John Fyfe, and Francois Engelbrecht

The Southern Annular Mode (SAM) is the dominant mode of midlatitude atmospheric circulation variability in the Southern hemisphere. In the future, the SAM trend is expected to be the net result of opposing effects from increasing greenhouse gases (GHG) and ozone recovery. Different greenhouse gas scenarios, which induce different rates of surface and atmospheric temperature change, are therefore associated with different future SAM trends (Barnes et al., 2014). Since the magnitude of warming due to GHGs is an important component of this response, one might expect to find a relationship between equilibrium climate sensitivity (ECS) and future Southern hemisphere circulation trends. In CMIP5, the relationship between the SAM and the level of tropospheric warming across models was found to be strongest in the summer and autumn and could explain around 20% of the intermodel spread (Grise and Polvani, 2014). The spread is more strongly correlated with differences in meridional temperature gradients (Harvey et al., 2014).

Many of the latest CMIP6 models show a larger equilibrium climate sensitivity (ECS) of up to ~5.5 K (Forster et al., 2019) compared to a maximum of ~4.7 K in CMIP5. This raises the important question of how a higher level of warming affects projections of the SH midlatitude circulation. In this study, we examine the response of the SAM in CMIP6 models and quantify its relationship to ECS and temperature gradients. Our starting hypothesis is that stronger surface warming will induce a larger increase in tropical free tropospheric temperatures, and hence all being equal, a larger tropics-to-pole temperature gradient and a more positive SAM trend. However, results show that despite the higher level of warming in the CMIP6 models, there is a smaller positive trend in SAM index than in CMIP5 indicating a different relationship between warming and midlatitude circulation trends in CMIP6. We attempt to explain potential reasons for these differences.


Barnes, E.A., N.W. Barnes, and L.M. Polvani, 2014: Delayed Southern Hemisphere Climate Change Induced by Stratospheric Ozone Recovery, as Projected by the CMIP5 Models. J. Climate, 27, 852–867, https://doi.org/10.1175/JCLI-D-13-00246.1

Forster, P.M., Maycock, A.C., McKenna, C.M. et al. (2019), Latest climate models confirm need for urgent mitigation. Nat. Clim. Chang. (2019) doi:10.1038/s41558-019-0660-0

Grise, K. M., and Polvani, L. M. (2014), Is climate sensitivity related to dynamical sensitivity? A Southern Hemisphere perspective, Geophys. Res. Lett., 41, 534– 540, doi:10.1002/2013GL058466.

Harvey, B.J., Shaffrey, L.C. & Woollings, T.J. (2014) Equator­-to-­pole temperature differences and the extra­tropical storm track responses of the CMIP5 climate models, Clim Dyn, 43: 1171. https://doi.org/10.1007/s00382-013-1883-9

How to cite: Wood, T., Maycock, A., McKenna, C., Chrysanthou, A., Fyfe, J., and Engelbrecht, F.: Revisiting the relationship between dynamical sensitivity and climate sensitivity in the Southern hemisphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16108, https://doi.org/10.5194/egusphere-egu2020-16108, 2020.

D3457 |
Luke Davis, David Thompson, and Thomas Birner
The dry dynamical core represents one of the simplest possible numerical models for studying the response of the extratropical circulation to climate change. In the model, the circulation is forced by relaxing temperature to a notional “equilibrium” using linear damping. The linear damping coefficient plays an essential role in governing the structure of the circulation. But despite decades of research with the dry dynamical core, the role of the damping coefficient in governing the circulation has received relatively little scrutiny.

In this work, we systematically vary the damping coefficient in a dry dynamical core in order to understand how the amplitude of the damping influences extratropical dynamics. Critically, we prove that the local climate feedback parameter is proportional to the damping coefficient – that is, the damping timescale is a measure of climate sensitivity for the dry atmosphere. The key finding is that the steady-state extratropical circulation responds to changes in this climate sensitivity.

Longer damping timescales (i.e. higher climate sensitivities) lead to a less dynamically active extratropical circulation, stronger and more persistent annular modes, and equatorward shifts in the jet. When perturbed with climate change-like forcings, changing the damping timescale can also change the dynamical response to the forcing. We argue that understanding the response of the circulation to climate change is critically dependent on understanding its climate sensitivity, and consider how climate sensitivity might be inferred from its effect on the circulation in the dry model and more complex general circulation models.

How to cite: Davis, L., Thompson, D., and Birner, T.: Connections between climate sensitivity and the extratropical circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22522, https://doi.org/10.5194/egusphere-egu2020-22522, 2020.

D3458 |
Roland Walz, Hella Garny, and Thomas Birner

A dry dynamical-core model is used to investigate the regime behavior of the polar vortex under the influence of tropical upper-tropospheric warming. Up to 5 K temperature increase in this region, the polar vortex strength and variability hardly changes. Only for temperature increases above 8 K the polar night jet speeds up by approximately 20 m s−1 and the probability of sudden stratospheric warmings is strongly reduced.

A comparison of climatological-mean differences of the zonal-mean zonal winds between the two regimes and the first empirical orthogonal function of the zonal-mean zonal wind closest to the regime transition at around 7.5 K temperature increase reveals that the system oscillates between both regimes at the regime transition. Every regime is present for a long time accounting for the peaked autocorrelation time scale being distinctive of a regime transition. From a dynamical point of view the strong polar vortex regime is characterized by less negative Eliassen-Palm (EP) flux divergence in the stratosphere and an equatorward refraction of EP flux in the midlatitudes compared to the weak polar vortex regime.

In order to quantify the influence of the polar vortex on the tropospheric circulation during tropospheric warming, another set of tropical upper-tropospheric heating simulations without a polar vortex is performed. This reveals that the latitudes of the tropospheric jets in both sets of simulations coincide for tropical upper-tropospheric warmings up to 5 K, or equivalently, when the polar vortex is in its weak regime. However, when the polar vortex starts to transition to the strong regime, i.e. for tropospheric warmings above 5 K, the poleward contraction of the tropospheric jet is strongly enhanced compared to the set of simulations without polar vortex.

How to cite: Walz, R., Garny, H., and Birner, T.: Polar vortex regimes in a simple general circulation model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4961, https://doi.org/10.5194/egusphere-egu2020-4961, 2020.

D3459 |
Iris Manola, Dim Coumou, Andrea Alessandri, Edouard Davin, Suqi Guo, Felix Havermann, Steven De Hertog, Quentin Lejeune, Inga Menke, Julia Pongratz, Carl Schleussner, Sonia Seneviratne, and Wim Thiery

Land cover and land management (LCLM) changes have a high potential to influence the biogeophysical and biogeochemical earth system processes. The interaction of soil and vegetation with the atmosphere alternates the water, energy and momentum balance, in turn affecting the climate locally, as well as the climate of distant regions through teleconnection pathways. This, among others, might benefit or oppose risks to local and global breadbasket regions, impacting the crop yields.

In this study, we conduct model experiments to assess the local and remote impact of LCLM changes, in particular global re-/afforestation and deforestation, with a focus on the large-scale boreal summer atmospheric circulation. We hypothesize that due to the dominant role of land-atmosphere feedbacks in this season, robust dynamical transformations take place due to the LCLM changes. The idealized model experiments consist of three fully coupled Earth System Models (EC-EARTH, MPI-ESM and CESM) that run under constant 2015 greenhouse forcing for 150 years. Globally the LCLM changes go through a sequence of unchanged grid boxes in a checkerboard approach as recent studies have done, in order to accurately separate the local from the non-local effects.

How to cite: Manola, I., Coumou, D., Alessandri, A., Davin, E., Guo, S., Havermann, F., De Hertog, S., Lejeune, Q., Menke, I., Pongratz, J., Schleussner, C., Seneviratne, S., and Thiery, W.: Impacts of global re-/afforestation and deforestation on large scale atmospheric circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19444, https://doi.org/10.5194/egusphere-egu2020-19444, 2020.

D3460 |
Binhe Luo, Dehai Luo, Aiguo Dai, and Lixin Wu

Winter surface air temperature (SAT) over North America exhibits pronounced variability on sub-seasonal-to-interdecadal timescales, but its causes are not fully understood. Here observational and reanalysis data from 1950-2017 are analyzed to investigate these causes. Detrended daily SAT data reveals a known warm-west/cold-east (WWCE) dipole over midlatitude North America and a cold-north/warm-south (CNWS) dipole over eastern North America. It is found that while the North Pacific blocking (PB) is important for the WWCE and CNWS dipoles, they also depend on the phase of the North Atlantic Oscillation (NAO). When a negative-phase NAO (NAO-) concurs with PB, the WWCE dipole is enhanced (compared with the PB alone case) and it also leads to a warm north/cold south dipole anomaly in eastern North America; but when PB occurs with a positive-phase NAO (NAO+), the WWCE dipole weakens and the CNWS dipole is enhanced. In particular, the WWCE dipole is favored by a combination of eastward-displaced PB and NAO- that form a negative Arctic Oscillation. Furthermore, a WWCE dipole can form over midlatitude North America when PB occurs together with southward-displaced NAO+.The PB events concurring with NAO- (NAO+) and SAT WWCE (CNWS) dipole are favored by the El Nio-like (La Nia-like) SST mode, though related to the North Atlantic warm-cold-warm (cold-warm-cold) SST tripole pattern. It is also found that the North Pacific mode tends to enhance the WWCE SAT dipole through increasing PB-NAO- events and producing the WWCE SAT dipole component related to the PB-NAO+ events because the PB and NAO+ form a more zonal wave train in this case.

How to cite: Luo, B., Luo, D., Dai, A., and Wu, L.: North Hemispheric winter air temperature variability and its linkage to North Pacific and North Atlantic SST modes?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16, https://doi.org/10.5194/egusphere-egu2020-16, 2020.

D3461 |
Paul Loikith, Valerie Thaler, Luana Albertani Pampuch, C. Roberto Mechoso, Armineh Barkhordarian, and Huikyo Lee

A multivariate assessment of climate model projections over South America from the CMIP5 archive is presented. Change in near-surface temperature, precipitation, evapotranspiration, integrated water vapor transport (IVT), sea level pressure, and wind at multiple pressure levels is quantified across the multi-model suite and an assessment of model-to-model agreement on projected change performed. All models project warming by the mid- and late-21st century throughout the continent, with the highest magnitude projected over tropical regions. The CMIP5 models are in strong agreement that precipitation will decrease in all seasons over portions of Patagonia, especially along the northern portions of the current-climate mid-latitude storm track. This is consistent with a robustly projected poleward shift of the Pacific extratropical high and mid-latitude storm track indicated by a systematic increase in sea level pressure and decrease in westerly wind over Patagonia. Decreased precipitation for the months of September, October, and November is also projected, with strong model agreement, over portions of northern and northeastern Brazil, coincident with decreases in sea level pressure and increases in evapotranspiration. IVT is broadly projected to decrease over southern South America, coincident with the projected poleward shift of the mid-latitude storm track indicators, with increases projected in the vicinity of the South Atlantic Convergence Zone in austral spring and summer. Further decomposition of the thermodynamic and dynamic components to this change in IVT indicate that the projected decreases in the mid-latitudes are primarily driven by changes in circulation (i.e. dynamic) while the sub-tropical and tropical changes have a predominantly thermodynamic origin. Results provide a comprehensive picture of climate change across South America and highlight where projections should be interpreted with the most confidence.

How to cite: Loikith, P., Thaler, V., Albertani Pampuch, L., Mechoso, C. R., Barkhordarian, A., and Lee, H.: A multivariate assessment of climate change projections over South America using CMIP5 models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-186, https://doi.org/10.5194/egusphere-egu2020-186, 2020.

D3462 |
Evgeniya Durneva

Global climate changes particularly observed in the Arctic region are influenced on the formation of circulation in the atmosphere. The planetary high-altitude frontal zone for midlatitudes has analyzed from 1991 to 2019 in the summer period, on July. Deviations poleward from the normal of high-altitude frontal zone and jet stream have observed, particularly marked over the Eurasia during last decades. Changes in the form and decreasing of intensity of high-altitude jet streams are noted, which further contribute to the formation of blocking anticyclones and increasing in the incidence of anomalous weather events.

The case of July 2018 is presented in this work. The anomalous high temperature in Scandinavia and north area of the European part of Russia have observed due to formation of the blocking over this territory. The main reason for the formation of blocking is the instability of the jet stream. The characteristics (intensity, position relative to the North Pole and form) of the arctic and midlatitudes jet stream have analyzed.

How to cite: Durneva, E.: Variability of planetary high-altitude frontal zone and jet stream in the Northern hemisphere from 1991 to 2019 in the summer period, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-229, https://doi.org/10.5194/egusphere-egu2020-229, 2020.

D3463 |
Masaru Inatsu, Takuya Aikawa, and Naoto Nakano

This paper proposes a new method to identify atmospheric blocking development without the time filtering used in previous studies. A mode-decomposed vorticity equation is formulated from the principal components (PCs) of 500-hPa geopotential height by applying a new idea; the orthonormality of PCs allows any variable to be decomposed into a projection corresponding to the PCs. To test this, sectorial blocking episodes in Northern Hemisphere winter were identified by Barriopedro’s method. A blocking index was defined for each longitudinal range as the linear combination of the 10 largest PCs by means of the composite for the blocking episodes. Blocking development was diagnosed, in terms of the low modes of PC1–PC10 and the high modes of PC11–PC50. The results suggest that the intensification of blocking over the North Pacific and Eurasia is associated with nonlinear interaction among high modes, whereas the intensification (decay) of North Atlantic blocks is related mainly to enhanced nonlinear interaction among low-frequency (high-frequency) eddies. This main result is insensitive to the choice of definition for blocks and the choice of the mode separation boundary.

How to cite: Inatsu, M., Aikawa, T., and Nakano, N.: Mode-Decomposed Equation Diagnosis for Atmospheric Blocking Development, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1796, https://doi.org/10.5194/egusphere-egu2020-1796, 2020.

Chat time: Thursday, 7 May 2020, 10:45–12:30

D3464 |
Bernat Jiménez-Esteve and Daniela I.V. Domeisen

El Niño Southern Oscillation (ENSO) can exert a remote impact on North Atlantic and European (NAE) winter climate. This teleconnection is driven by the superposition and interaction of different influences, which are generally grouped into two main pathways, namely the tropospheric and stratospheric pathways. In this study, we focus on the tropospheric pathway through the North Pacific and across the North American continent. Due to the possible non-stationary behaviour and the limited time period covered by reanalysis data sets, the potential nonlinearity of this pathway remains unclear. In order to address this question, we use a simplified physics atmospheric model forced with seasonally varying prescribed sea surface temperatures (SST) following the evolution of different ENSO phases with linearly varying strength at a fixed location. To isolate the tropospheric pathway the zonal mean stratospheric winds are nudged towards the model climatology. The model experiments indicate that the tropospheric pathway of ENSO to the North Atlantic exhibits significant nonlinearity with respect to the tropical SST forcing, both in the location and amplitude of the impacts. For example, strong El Niño leads to a significantly stronger impact over the North Atlantic Oscillation (NAO) than a La Niña forcing of the same amplitude. For La Niña forcings, there is a saturation in the response, with no further increase in the NAO impact even when doubling the SST forcing, while this is not the case for El Niño. These findings may have important consequences for long-range prediction of the North Atlantic and Europe.

How to cite: Jiménez-Esteve, B. and Domeisen, D. I. V.: Nonlinearity in the Tropospheric Pathway of ENSO to the North Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2703, https://doi.org/10.5194/egusphere-egu2020-2703, 2020.

D3465 |
Chang-Hyun Park, Seok-Woo Son, and Jung Choi

During El Niño winters, East Asia and western North America become anomalously warm because of the combined effect of anti-cyclonic circulation anomaly over Kuroshio Extension and Philippine sea, and an enhanced Aleutian Low. However, this El Niño-Southern Oscillation (ENSO)-North Pacific teleconnection disappears in early January. In this study, we suggest that this breakdown in regional teleconnection is partly due to Madden-Julian Oscillation (MJO). In early December of El Niño winters, MJOs frequently form and reach at Western Pacific, causing positive intraseasonal Pacific North American (PNA)-like teleconnection, which is same pattern to the El Niño teleconnection. In mid-December, however, as MJOs are frequently organized over Indian Ocean, it causes a destructive interference, cancelling El Niño teleconnection in early January. Although weak and not statistically significant, this sharp decline of ENSO teleconnection in early January also appears in La Niña winters. A preference of MJO organization and its propagation in ENSO winters are explained by moist static energy anomalies in the west Indian Ocean. This result suggests that MJO is important for predicting ENSO teleconnection on intraseasonal scales.

How to cite: Park, C.-H., Son, S.-W., and Choi, J.: A breakdown of ENSO-North Pacific Teleconnection in early January , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2941, https://doi.org/10.5194/egusphere-egu2020-2941, 2020.

D3466 |
Lei Du and Riyu Lu

The present study investigates the intraseasonal variations of meridional winds over North Pacific during summer based on reanalysis datasets. It is shown that the band of 10-30 days is the main component of total intraseasonal varaitions. We identified a teleconnection pattern over North Pacific at this band . This teleconnection pattern is characterized by a zonally-oriented wave-like structure with a zonal wavenumber 5, and does not show a phase-locking feature. In addition, the anomalies associated with the teleconnection pattern exhibit a roughly baratropic structure. Further analyses suggest that the teleconnection pattern can gain energy from the basic flow through the baroclinic energy conversion, while the barotropic energy conversion plays a trivial role.

How to cite: Du, L. and Lu, R.: A teleconnection pattern of 10-30-day atmospheric oscilations over North Pacific during summer: Chracteristics and maintainance mechanism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3187, https://doi.org/10.5194/egusphere-egu2020-3187, 2020.

D3467 |
Xiang Li and Hui Gao

    Under the global warming scenarios, the air temperatures (T2m) in China in boreal winter shows a remarkable increasing trend since the 1980s, which is quite similar with the change of the globe. But in Northeast China (NEC), the temperature displays an opposite characteristics with an obvious decreasing trend in recent two decades. Results of the empirical orthogonal functions (EOF) of T2m in China indicate that the first leading mode is a consistent positive or negative temperature departures in the whole country, but the variance of this mode show a weakening tendency. The second leading mode of T2m in China shows a seesaw temperature anomaly pattern in NEC and in other regions of eastern China. Different from the 1st EOF mode, variances of this mode show an intensifying tendency. Both statistical analysis and case studies of 20 winters during 2000 to 2019 indicate that this opposite change in NEC may be related to the decadal relationship between the Siberian high and the Arctic oscillation. Previous studies explored that there was a significant negative correlation between the two factors, but this relationship was significantly weakened in the past two decades, which led to the independent influences from the two circulation members on the temperature in NEC, and consequently resulted in an inconsistent variation in the region.

How to cite: Li, X. and Gao, H.: Why Northeast China Has a Cooling Trend in 21st century?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3228, https://doi.org/10.5194/egusphere-egu2020-3228, 2020.

D3468 |
Tuantuan Zhang, Xingwen Jiang, Chi-Yung Tam, Junwen Chen, Ngar-Cheung Lau, Song Yang, and Zunya Wang

This is a consensus that snow over the Tibetan Plateau (TP) modulates the regional climate significantly. Possible causes for the interannual variability of snow over the TP, however, are under debate, especially regarding the independent roles of El Niño-Southern (ENSO) and Indian Ocean dipole (IOD). Based on in-situ observational data analyses and model simulations, our study shows that impacts of ENSO and IOD on snow depth (SD) over the TP are different during early winter. In particular, ENSO mostly affects SD over the eastern TP, while IOD affects SD over the central-western TP. Both above-normal snowfall and cold temperature anomaly contribute to deeper-than-normal SD, with the former playing a more important role. Diabatic cooling of the suppressed convection over the western North Pacific that related to the positive phase of ENSO could excite an anomalous cyclonic circulation and strong cold temperature anomalies over the eastern TP. There is an enhanced moisture transported over the eastern TP from the tropics due to the anomalous cyclonic circulation; along with strong cold temperature anomalies, resulting in above-normal snowfall in the eastern TP. On the other hand, anomalous convection over the western Indian Ocean related to the positive IOD could generate a wave-train propagating northeastward and induce an anomalous cyclonic circulation over the central-western TP. The associated anomalous circulation transports extra moisture from the tropics to the central-western TP, providing conditions favorable for more snowfall over the central-western TP. Opposite conditions tend to occur during negative phases of ENSO and IOD.

How to cite: Zhang, T., Jiang, X., Tam, C.-Y., Chen, J., Lau, N.-C., Yang, S., and Wang, Z.: Impacts of ENSO and IOD on Snow Depth over the Tibetan Plateau: Roles of Convections over the Western North Pacific and Indian Ocean , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3630, https://doi.org/10.5194/egusphere-egu2020-3630, 2020.

D3469 |
Hasi Aru

The western Pacific pattern (WP) is one of the most prominent teleconnection patterns over the Northern Hemisphere (NH) in boreal winter. There exist several methods employed to identify the WP in the literature. This study compares eight WPs defined by different methods. Correlation coefficients among the eight WP indices (WPIs) show considerable spreads, though most of them are statistically significant. The meridional dipole structure of WP can be captured by all of the WPIs, but it shows large spreads in the locations of the centers. Several WPIs produce a significant correlation with the winter Arctic Oscillation, with marked signals of atmospheric anomalies over the Arctic region. Connections of the WPs with the simultaneous winter El Niño-Southern Oscillation (ENSO) depend largely upon their definitions. Impacts of the WPs on the surface air temperature over many parts of Eurasia and North America are also sensitive to their definitions. Differences in the surface air temperature anomalies are closely related to differences in the spatial structure of the WPs. Finally, we define a new WP index as differences in the area-average 500-hPa geopotential height anomalies between subtropics and mid-latitude of northwestern Pacific. This newly defined WP index has a close relation with the above eight WPIs, the tropical Pacific sea surface temperature and surface air temperature anomalies over Eurasia and North America.

How to cite: Aru, H.: Comparisons of different definitions of the western Pacific pattern and associated winter climate anomalies in Eurasia and North America, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4297, https://doi.org/10.5194/egusphere-egu2020-4297, 2020.

D3470 |
Sara Ivasić and Ivana Herceg Bulić

Atmospheric variability and predictable components over North Atlantic-European area were analyzed using an atmospheric general circulation model of intermediate complexity (ICTP AGCM). In order to extract individual modes of variability occurring in the ensemble of numerical simulations, EOF analysis was applied onto the fields of the 200 hPa geopotential height and total precipitation. The same variables were selected for the signal-to-noise optimal patterns method, which identifies the patterns that maximize the signal-to-noise ratio, following Straus et al. (2003).

To detect the potential impact of tropical ocean SSTs, five experiments based on a 35-member ensemble of simulations for the 1855 – 2010 period were conducted. Each experiment was forced with observed SST anomalies prescribed in different ocean areas: the experiment with climatological SSTs (i.e. no SST forcing), SST anomalies prescribed globally, SST forcing prescribed in the entire tropical zone, SST forcing constrained to the tropical Atlantic, and the experiment with SST forcing constrained to the tropical Pacific.

SST forcing impacts the interannual variability of the geopotential height and total precipitation, represented with EOF1 and EOF2 patterns, only in the frequency of occurrence of a certain atmospheric mode. In the winter season the first EOF pattern projects onto the NAO, while the second EOF pattern projects onto the Atlantic ridge.

The signal-to-noise optimal patterns method has shown that the optimal patterns and signal-to-noise ratio are affected by the boundary forcing of the oceans.

How to cite: Ivasić, S. and Herceg Bulić, I.: Impact of tropical ocean SSTs on the variability and predictable components of seasonal atmospheric circulation in the North Atlantic – European area , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4542, https://doi.org/10.5194/egusphere-egu2020-4542, 2020.

D3471 |
Marianna Benassi, Giovanni Conti, Silvio Gualdi, Paolo Ruggeri, Javier Garcia–Serrano, Froila Palmeiro, Lauriane Battè, and Constantin Ardilouze

El Nino Southern Oscillation (ENSO) represents the major driver of interannual climate variability at the global scale. Observational and model-based studies have fostered a long-standing debate on the shape and the intensity of ENSO influence over the Euro-Mediterranean sector. Indeed, the detection of this signal is strongly affected by the large variability which characterizes the atmospheric circulation in the North Atlantic and European sector.

Different mechanisms have been proposed as involved in the propagation of ENSO signal from low to mid latitude, and we want to investigate if and how the low frequency variability of North Pacific sea-surface temperature (SST) may affect their efficacy. In this work, we study how the different phases of the extratropical SST pattern linked to the Pacific Decadal Oscillation (PDO) modulates the ENSO fingerprint over the Euro-Mediterranean region.

A set of idealized sensitivity experiments designed in the framework of the MEDSCOPE project has permitted to identify the ENSO teleconnection over the Euro-Mediterranean domain and to reveal the potential modulating role of the different phases of the extratropical PDO SST forcing.

In order to place this process in a dynamical framework, a tropospheric pathway has been proposed. The propagation of planetary waves from low to mid latitude has been investigated, by looking at the sensitivity of this mechanism to different underlying mean state.

These results allow to gain a deeper understanding of the links between mid-latitude climate variability and tropical forcing and of the processes ruling the low-mid latitude teleconnection in the Northern Hemisphere. Moreover, a clearer insight of these processes may lead to a new comprehension of possible sources of predictability for the Euro-Mediterranean domain over different time scales.


How to cite: Benassi, M., Conti, G., Gualdi, S., Ruggeri, P., Garcia–Serrano, J., Palmeiro, F., Battè, L., and Ardilouze, C.: ENSO teleconnection over the Euro-Mediterranean sector: the role of extratropical Pacific modulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4891, https://doi.org/10.5194/egusphere-egu2020-4891, 2020.

D3472 |
Zuowei Xie, Robert Black, and Yi Deng

The roles of planetary and synoptic-scale waves in extreme cold wave (ECW) events over the southeastern (SE) and northwestern (NW) United States (US) are studied using a spherical harmonic decomposition in conjunction with piecewise tendency diagnosis (PTD). Planetary waves and synoptic waves jointly work together to initiate ECW events. Notably, the planetary waves not only provide a direct contribution to circulation field enacting ECW events but also alter the background circulation field in such a manner that promotes synoptic waves growth via increases in regional barotropic deformation. The SE-ECW events, concurrent with the Northern Hemisphere annular mode (NAM) negative phase, feature high latitude intensification and subsequent southeastward movement of cold surface air temperature (SAT) anomalies. The planetary-scale pattern provides a sizable contribution to the total wave pattern on both sea level pressure (SLP) and upper level. Moreover, the negative NAM planetary anomaly acts to displace the jet equatorward and thereby increases the barotropic deformation of the synoptic-scale anomaly over southeastern US. PTD confirms that the planetary-scale barotropic deformation plays a key role in deepening the negative height anomaly with a secondary contribution from baroclinic growth. In contrast, NW-ECW events feature a regional SAT cold anomaly that intensified in situ in association with a quasi-stationary positive SLP anomaly with a substantial planetary-scale wave component. The upper level circulation is characterized by a pronounced anomalous ridge over the Gulf of Alaska and a northeast-southwest tilted negative height anomaly to its east. The negative height anomaly axis is orthogonal to the planetary-scale dilatation, result in a stronger planetary barotropic deformation of the incipient negative height anomaly.

How to cite: Xie, Z., Black, R., and Deng, Y.: Planetary and synoptic-scale dynamic control of extreme cold wave patterns over the United States, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5549, https://doi.org/10.5194/egusphere-egu2020-5549, 2020.

D3473 |
Jil Kiefer, Christina Karamperidou, and Pedro DiNezio

Sudden and strong positive sea surface temperatures (SST) anomalies in the far eastern Pacific that are tied to the rather narrow NINO1+2 (0-10°S, 90°W-80°W) region are known as coastal El Niño events. In contrast to the well-studied features of typical basin-scale ENSO events, the frequency, origin and relevant processes of coastal El Niños are largely unknown. Here, we analyze their characteristics and future behavior using observational data and simulations with the Community Earth System Model (CESM), which exhibits skill in simulating precipitation, wind and SST fields associated with coastal El Niños. We find that tropical Pacific basin-scale ocean dynamics – in sharp contrast to a typical El Niño event  – play no major role in the evolution of a coastal El Niño. On the other hand, we find that atmospheric circulation anomalies from the Southern Hemisphere lead the evolution of coastal El Niño events, by causing warm SST anomalies that then propagate into the NINO1+2 region. Once initiated, local thermodynamical feedback processes such as cloud radiation feedbacks are responsible for the growth and decay of the events. Greenhouse gas forcing leads to an increase in the frequency of coastal El Niño events and a shift of their peak month in CESM simulations, related to a shallowing of the thermocline and changes in Rossby wave forcing from the Southern Hemisphere. In conclusion, based on our results, we will demonstrate the potential for increased predictability of coastal El Niño events, whose intense coastal SST warming and associated extreme precipitation poses a serious threat for local communities via loss of life and severe economic damage.

How to cite: Kiefer, J., Karamperidou, C., and DiNezio, P.: Dynamics of Coastal El Niño Events in present and future climates., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6179, https://doi.org/10.5194/egusphere-egu2020-6179, 2020.

D3474 |
Chen Shi, Wang Kaicun, and Zhou Chunlüe

Heatwave is affected by large-scale atmospheric circulation on temperature-related climates in the context of global warming. Recently Northern China have experienced an increase in heatwaves which is partly due to the atmospheric circulation. This study aims to address the influence clearly. Northern China heatwaves are computed on excess hot factor (EHF) and the five EHF indexes are studied afterwards to get a picture of heatwaves in summer Northern China. China circulation patterns are classified into nine typical circulation patterns on self-organizing map (SOM) which then can be described quantitatively by pattern factors: frequency, persistence and maximum persistence. Pearson correlation analysis and stepwise regression analysis are applied for exploring the impact. Results show the spatial pattern of the times of individual heatwave event (HWN) and the days of the longest heatwave duration (HWD) are high value everywhere in Northern China. The overall EHF indexes all rising in time series (P<0.05) and the regional heatwave occurrence have trends of 0.79 day per year (P<0.05). However, the factors of the patterns show inconspicuous tendency. Two patterns with significant correlations (P<0.05) are proved to be suggestive of Okhotsk Sea high and West Pacific Subtropical High. It declares that the Okhotsk Sea high favors Northern China heatwave occurrence rather than subtropical high: the warm center over Okhotsk Sea transfer heat upper and west, generating the high temperature and persist high pressure system, causing heatwave happening in summer Northern China. The two related atmospheric circulation patterns explain 38% of the heatwave occurrence based on stepwise regression model, the Okhotsk Sea high gets the coefficient of 0.443 and the subtropical high is -0.347.  

How to cite: Shi, C., Kaicun, W., and Chunlüe, Z.: The impact of large-scale atmospheric patterns on Heatwaves in summer Northern China based on Self-Organizing Map, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6209, https://doi.org/10.5194/egusphere-egu2020-6209, 2020.

D3475 |
Sahil Sharma and Kyung-Ja Ha

The equatorial zonal asymmetric (Walker) circulation causes changes in the tropical rainfall pattern which induces devastation flood and drought that considerably impact the lives of millions of people. However, understanding of changes in zonal circulation is not yet certain. Here we examine the robustness of changes in Indian Walker Circulation (IWC) characteristics using different reanalysis and observation datasets in terms of the linear trends of IWC. The meridional (5oS:5oN)  averaged vertical velocity using different datasets are used to precisely locate the ascending (94oE:104oE, eastern) and descending (35oE:45oE, western) branch of IWC. We analyzed the zonal sea level pressure (SLP) gradient, velocity potential (VP) at 850 and 200 hPa, surface zonal wind (SZW) and zonal mass stream function (ZMSF) anomalies over the period of 1980–2017. We found that the magnitude of ZMSF representing anticlockwise circulation has an increasing trend in all the datasets. This kind of change is physically in agreement with the changes of SLP and SZW (an increasing trend in westerlies over the central IO) while the VP shows the decreasing trend which is in agreement with the strengthening of IWC during the recent decades. JRA55 is the most reliable which shows the significant and highest trend among all other datasets. The change point detection using the Pettitt method is applied to the normalized mean of all datasets which determines that in the post-1997-98 there is a significant strengthening of IWC as compared to the pre-1997-98 which demonstrates that IWC is highly sensitive by super El-Nino. The attribution of this strengthening can be examined using the CMIP5/6 datasets to determine the relative contribution of anthropogenic warming and natural variability.

How to cite: Sharma, S. and Ha, K.-J.: Equatorial atmospheric zonal circulation changes over India Ocean during recent decades, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6285, https://doi.org/10.5194/egusphere-egu2020-6285, 2020.

D3476 |
Yuting Wu, Xiaoming Hu, Ziqian Wang, Zhenning Li, and Song Yang

The surface temperature cold bias over the Tibetan Plateau (TP) is a long-lasting problem in both reanalysis data and climate models. While previous studies have mainly focused on local processes for this bias, the TP surface temperature is also closely related to tropical SST in both observations and Coupled Model Inter-comparison Project (CMIP5) models. This study investigates the role of tropical SST climatological bias in the TP surface temperature cold bias, and analysis of CMIP5 models suggests that the surface temperature cold bias over the TP is more obvious (about 4 K) in winter, with an east-west distribution pattern, than in summer (about 1 K), with a south-north distribution pattern. Considering that the tropical SST bias in CMIP5 models may be an important source of the TP surface temperature cold bias, a series of model experiments were conducted by the NCAR CAM4 to test the hypothesis. Model experiment results show that the tropical SST bias can reproduce cold bias over the TP, with 2 K in winter and about 0.5 K in summer. The mechanisms for TP surface temperature cold bias are different in winter and summer. In winter, tropical SST bias influences the TP surface temperature mainly by anomalous snow cover, while anomalous precipitation and clouds are more important for the temperature bias in summer.

How to cite: Wu, Y., Hu, X., Wang, Z., Li, Z., and Yang, S.: Remote Sources of Surface Temperature Cold Bias over the Tibetan Plateau: Role of Tropical SST Climatological Bias, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6380, https://doi.org/10.5194/egusphere-egu2020-6380, 2020.

D3477 |
Sally Dacie and Jiawei Bao

Using just a few simple assumptions and a widely used radiative transfer scheme, we try to explain why rainfall increases in a warming climate and predict how the tropical mean circulation changes to accommodate this. We use konrad, a 1D radiative-convective equilibrium model, in which convection is handled simply by specifying the tropospheric lapse rate and enthalpy conservation (similar to Manabe and Wetherald 1967). Considering energy balance at the surface or equivalently in the whole of the atmosphere, evaporation or condensation rates can be calculated (as in Jeevanjee and Romps 2018), giving precipitation increases of 2.0-2.7% per Kelvin increase in surface temperature. The direct radiative effect of carbon dioxide results in a decrease in precipitation, but the warming it induces leads to an overall increase in precipitation at the new equilibrium state of 1.4-2.0%/K. Thus, in agreement with global modelling studies (eg. Flaeschner et al 2016), we expect that the continual increase in atmospheric carbon dioxide in the real world is suppressing the increase in mean precipitation that will occur in the long term. To derive the convective mass flux as well as the downwelling and upwelling velocities and area fractions from our single column model output, we think of the atmosphere as two distinct regions, a saturated moist region of upward motion and a non-saturated region where adiabatic motion balances radiative cooling. The mass flux decreases in a warming climate as the increase in water vapour available for condensation is larger than the increase in condensation rate required to balance radiative cooling. Further, using our simplistic approach we find an increase in upwelling area in a warming climate. This has implications for convective aggregation and how it may change with climate change.

How to cite: Dacie, S. and Bao, J.: A simple derivation of tropical mean precipitation and circulation changes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7508, https://doi.org/10.5194/egusphere-egu2020-7508, 2020.

D3478 |
Pedro M. Sousa, Alexandre M. Ramos, Ricardo M. Trigo, Christoph C. Raible, Martina Messmer, Joaquim G. Pinto, and Ricardo Tomé

Moisture transport and Atmospheric Rivers (ARs) over the Northeastern Atlantic are a very relevant process for the inter-annual variability of precipitation over Western Europe. Based on a long-term transient simulation (850-2100CE) from the CESM model, we have showed that moisture transport towards Western Europe (using the vertically integrated horizontal water vapor transport, IVT) has been increasing significantly since pre-industrial period, in a clear association with the global warming trend. Both current and projected changes (using RCP 8.5) significantly exceed the range given by inter-annual to inter-decadal internal/external variability observed during the last millennium.

We have checked the emergence of the temperature, IVT and precipitation signals in Iberia and the UK, showing that while the first two have now clearly emerged from the pre-warming state, precipitation series are still slightly below that threshold. Nevertheless, projections clearly show an increase in rainfall at higher latitudes (i in phase with a warmer and moister atmosphere); and a decrease at lower latitudes decoupled from the overall increase in moisture availability. Additionally we have explored the role played by large-scale circulation and atmospheric dynamics for these contrasting projections. Overall, results show that a poleward migration of moisture corridors and ARs explain a significant fraction of these projected trends. Based on the Clausius–Clapeyron relation we have separated the thermodynamical from dynamical changes. We also show how that a significant increase in subtropical anticyclonic activity over Iberia is responsible for: i) dynamical circulation changes; ii) a shortening of the wet season; iii) to less efficient precipitation regimes in the region. These results highlight the urge to adapt to a drying trend in Mediterranean-type climates, as a consequence of Global Warming.


The financial support for this work was possible through the following FCT project: HOLMODRIVE - North Atlantic Atmospheric Patterns influence on Western Iberia Climate: From the Lateglacial to the Present [PTDC/CTA-GEO/29029/2017]

How to cite: Sousa, P. M., Ramos, A. M., Trigo, R. M., Raible, C. C., Messmer, M., Pinto, J. G., and Tomé, R.: Using a long-term climate simulation to address future changes in Western Europe precipitation regimes due to global warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7550, https://doi.org/10.5194/egusphere-egu2020-7550, 2020.

D3479 |
Masoud Rostami, Konstantin Klein, Christian Wegener, Yaping Shao, and Gerd-Christian Weniger

Heinrich events are recognized as the dominant periods of extreme cold terrestrial climate conditions during the last glacial period. The role of climate forcing alone upon Human Existence Potential (HEP) during extreme events, e.g. Heinrich and Dansgaard-Oeschger events, is not yet sufficiently resolved. By reproducing climate variables during the two extreme cold and warm cycles by means of an Earth System Model, employing an improved HEP model, and utilizing archaeological excavation sites, we report the spatial distribution of HEP over Europe during both cold stadials and warm interstadials corresponding to the two Upper Palaeolithic technocomplexes: Late Gravettian and Aurignacian. By introducing some other diagnostics like Environmental Human Catchment, which is defined as an area delimited by low HEP, cooling-aridity index, and Least Cost Path among colonized people, we shed light into population dynamics in this epoch. Consecutive extreme cold and warm cycles, corresponding to contraction-expansion of HEP, supports the hypothesis of repetitive depopulation–repopulation cycles of habitats. Regarding the controversial issue of late survival location of Neanderthals, we illustrate that western coastlines had such a suitable and stable HEP scores for all human taxa including Neanderthals to survive during Heinrich events.

How to cite: Rostami, M., Klein, K., Wegener, C., Shao, Y., and Weniger, G.-C.: Impacts of Heinrich events upon Human existence potential in Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7778, https://doi.org/10.5194/egusphere-egu2020-7778, 2020.

D3480 |
Peter Yu Feng Siew, Camille Li, Stefan Sobolowski, and Martin King

An observed relationship linking Arctic sea ice conditions in autumn to the North Atlantic Oscillation (NAO) index the following winter has potential relevance for seasonal predictions of European and North American climate. The physical pathway most often invoked to explain this particular teleconnection passes through the stratosphere. A Causal Effect Networks (CEN) approach is used to explore this stratospheric pathway between late autumn Barents-Kara sea ice and the February NAO, focusing on its seasonal evolution, timescale-dependence, and robustness. This pathway is statistically detectable in the satellite period, explaining 26% of the interannual variability in the February NAO. However, a bootstrap-resampling test reveals that the pathway is highly intermittent: the whole pathway emerges in only 15% of the bootstrapped samples. The intermittent nature of the pathway is consistent with the weak signal-to-noise ratio of the atmospheric response in the sea ice perturbation experiments, and suggests that a background state is important in determining whether the pathway is active. Higher frequency synoptic interactions between Barents-Kara sea ice and sea level pressure over Urals potentially interfere with the stratospheric pathway. Such interference likely reduces the potential for using the ice-NAO relationship for predicting midlatitude winter climate. This study helps quantify the robustness of linkages within the stratospheric pathway, and provides insight into which linkages are most subject to sampling issues within the relatively short observational record.

How to cite: Siew, P. Y. F., Li, C., Sobolowski, S., and King, M.: Intermittency of Arctic-midlatitude teleconnections: the stratospheric pathway between autumn sea ice and the winter NAO, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7905, https://doi.org/10.5194/egusphere-egu2020-7905, 2020.

D3481 |
Yongkun Xie, Guoxiong Wu, and Yimin Liu

The three-dimensional connections between Eurasian cooling and Arctic warming since 1979 were investigated using potential vorticity (PV) dynamics. We found that Eurasian cooling can be regulated by Arctic warming through PV adaptation and PV advection. Here, PV adaptation refers to the adaptation of PV to forcing and coherent dynamic/thermodynamic adaptation to PV change. In a PV perspective, first, the anticyclonic circulation change over the Arctic is produced by a negative PV change through PV adaptation, in which the change means the linear trend from 1979~2017. The negative PV change is directly regulated by Arctic warming because the vertical structure of Arctic warming is stronger at lower levels, which generates a negative PV change through the diabatic heating effect. Second, the circulation change produces a change in horizontal PV advection due to the existence of climatological PV gradients. Thus, as a balanced result, both the circulation change and PV change extend to mid-latitude through horizontal PV advection and PV adaptation. Eventually, Eurasian cooling at the surface and in the lower troposphere is dominated by PV changes at the surface through PV adaptation. Meanwhile, enhanced Eurasian cooling in the middle troposphere is dominated by top-down influences of upper-level PV change through PV adaptation. Nevertheless, the upper-level PV changes are still contributed by horizontal PV advection associated with Arctic warming. Overall, the general dynamics connecting Eurasian cooling with Arctic warming is demonstrated in a PV view.

How to cite: Xie, Y., Wu, G., and Liu, Y.: Eurasian cooling linked with Arctic warming: Insights from PV dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8416, https://doi.org/10.5194/egusphere-egu2020-8416, 2020.

D3482 |
Martin Stendel

The equivalent potential temperature Θe is a useful measure of the total heat content in the atmosphere, as it is conserved during both dry adiabatic and wet adiabatic processes. It is defined as letting an air parcel expand pseudo-adiabatically until all the water vapour has condensed, release and precipitate all its latent heat and compress it dry-adiabatically to the standard pressure of 1000 hPa.

Changes in surface or air temperature can thus be related to changes in humidity. For example, the relative contributions of temperature and humidity changes in tropical cyclones can be addressed, Arctic amplification due to the fact that saturation mixing ratio follows an exponential curve with temperature can be investigated, and by considering Θe in different vertical levels, an assessment of changes in convective stability can be made.

We have conducted a very long climate simulation with a global model interactively coupled to a Greenland ice sheet component. An extended RCP8.5 scenario is applied, where emissions of greenhouse gases continue to increase and then eventually level out around 2250. The model is then run for another 1000 years. With such an extreme forcing, all Arctic sea ice has completely disappeared, and a large part of the Greenland Ice Sheet has melted at the end of the simulation.

We examine changes in the total heat content based on observations and model data for past and present as well as for future climate. Daily data, allowing the identification of individual weather systems will be discussed for time slices with a seasonally and later a totally ice-free Arctic.

How to cite: Stendel, M.: Trends in total heat content in a very long climate change simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9526, https://doi.org/10.5194/egusphere-egu2020-9526, 2020.

D3483 |
Jerome Sauer, Johanna Baehr, and Nedjeljka Žagar

Sea ice alters the surface albedo and modulates the heat, moisture and momentum exchange between the ocean and the atmosphere. Various studies suggest an influence of the sea ice on the atmospheric circulation, whereby the focus is often on simultaneous connections and Arctic-wide sea ice conditions. Sea ice has a strong memory and we thus hypothesize a potential feedback on the atmosphere also at higher lags. Using ERA5 reanalysis data between 1983 and 2017, the present work investigates a potential connection of the summer atmospheric circulation over Eurasia to winter sea ice anomalies southwest of Greenland. Composites of the June-July geopotential height pattern show a wave-train structure throughout the troposphere and the resulting circulation anomalies are found to influence the two metre temperatures over northeastern Europe and northern Russia. These anomalies are significantly correlated with December-January sea ice anomalies. Persistent sea surface temperature (SST) anomalies associated with the strong ice memory indicates that the winter signal is partly stored in the Labrador Sea. The observations indicate a response in the June-July 500 hPa vertical velocity in proximity of the strongest SST anomalies that is dynamically consistent with the lower-level and upper-level divergence pattern. The result suggests that the vertical velocity potentially connects a vorticity forcing in the upper troposphere to near-surface conditions over the Labrador Sea that originate from the preceeding winter.
A further analysis shows a particularly pronounced wave-train signal when the December-January ice anomalies appear in phase with a strong North Atlantic Oscillation (NAO) index. Those years are characterized by extensive and persistent SST anomalies in the North Atlantic bearing similarities with the tripole pattern that is known to be associated with the NAO. The SST signal is accompanied by widespread heat flux anomalies hinting at a further influence coming from the central North Atlantic. The study provides a first analysis of two possible factors that potentially contribute to the linkage between winter sea ice and the summer atmospheric circulation.

How to cite: Sauer, J., Baehr, J., and Žagar, N.: Potential linkage between the atmospheric summer circulation over Eurasia and preceding sea ice anomalies southwest of Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10464, https://doi.org/10.5194/egusphere-egu2020-10464, 2020.

D3484 |
Piia Post and Andreas Lehmann

A detailed assessment of climate variability of the Baltic Sea area for the period 1958-2008 (Lehmann et al. 2011) revealed that changes in the warming trend since the mid-1980s, were associated with changes in the large-scale atmospheric circulation over the North Atlantic. The analysis of winter sea level pressure (SLP) data highlighted considerable changes in intensification and location of storm tracks, in parallel with the eastward shift of the North Atlantic Oscillation (NAO) centres of action. Additionally, a seasonal shift of strong wind events from autumn to winter and early spring exists for the Baltic area. Lehmann et al. (2002) showed that different atmospheric circulation regimes force different circulation patterns in the Baltic Sea. Furthermore, as atmospheric circulation, to a large extent, controls patterns of water circulation and biophysical aspects relevant for biological production, such as the vertical distribution of temperature and salinity, alterations in weather regimes may severely impact the trophic structure and functioning of marine food webs (Hinrichsen et al. 2007). To understand the processes linking changes in the marine environment and climate variability, it is essential to investigate all components of the climate system which of course include also the large-scale atmospheric circulation. Now, since extended time series data (1948-2018) for additional 20 years are available, it is interesting to investigate recent changes/shifts of the large-scale atmospheric conditions and their impact on the wind climate over the Baltic Sea area.

How to cite: Post, P. and Lehmann, A.: Impact of large-scale atmospheric circulation changes over the North Atlantic on the wind climate of the Baltic Sea area for the period 1948/49-2018/19, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10662, https://doi.org/10.5194/egusphere-egu2020-10662, 2020.

D3485 |
Kamilya Yessimbet and Andrea Steiner

Winter weather and extreme events at mid-latitudes are determined by the atmospheric circulation variability, which is closely related to jet stream configuration and atmospheric blocking. In January 2019, record-breaking snowfall in the Northern Alps affected Austria and Germany. The event is linked to a typical weather regime of blocking over the North Atlantic and southward meridional moisture transport from the high latitudes to the Alps. This study investigates the synoptic conditions prior and during the event addressing possible forcing mechanisms for the extreme snowfall occurrence.

We analyzed the atmospheric conditions using the ERA-5 reanalysis dataset investigating geopotential height (GPH), pressure, temperature, and wind fields. For blocking detection, we applied a classical algorithm based on the reversal of mid-latitude 500 hPa GPH gradients. Evolution of surface conditions and snowfall impacts was studied based on the European daily high-resolution gridded dataset (E-OBS) and snow data provided by the Austrian weather service.

Tropospheric analysis revealed that a persistent blocking high over the North Atlantic played a major role in the meridional elongation of upper-level streams. A low-pressure system, embedded in the strongly meandering jet stream’s trough, modulated the moisture flow directly towards the Alpine mountains leading to record-breaking snowfall.

Prior to the event, a major sudden stratospheric warming (SSW) took place at Northern high latitudes. We discuss the initial atmospheric conditions including SSW, blocking, and impacts on surface weather in Europe, and particularly in the Alpine region.

How to cite: Yessimbet, K. and Steiner, A.: Heavy Alpine snowfall in January 2019 connected to atmospheric blocking, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10705, https://doi.org/10.5194/egusphere-egu2020-10705, 2020.

D3486 |
Hakan Sert, Orkun Temel, Cem Berk Senel, and Ozgur Karatekin

In this study, we present a three-dimensional global circulation model (GCM) to investigate the environmental effects of an asteroid impact on the global Earth system. The model is applied to model the atmospheric response of the Cretaceous–Paleogene (K–Pg) extinction event which took place 66 million years ago and resulted in the mass extinction of various animal and plant species. The atmospheric model is developed based on the planetWRF model. First, the paleoclimate model is validated using the proxy data. Then, the sensitivity to atmospheric co2 concentration is investigated. The radiation parameterization scheme of the planetWRF model is modified to include the effect of various climate-active aerosols and gases released after the impact event. The model is also coupled both to a simple one-dimensional ocean mixed layer and a three-dimensional ocean circulation model. Both the atmospheric and oceanic response is investigated.

How to cite: Sert, H., Temel, O., Senel, C. B., and Karatekin, O.: A global circulation model for the asteroid impact simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10832, https://doi.org/10.5194/egusphere-egu2020-10832, 2020.

D3487 |
Jie Cao

A new teleconnection pattern (the BEAP) across the Bay of Bengal‐East Asia‐Pacific region in boreal summer is revealed in this study using mainly ERA‐Interim reanalysis data from the European Centre for Medium‐Range Weather Forecasts. The BEAP index (BEAPI) is defined as the signed sum of standardized apparent moisture sinks at five centers along the pathway. Correlation analysis of the apparent heat sources and apparent moisture sinks has verified the existence of the BEAP teleconnection. Variations in BEAP can affect precipitation anomalies resulting from the anomalous moisture transport and the antiphase surface temperature variation. Wave flux analysis has verified the Rossby wave propagation route that originates around the central Bay of Bengal and extends across North China to the West Pacific. La Niña‐type sea surface temperature anomalies (SSTAs) appearing simultaneously in the same season can excite a positive BEAP pattern by enhancing convection over the Bay of Bengal, while El Niño‐type SSTAs have the opposite effect. Significant correlation between the BEAPI and the SSTAs can last from early summer to early winter. Numerical experiments confirm the BEAP teleconnection pattern and the associated physical processes.

How to cite: Cao, J.: Bay of Bengal‐East Asia‐Pacific Teleconnection in Boreal Summer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12432, https://doi.org/10.5194/egusphere-egu2020-12432, 2020.

D3488 |
Yingmo Zhu

This study examines how the thermodynamic anomaly in Arabian Sea (AS)–Bay of Bangle (BOB) relates to Yunnan precipitation in the rainy season. The observational diagnosis basing on data sets of atmospheric circulation reanalysis, precipitation from 124 stations in Yunnan and outgoing longwave radiation indicates that, when the thermodynamic anomaly in the AS–BOB is weaker during rainy-season, an anomalous anticyclone will control the AS–BOB. An anomalous cyclone in Yunnan resulted from the anomalous anticyclone in the AS–BOB induces anomalous water vapor converging with anomalous cold air in the same region. As a result, heavier-than-normal precipitation occurs in Yunnan in rainy-season. When the thermodynamic anomaly in the AS–BOB is stronger, the opposite configuration of anomalous circulation will cause less-than-normal precipitation in Yunnan. The results of several numerical experiments obtained from a linear baroclinic model support the key physical processes revealed in the observational diagnosis.

How to cite: Zhu, Y.: Relationship between Thermodynamic Anomalies in Arabian Sea–Bay of Bangle on Rainy-season Precipitation in Yunnan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12651, https://doi.org/10.5194/egusphere-egu2020-12651, 2020.

D3489 |
Ziniu Xiao

The intensity index of Kuroshio Extension and northern front zones (KEF) is defined as the area average of SST meridional gradient by using Hadley Center’s surface sea temperature dataset (1949-2014), and the Kuroshio Extension frontal intensity index (KEFI) has seasonal to interdecadal variations. In winter, the KEFI has significant positive correlation with transient variances in the North Pacific storm tracks area, and the positive relationship appears when KEFI lead storm tracks one month which indicates the intensity of KEF could influence storm tracks in winter. To investigate the possible mechanism, we found: when the winter SST front is stronger, the more significant difference between ocean-air heat flux in both sides of KEF could strengthen the near-surface temperature gradient, which maintains the near-surface baroclincity and benefits the transient heat transport, promote the develop of transient eddies at last. Additional, the large-scale circulation also be response to KEF in winter: when the KEF is stronger, the Aleutian is deepen, the subtropical high is strengthen, the 500 hPa potential high is increased (decreased) in south (north), the subtropical jet is weaker and wider. It is found that the oceanic fronts promote storm tracks by transporting heat upward and maintaining the air temperature gradient in winter. In further, the significant correlation was found between the Kuroshio Extension Oceanic Front intensity and the temperature over North America in autumn and winter.

How to cite: Xiao, Z.: The Impact of Kuroshio Extension Fronts Variation on the Pacific Storm Tracks and the relationship with Temperature over North America, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12735, https://doi.org/10.5194/egusphere-egu2020-12735, 2020.

D3490 |
Emily Vosper, Eunice Lo, Dann Mitchell, and Kerry Emanuel

Hurricanes are among the most destructive extreme weather events affecting humanity, in both social and economic terms. Hurricane Dorian (2019) caused widespread devastation when it stalled over the Bahamas as a category five hurricane bringing the most rainfall to the country from a hurricane in the reliable observation period, whilst secondary events such as flooding, landslides and disease left tens of thousands of people homeless. Climate change has been shown to influence hurricane activity, but so far there have been few studies that have explored hurricane response under the Paris Agreement goals especially in the case of stalling hurricanes. Here we show that extreme hurricane rainfall events, which affect the Caribbean region, are more likely in both of the Paris Agreement scenarios compared to the present climate with five of the six 100-year hurricanes studied occurring more often in these simulations. In particular, we show a currently one in 100-year rainfall event affecting the Bahamas is at least three times as likely under the Paris Agreement goals compared to the present climate.

How to cite: Vosper, E., Lo, E., Mitchell, D., and Emanuel, K.: Projecting the Reoccurrence of one in 100-year Caribbean Hurricanes under the Paris Agreement Goals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13616, https://doi.org/10.5194/egusphere-egu2020-13616, 2020.

D3491 |
Wan-Ling Tseng, Huang-Hsiung Hsu, Li-Chiang Jiang, Chiung-Wen June Chang, Ben-Jei Tsuang, and Chia-Ying Tu

Global warming’s impact on the Madden-Julian Oscillation (MJO) is assessed using one of the few models capable in reproducing its key features. In a warmer climate predicted for the end of the century, it has been proved the MJO increases in amplitude and frequency, showing a more circumglobal propagation tendency. Here, we examine the MJO teleconnection and its extratropical response under the warmer climate by the time-slide experiments. The extratropics impact on different phase is shifted through the change of the mean atmospheric circulation. The strengthening of the midlatitude jet stream leads to the zonal extended wave propagation. It results the stronger variability of the atmospheric river to the America west coast. Moreover, the relationship with the NAO and PNA is weaker but the stronger fluctuation is shown in the polar area. This suggests the teleconnection of the North America weather by the tropical convection is going to change in the warming climate. It is essential to consider in the further projection and subseasonal to seasonal forecast.

How to cite: Tseng, W.-L., Hsu, H.-H., Jiang, L.-C., Chang, C.-W. J., Tsuang, B.-J., and Tu, C.-Y.: The change of MJO teleconnection under the global warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13817, https://doi.org/10.5194/egusphere-egu2020-13817, 2020.

D3492 |
Erin Walker, Daniel Mitchell, William Seviour, Paul Valdes, and Mat Collins

Accurately determining extratropical cyclone paths is key in determining regional impacts associated with precipitation and wind. It is known that the stratosphere plays an important role in atmospheric dynamics and can extend its influence down to the surface. Despite this, many attribution studies have not included a stratosphere in their experiments. We believe that not considering the stratosphere could affect the results of these experiments, so the role it has on North Atlantic storm tracks is analysed using an idealised, atmospheric only model named Isca. With the aim of identifying clear implications of including the stratosphere in storm track analysis in the North Atlantic basin, a large ensemble formed of 4 separate experiments is set up for the winter of 2013/2014. The four experiments are as follows; 1) no vertical layers in the stratosphere, 2) vertical levels extended to the upper stratosphere, 3) doubling of vertical levels throughout the atmosphere, and finally, 4) an increase of vertical levels at the tropopause. We expect that including the stratosphere, in addition to increasing vertical resolution, will help improve model representation of storm tracks and their intensities during the 2013/2014 winter. The results of this study hope to highlight how the inclusion of the stratosphere and increased vertical resolution can lead to the improvement in modelling storm track statistics, which in turn will help to make more reliable attribution statements in the future.

How to cite: Walker, E., Mitchell, D., Seviour, W., Valdes, P., and Collins, M.: The role of increasing vertical resolution on the detection and attribution of North Atlantic storms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16144, https://doi.org/10.5194/egusphere-egu2020-16144, 2020.

D3493 |
Paolo Ghinassi, Federico Fabiano, Virna L. Meccia, and Susanna Corti

Rossby waves play a fundamental role for both climate and weather. They are in fact associated with heat, momentum and moisture transport across large distances and with different types of weather at the surface. Assessing how they are represented in climate models is thus of primary importance to understand both predictability and the present and future climate. In this study we investigate how ENSO and the AMV affect the large scale flow pattern in the upper troposphere of the Northern Hemisphere, using reanalysis data and data from the PRIMAVERA simulations.

The upper tropospheric large scale flow is investigated in terms of the Rossby wave activity associated with persistent and recurrent patterns over the Pacific-North American and Euro-Atlantic regions during winter, the so called weather regimes. In order to quantify the vigour of Rossby wave activity associated with each weather regime we make use of a recently developed diagnostic based on Finite Amplitude Local Wave Activity in isentropic coordinates, partitioning the total wave activity into the stationary and transient components. The former is associated with quasi-stationary, planetary Rossby waves, whereas the latter is associated with synoptic scale Rossby wave packets. This allows one to quantify the contribution from stationary versus transient eddies in the total Rossby wave activity linked to each weather regime.

In this study we explore how ENSO and the AMV affect both the weather regimes frequencies and the upper tropospheric waviness in the Pacific and Atlantic storm tracks, respectively. Furthermore we analyse how both the stationary and transient wave activity component modulate the onset and transition between different regimes.

How to cite: Ghinassi, P., Fabiano, F., Meccia, V. L., and Corti, S.: The impact of ENSO and the Atlantic Multidecadal Variability (AMV) on the upper tropospheric large scale flow in the PRIMAVERA models., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18113, https://doi.org/10.5194/egusphere-egu2020-18113, 2020.

D3494 |
Elzina Bala, Aiko Voigt, and Peter Knippertz

One of the grand challenges of climate is predicting and modeling tropical rainfall. Here, we address a specific problem of this grand challenge, namely how does the vertical structure of the atmosphere affect the tropical circulation and the position of the ITCZ during the seasonal cycle and in response to increased CO2. The tropical circulation can be described by the column-integrated budget of moist static energy (MSE). We use this framework in the TRAC-MIP model ensemble to investigate the role of the vertical structure of the tropical atmosphere in setting the anti-correlation between the ITCZ location and the atmospheric energy transport.

TRACMIP "The Tropical Rain belts with an Annual cycle and Continent - Model Intercomparison Project" is a set of idealized simulations that are designed to study the tropical rain belt response to past and future forcings. TRACMIP includes 13 comprehensive CMIP5-class atmosphere models and one simplified atmospheric model. Importantly, TRACMIP includes a slab ocean with prescribed ocean heat transport. This leads to a closed surface energy balance and forces the annual-mean ITCZ to be north of the equator, consistent with today’s climate.

We use the MSE budget framework to diagnose the seasonal evolution of vertical velocity from the energetic terms in the MSE budget equation. We obtain a diagnostic expression for the vertical velocity. By means of the MSE budget framework we estimate the efficiency of exporting energy from the atmospheric column, which is defined as the gross moist stability (GMS). The GMS characterizes the stability of the tropical troposphere related to moist convective processes in the tropospheric column. We use the MSE and GMS analysis to disentangle the impact of deep and shallow circulations on energy transport, vertical velocity and hence precipitation in an objective manner.

Through this work we aim to elucidate to what extent model uncertainty in simulations of future ITCZ changes are caused by model differences in the vertical structure of the atmosphere. We also hope to use the results to advance our understanding of the tropical climate and to assess the plausibility of simulated changes in tropical rainfall.

How to cite: Bala, E., Voigt, A., and Knippertz, P.: An MSE budget view on seasonal and CO2-induced ITCZ shifts in the TRAC-MIP model ensemble, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18239, https://doi.org/10.5194/egusphere-egu2020-18239, 2020.

D3495 |
Fabio D'Andrea and Paolo Davini

We present a comprehensive analysis of the representation of winter and summer Northern Hempishere atmospheric blocking in global climate simulations in both present and future climate. Three generations of climate models are considered: CMIP-3 (2007), CMIP-5 (2012) and CMIP-6 (2019).
All models show common and extended underestimation of blocking frequencies, but a reduction of the negative biases in successive model generations is observed. However, in some specific regions and seasons as the winter European sector, even CMIP-6 models are not yet able to achieve the observed blocking frequency. For future decades the vast majority of models simulates a decrease of blocking frequency in both winter and summer, with the exception of summer blocking over the Urals and winter blocking over Western North America. Winter predicted decreases may be even larger than currently estimated considering that models with larger blocking frequencies  hence generally smaller errors - show larger reduction. Nonetheless trends computed over the historical period are weak and often contrasts with observations: this is particularly worrisome for summer Greenland blocking where models and observation significantly disagree. Finally, the intensity of global warming is related to blocking changes: wintertime European blocking is expected to decrease following larger global mean temperatures, while Western Russia summer blocking is expected to increase.

How to cite: D'Andrea, F. and Davini, P.: Northern Hemisphere atmospheric blocking simulation in present and future climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18294, https://doi.org/10.5194/egusphere-egu2020-18294, 2020.

D3496 |
Federico Fabiano, Ignazio Giuntoli, Paolo Ghinassi, and Susanna Corti

Weather Regimes (WRs) are preferred large-scale atmospheric circulation patterns found in mid-latitude regions like the North-Atlantic or North-Pacific, that have a certain degree of recurrence and persistence. From a nonlinear dynamical system view, they can be seen as the attractors of the chaotic atmospheric flow at mid-latitudes. In simple nonlinear dynamical systems, under a small external forcing the attractors remain fixed at first-order, but the frequency of occurrence of the different dynamical regimes changes, with some regimes becoming more populated. By analogy, a similar response to forcing has also been hypothesized for the WRs in complex GCMs. Here we test this hypothesis in the climate models participating to CMIP6, looking for the change of the WRs frequency in the future climate (2050-2100) under different scenarios, with respect to the historical simulations (1964-2014).

WRs also constitute a suitable framework to study the impacts and occurrence of extreme weather. In this sense, each WR is characterized by a large-scale circulation pattern, that drives different climatic conditions over specific regions, and long-lasting WRs are often connected with extreme temperature or precipitation anomalies. Therefore, the projected change in the frequency of the WRs also produces an intensification of the impacts connected with the regimes occurring more often. For each model analyzed, we also study the impacts related with each WR and their change with future climate under the different scenarios.

How to cite: Fabiano, F., Giuntoli, I., Ghinassi, P., and Corti, S.: Projected change in frequency of Weather Regimes in CMIP6 models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18722, https://doi.org/10.5194/egusphere-egu2020-18722, 2020.