CL4.19

Hemisphere-wide remote Rossby wave responses of the upper-level flow to soil moisture anomalies have been reported for the Northern Hemisphere. Model experiments varying soil moisture over North America point to the involvement of both linear and non-linear wave dynamics. Here three sets of model experiments are performed with the Community Earth System Model to study the role of soil moisture anomalies as a boundary forcing for the formation of extra-tropical upper-level Rossby wave patterns during Southern Hemisphere summer.

In the model experiments, soil moisture over Australia is set to +1STD (wet) and to -1STD (dry) of the ERA-Interim reanalysis climatology for the years 2009 to 2016. With his set-up 50 ensemble members are run and the wet and dry simulations compared. The local response to the soil moisture forcing is a positive heating anomaly in the dry simulations that results in a thermal low-like circulation anomaly with an anomalous surface low and an anomalous upper-level anticyclone.

A circum-hemispheric flow response is identified both in the extra-tropical upper-level flow and in the surface storm tracks that overall resembles a positive Southern Annular Mode-like flow anomaly in the dry simulations. The structure of this atmospheric response strongly depends on the background flow. During two El Niño summers the response is strongly influenced by nonlinear Rossby wave forcing, while during two La Niña summers the flow response resembles a circum-hemispheric wave train reflecting linear wave propagation.

How to cite: Romppainen-Martius, O., Wehrli, K., and Rohrer, M.: Local and remote Southern Hemisphere extratropical circulation responses to soil moisture anomalies in Australia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-278, https://doi.org/10.5194/egusphere-egu21-278, 2021.

In this study, we aim at identifying dynamical differences between short blocks, which last only five days, and long blocks, which last at least ten days, to better characterise long blocks. We show that long blocks often involve cyclonic Rossby wave breaking, while short blocks are equally associated with cyclonic and anticyclonic wave breaking. This main result is reproduced in several coupled climate models. We propose three mechanisms that might explain the lower number of long anticyclonic blocks: 1/ a downstream reinforcement of the anticyclone during anticyclonic blocks might be associated with a stronger downstream advection of the block; 2/ the mean zonal wind is reinforced by synoptic eddies towards a more northward position during anticyclonic blocks, whereas synoptic eddies force the mean zonal wind to the south of the block during cyclonic blocks, which has been previously shown to be associated with more persistent weather patterns; 3/ strong and/or sustained eddy feedback is needed to maintain long anticyclonic blocks. All these parameters combined might explain why blocks last longer and why anticyclonic blocks are less present at extreme durations.

How to cite: Drouard, M., Woollings, T., Sexton, D., and McSweeney, C.: Contrasting dynamics of short and long blocks in the Northern Hemisphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1165, https://doi.org/10.5194/egusphere-egu21-1165, 2021.

The North Atlantic Oscillation (NAO), coexistent meridional oscillation of subpolar Icelandic low and the subtropical Azores high dominates the Northern Hemispheric winter climate. Variability in the circulation of NAO may activate the extreme weather events, such as the enhanced zonal winds, in northeast America, Atlantic and Eurasia. On the other hand variability in the zonal wind patterns effects the position of the NAO events. It is more relevant to investigate the interaction between NAO and the weather patterns during the winter time since NAO is powerful during winter. Hence the wintertime weather systems are highly altered by such an impact. Analysis indicate that negative and positive phases of NAO mainly modulate the local cyclonic and anticyclonic wave characteristics and hence the zonally asymmetric circulation of the middle atmosphere. Zonal asymmetries in the weather patterns originate from ocean-continent temperature gradients and topographical contrasts after all solar incident radiation is almost uniform over the longitudes. Thus zonally asymmetric patterns for certain variables such as zonal winds show strong seasonal dependence and highly correlate with the climatological position of the NAO mainly in the winter hemisphere. In this study longitudinal differences in the zonal wind is analyzed in order to observe its strong influence on the evolution of NAO. Zonal asymmetries of zonal wind is examined by evaluating the deviation from zonal mean of the long term annual average of both winter and spring months from December to April. Zonal winds up to 100km for winter and spring is examined between 2006-2100 using CMIP5 MPI-ESM-MR RCP4.5 scenario for the extratropical and the polar latitudes. Additionally ERA5 reanalysis data is used to identify the ability of CMIP5 Reference Period (RP) data to capture the observed patterns for the years from 1979 to 2005.

Acknowledgements: This study is supported by TUBİTAK (The Scientific and Technology Research Council of Turkey), The Scientific and Technological Research Projects Funding Program, 1001. The projects number is 117Y327.

How to cite: İlhan, A., Demirhan, D., and Ünal, Y.: Observed and simulated zonally asymmetric zonal wind patterns under NAO conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7743, https://doi.org/10.5194/egusphere-egu21-7743, 2021.

Understanding the formation and evolution mechanisms of Ural blocking (UB) is of great importance for the prediction of UB and relevant extremes in east Asia. Using the 6-hourly ERA-Interim reanalysis data, this study quantifies the conservative and nonconservative processes in the lifecycle of UB through the lens of the hybrid Eulerian-Lagrangian local finite-amplitude wave activity (LWA) diagnostics. It is found that (i) as a wave activity source, eddy heat flux works to not only initiate the UB, but also prevent the wave activity of the blocking from dispersing downstream---the key characteristic of blocking; (ii) both the wave propagation and wave advection mechanisms are indispensable for the evolution of UB, playing a tug-of-war on the downstream development of wave activity; (iii) throughout the lifespan of UB, diabatic heating provides the most important damping mechanism for the wave activity both upstream and downstream.

How to cite: Wang, M., Zhang, Y., and Lu, J.: The evolution mechanisms of Ural blocking from the lens of local finite-amplitude wave activity budget analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7195, https://doi.org/10.5194/egusphere-egu21-7195, 2021.

In this study we aim to assess how the upper tropospheric Rossby wave activity is represented in the PRIMAVERA models. The low and high resolution historical coupled simulations will be compared with ERA5 reanalysis (spanning the 1979-2014 period) to enlighten model deficiencies in representing the spatial distribution and temporal evolution of Rossby wave activity and to emphasize the benefits of increased resolution. Our analysis focuses on the wintertime large scale circulation over the Euro-Atlantic sector.

A diagnostic based on Local Wave Activity (LWA) in isentropic coordinates is used to identify Rossby waves and to quantify their amplitude. LWA is partitioned into its stationary and transient components, to distinguish the contribution from planetary versus synoptic scale waves (i.e. wave packets). This diagnostic is then combined with another one to identify persistent and recurrent large scale circulation patterns, the so called weather regimes. Weather regimes in the Euro-Atlantic sector are identified with the usual approach of EOF decomposition and k-mean clustering applied to daily anomalies of Montgomery streamfunction, in order to have a consistent framework with LWA (which is defined in isentropic coordinates). A composite of transient LWA is realised for each weather regime to obtain the spatial distribution of Rossby wave activity associated with each weather regime.

Results show a marked intermodel variability in the ability of reproducing the correct (i.e. the one observed in reanalysis data) LWA distribution. Many of the models in fact fails to reproduce the localized (in space) maxima of LWA associated with each weather regime and to distribute LWA over a larger region compared to reanalysis. High resolution helps to correct this bias in the majority of the models, in particular in those where the low-resolution LWA distribution was already close to reanalysis. Finally, the temporal behaviour of the spatially averaged LWA in the examined period is discussed.

How to cite: Ghinassi, P., Fabiano, F., and Corti, S.: Rossby wave activity associated with Euro-Atlantic weather regimes in the PRIMAVERA historical runs., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9605, https://doi.org/10.5194/egusphere-egu21-9605, 2021.

Long term projections of the Northern Hemisphere winter climate show an overall increase of surface temperatures that is particularly amplified at Arctic latitudes. On top of this, projections agree in predicting a faster temperature increase on land than on sea surface, therefore a reduced winter land-sea contrast in the mid latitudes. Despite the robustness of this feature in climate projections, the response of the atmospheric system to a strongly reduced winter land-sea contrast has been scarcely investigated. Here, we study how it affects the low and high frequency variability in the extratropics using a simplified GCM, with a focus on the Atlantic and Pacific jets. Moreover, different sea surface temperatures are applied to the North Atlantic and North Pacific basins in order to simulate the presence of a differential warming, as in the well-known scenario of a North-Atlantic warming hole. 

How to cite: Portal, A., Pasquero, C., D'Andrea, F., Davini, P., and Hamouda, M.: Reduced winter land sea contrast and its effects on mid-latitude atmospheric variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1790, https://doi.org/10.5194/egusphere-egu21-1790, 2021.

South situations or days in which the South Wind (SE-S-SW) constitutes the predominant wind direction (mode) and one of the factors more determinants of the climatic conditions in terms of temperature and relative humidity in the autumn-winter period of the geographical region of southwestern Europe around the central territorial axis of the Bay of Biscay-Gascony.

According to our conclusions on the official data analyzed in thirteen meteorological stations in this region of southwestern Europe, for the 1961-2010 annual series, more than 43% of the autumn-winter days with prevailing winds from the South, or South Wind, register average temperature values ​​(T) higher than their respective autumn-winter average. Likewise, in eleven of the thirteen stations analyzed, for the same annual series, the average relative humidity (H) record corresponding to the set of autumn-winter days with predominant South Wind is lower than the respective mid autumn-winter and in the other two seasons both records are equal.

In the stations of the coastal region, such as Bilbao and Gijón, for the 1961-71 annual series, with an atmospheric circulation characterized, in all autumn-winter periods, by negative mean values ​​of the North Atlantic Oscillation index (NAO), the percentage frequency of South Wind situations is higher than that corresponding to the coastal stations of San Sebastián, Santander and Biarritz in the 1971-2010  annual series, as well as with respect to the percentage frequency for any other station and in both series.

The climatic and environmental conditions of this region of southwestern Europe are strongly affected by the tempering influence of the South Atlantic winds, following a process of orographic condensation-desiccation to windward and subsidence, adiabatic compression and rapid movement along the slopes, downwind of the Cantabrian-Pyrenean mountain range (Foehn effect).

Thus, the general atmospheric circulation over the region favors, especially in the autumn-winter period, the advections of humid and unstable air masses (storms and fronts) coming from the middle and subtropical latitudes of the North Atlantic, which generate anabatic south winds on the Iberian Peninsula, heading towards the Western Cantabrian-Pyrenean region and through it towards the continental Atlantic façade, but already more tempered and parched.

These southern situations are generated under conditions of atmospheric circulation and synoptic configuration defined by the interrelation between multiple oceanic and atmospheric patterns (in addition to solar, orographic factors...) that also largely determine the climatology of the entire oceanic and continental North Atlantic región.

As a great diversity of studies carried out have been collecting and demonstrating, the ocean-atmospheric patterns, both planetary and regional (ENSO, AMO, NAO, WeMO...) and the teleconnection between events or climatic phenomena generated by they and even in a very distant between them, constitute fundamental factors to define the atmospheric circulation and the climatology of the North Atlantic-Western Europe region (NAWE).

The empirical vision of the teleconnection between these ocean-atmospheric patterns requires the analysis of the significant statistical correlation coefficients between the indices of such factors or patterns, for which we will use the integrated program or set of programs "R".

How to cite: San Martin Orbe, P.: Analysis of statistical correlation between the indices of the most determinant ocean-atmospheric patterns in the climatology of the North Atlantic-Western Europe (NA-WE), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7373, https://doi.org/10.5194/egusphere-egu21-7373, 2021.

The Gulf Stream and Kuroshio regions feature strong sea surface temperature (SST) gradients that influence cyclone development and the storm track. Smoothing the SSTs in either the North Atlantic or North Pacific has been shown to yield a reduction in cyclone activity, surface heat fluxes, and precipitation, as well as a southward shift of the storm track and the upper-level jet. To what extent these changes are attributable to changes in individual cyclone behaviour, however, remains unclear. Comparing simulations with realistic and smoothed SSTs in the atmospheric general circulation model AFES, we find that the intensification of individual cyclones in the Gulf Stream or Kuroshio region is only marginally affected by reducing the SST gradient. In contrast, we observe considerable changes in the climatological mean state, with a reduced cyclone activity in the North Atlantic and North Pacific storm tracks that are also shifted equator-ward in both basins. The upper-level jet in the Atlantic also shifts equator-ward, while the jet in the Pacific strengthens in its climatological position and extends further east. Surface heat fluxes, specific humidity, and precipitation also respond strongly to the smoothing of the SST, with a considerable decrease of their mean values on the warm side of the SST front. This decrease is more pronounced in the Gulf Stream than in the Kuroshio region, due to the amplified decrease in SST along the Gulf Stream SST front.  Considering the pertinent variables occurring within different radii of cyclones in each basin over their entire lifetime, we find cyclones to play only a secondary role in explaining the mean states differences between smoothed and realistic SST experiments.

How to cite: Spengler, T., Tsopouridis, L., and Spensberger, C.: SST fronts along the Gulf Stream and Kuroshio affect the atmospheric winter climatology primarily in the absence of storms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3096, https://doi.org/10.5194/egusphere-egu21-3096, 2021.

Future changes in the mid-latitude wintertime atmospheric circulation are studied from a weather regimes perspective. The analysis is based on daily geopotential height at 500 hPa during the extended winter season (NDJFM) from both CMIP5 and CMIP6 historical and scenario simulations. The model performance in reproducing the observed weather regimes during the historical period in the Euro-Atlantic (EAT) and Pacific-North American (PAC) sectors is first evaluated, showing a general improvement of CMIP6 models in terms of regime patterns, frequencies and variance ratio. The projected circulation changes in the future climate (2050-2100) under the different scenarios are analysed in terms of the change in the frequency and persistence of the regimes. Significant positive trends are found for the frequency of NAO+ and negative trends for the Scandinavian Blocking and Atlantic Ridge regimes. This confirms the tendency for the zonalization of the circulation in the EAT sector, with decreased latitudinal variability of the jet stream. For the PAC sector, significant changes are seen for the Pacific Trough regime (increase) and the Bering Ridge (decrease), while there is no agreement in the response of the two PNA regimes. The spread among the model responses in the most extreme scenarios is analysed through a multi-linear regression approach and linked to different levels of warming in the polar stratosphere, the tropical upper troposphere, the North Atlantic and the Arctic.

How to cite: Fabiano, F., Meccia, V. L., Davini, P., Ghinassi, P., and Corti, S.: A regime view of future atmospheric circulation changes in Northern mid-latitudes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12654, https://doi.org/10.5194/egusphere-egu21-12654, 2021.

Persistent summer extremes, such as heatwaves and droughts, can have considerable impacts on nature and societies. There is evidence that weather persistence has increased in Europe over the past decades, in association to changes in atmosphere dynamics, but uncertainties remain and the driving forces are not yet well understood. 

Particularly for Europe, the jet stream may affect surface weather significantly by modulating the North Atlantic storm tracks. Here, we examine the hypothesis that high-latitude warming and decreased westerlies in summer result in more double jets, consisting of two distinct maxima of the zonal wind in the upper troposphere, over the Eurasian sector. Previous work has shown that such double jet states are related to persistent blocking-like circulation in the mid-latitudes. 

We adapt a dynamical perspective of heat extreme trends by looking at large scale circulation and in particular, changes in the zonal mean zonal wind in different levels of the upper troposphere. We define clusters of jet states with the use of Self-Organizing Maps and analyze their characteristics. We find an increase in frequency and persistence of a cluster of double jet states for the period 1979-2019 during July-August (in ERA5 reanalysis data). Those states are linked to increased surface temperature and more frequent heatwaves compared to climatology over western, central, and northern Europe. Significant positive double jet anomalies are found to be dominant in the days preceding and/or coinciding with some of the most intense historical heatwaves in Europe, such as those of 2003 and 2018. A linear regression analysis shows that the increase in frequency and persistence of double jet states may explain part of the strong upward trend in heat extremes over these European regions.

How to cite: Rousi, E., Kornhuber, K., Beobide Arsuaga, G., Luo, F., and Coumou, D.: Increased frequency of Eurasian double jets linked to summer heat extremes in Europe, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10104, https://doi.org/10.5194/egusphere-egu21-10104, 2021.

The Ferrel cell consists of the zonal mean vertical and meridional winds in the midlatitudes. The continuity of the Ferrel circulation and the zonal mean momentum and heat budgets imply a collocation of the eddy-driven jet and poleward eddy heat flux maxima, under certain assumptions, including the negligibility of diabatic heating. The latter assumption is questioned, since midlatitude storms are associated with latent heating in the midtroposphere. In this study, the heat budget of the Ferrel cell in both hemispheres is examined, using the JRA55 reanalysis data set. The diabatic heating rate is significant close to the center of the Ferrel cell during winter and at the ascending branch during summer in both hemispheres. The interannual variability shows a positive correlation between the diabatic heating rate in the midlatitude midtroposphere and the latitudinal separation between the eddy heat flux and the eddy-driven jet maxima during winter in both hemispheres.

How to cite: Lachmy, O. and Kaspi, Y.: The role of diabatic heating in Ferrel cell dynamics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15617, https://doi.org/10.5194/egusphere-egu21-15617, 2021.

The midlatitude storm tracks are one of the most prominent features of extratropical climate. Despite the theoretical expectation, based on baroclinic instability theory that baroclinic eddy strength correlates with jet intensity, there is a decrease in storm-track activity during midwinter over the Pacific compared to the shoulder seasons. Recent studies suggest this phenomenon is a result of the general circulation effect on the storm-track through interaction with the jet-stream. To isolate the effect of jet strength, we conduct a series of GCM experiments with a systematically varied jet intensity. The simulations are analyzed using Lagrangian tracking to understand the response from a single eddy perspective. The results of the Lagrangian analysis show that while the response of upper-level eddies is dominated by a reduction in the amount of tracked features, the lower-level eddies' response is also affected by a reduction in their lifetime. Analyzing the effect of the jet strength on the pairing between the upper- and lower-level eddies, we show how the jet intensification break the baroclinic wave structure and limits its growth. Furthermore, we show that these results can be settled with linear baroclinic instability models if the eddies' spatial scale is considered. The intensification of the jet and increase in the deformation radius shift the preferred scale for growth from the synoptic-scale toward the planetary-scale, consistent with the reduction in storm activity. This mechanism potentially explains the midwinter suppression of storm activity over the Pacific and the difference from the response over the Atlantic.

How to cite: Hadas, O. and Kaspi, Y.: Suppression of Baroclinic Eddies by Strong Jets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15342, https://doi.org/10.5194/egusphere-egu21-15342, 2021.

There is strong evidence that a positive feedback between the zonal-mean wind anomalies and the eddies (i.e. a positive feedback of EOF1 onto itself) is important for maintaining the wind anomalies associated with the annular modes. However, a recent study by Lubis and Hassanzadeh, (2021, JAS) shows that under some circumstances, EOF1 and EOF2 can interact and exert feedbacks on each other at some lag times, affecting the time scale of the annular modes. Building upon the seminal work of Lorenz and Hartmann (2001, JAS), we introduced a reduced-order model for coupled EOF1 and EOF2 that accounts for potential cross-EOF eddy-zonal flow feedbacks. Using the analytical solution of this model, we derive conditions for the existence of the propagating regime based on the feedback strengths. Using this model, and idealized GCMs and stochastic prototypes, we show that cross-EOF feedbacks play an important role in controlling the persistence of the annular modes by setting the frequency of the oscillation. We find that stronger cross-EOF feedbacks lead to less persistent annular modes. The underlying dynamics of the cross-EOF feedbacks for propagating annular modes in both reanalysis and an idealized GCM are also investigated. Using a finite-amplitude wave activity (FAWA) framework, we show that the cross-EOF feedbacks result from the out-of-phase oscillations of EOF1 (north-south jet displacement) and EOF2 (jet pulsation) leading to an orchestrated combination of equatorward propagation of wave activity (a baroclinic process) and nonlinear wave breaking (a barotropic process), which altogether act to reduce the total eddy forcings. The results highlight the importance of considering the coupling of EOFs and cross-EOF feedbacks to fully understand the natural and forced variability of the zonal-mean large-scale circulation.

Reference: Lubis, S. W., & Hassanzadeh, P. (2021). An Eddy–Zonal Flow Feedback Model for Propagating Annular Modes, Journal of the Atmospheric Sciences, 78(1), 249-267.

How to cite: Lubis, S. and Hassanzadeh, P.: An Eddy-Zonal Flow Feedback Model for Propagating Annular Modes and their Dynamics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5998, https://doi.org/10.5194/egusphere-egu21-5998, 2021.

The global hydrological cycle is expected to intensify under a warming climate. Since extratropical Rossby wave trains are triggered by tropical convection, this will impact the atmospheric circulation in the extratropics. Owing to the approximate linearity of the teleconnection pattern, we can use a method based in linear response theory to quantify this extratropical response using a step response function. We have examined the step response functions for a selection of CMIP5 pre-industrial control runs and reanalysis data,  in particular studying the response during the boreal winter. We found there to a large intermodel spread in the response pattern owing to differences in representations of the model basic state. In the current work, we use a 'perfect model' approach to conduct a systematic study of the performance of the linear response method in projecting future winter-time northern hemisphere circulation changes using the present day (1986-2005) model basic states, comparing these to those projected by CMIP5 models under a 3 degree rise in mean global temperature anomaly above pre-industrial. We demonstrate how, given a projected precipitation change pattern, the linear response theory method can compete with the models in providing faithful projections for the extratropical circulation changes.

How to cite: Senior, N., Matthews, A., and Joshi, M.: Projections of future atmospheric circulation changes using the extratropical linear step response to tropical precipitation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6528, https://doi.org/10.5194/egusphere-egu21-6528, 2021.

Observations of warm past climates and projections of future climate change show that the Arctic warms more than the global mean, particularly during winter months. Past warm climates such as the early Eocene had above-freezing Arctic continental temperatures year-round. In this work, we show that an enhanced increase of Arctic continental winter temperatures with increased greenhouse gases is a robust consequence of the smaller surface heat capacity of land (compared to ocean), without recourse to other processes or feedbacks. We use a General Circulation Model (GCM) with no clouds or sea ice and a simple representation of land. The equator-to-pole surface temperature gradient falls with increasing CO2, but this is only a near-surface phenomenon and occurs with little change in total meridional heat transport. The high-latitude land has about twice as much warming in winter than in summer, whereas high-latitude ocean has very little seasonality in warming. A surface energy balance model shows how the combination of the smaller surface heat capacity of land and the nonlinearity of the temperature dependence of surface longwave emission gives rise to the seasonality of land surface temperature change. The atmospheric temperature change is surface-enhanced in winter as the atmosphere is near radiative-advective equilibrium, but more vertically homogeneous in summer as the Arctic land gets warm enough to trigger convection. While changes in clouds, sea ice and ocean heat transport undoubtedly play a role in high latitude warming, these results show that surface-enhanced atmospheric temperature change and enhanced land surface temperature change in winter can happen in their absence for very basic and robust reasons.

How to cite: Henry, M. and Vallis, G.: Seasonality of Polar Warming in Climates with Very High Carbon Dioxide., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7424, https://doi.org/10.5194/egusphere-egu21-7424, 2021.

The interaction between large-scale tropical circulations and moist convection has been the focus of a number of studies. However, projections of how the large-scale tropical circulation may change under global warming remain uncertain because our understanding of this interaction is still limited.

Here, we use a cloud-resolving model (CRM) coupled with a supra-domain scale (SDS) parameterisation of the large-scale circulation to investigate how tropical circulations driven by sea-surface temperature (SST) gradients change in a future warmer climate. Two popular SDS parameterisation schemes are compared; the weak temperature gradient approximation and the damped-gravity-wave approximation. In both cases, the large-scale vertical velocity is related to the deviation of the simulated density profile from a reference profile taken from the same model run to radiative-convective equilibrium.

We examine how the large-scale vertical velocity profile varies with surface temperature for fixed background profile (relative SST) as well as how it varies with the surface temperature of the reference profile (background SST). The domain mean vertical velocity appears to be very top-heavy with the maximum vertical velocity becoming stronger at warmer surface temperatures. The results are understood using a simple model for the thermodynamic structure of a convecting atmosphere based on an entraining plume. The model uses a fixed entrainment rate and the relative humidity from the cloud-resolving model to predict a temperature profile. The vertical velocities calculated from these predicted temperature profiles is similar to the vertical velocity structures and their behaviour in a warmer climate that we see in the CRM simulations. The results provide insight into large scale vertical velocity structures simulated by SDS parameterisation schemes, providing a stepping stone to understanding the factors driving changes to the large-scale tropical circulation in a future warmer climate.

How to cite: Neogi, S. and Singh, M.: Understanding changes in tropical circulations in a future warmer climate using a cloud-resolving model and a conceptual model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1859, https://doi.org/10.5194/egusphere-egu21-1859, 2021.

The large-scale divergence field tilts eastward with latitude moving away from its near-equatorial maximum in the summer hemisphere. This tilt, observed for all hemispheres and seasons, is also apparent in a hierarchy of models of varying complexity, including the simple Gill model. Previous theoretical work has shown that the divergence tilt determines the sign of the divergent momentum flux in the deep tropics, suggesting a possible connection to wave propagation.

In this presentation,  we show that changes in the divergence tilt are one of two primary drivers of the interannual eddy momentum flux variability in the tropics. We also show that interannual changes in the divergence tilt are strongly correlated with the West Pacific Oscillation, with an associated large extratropical impact. The dynamical mechanisms behind this association are also discussed.

How to cite: Zurita-Gotor, P.: The interannual variability of the tropical divergence tilt, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11171, https://doi.org/10.5194/egusphere-egu21-11171, 2021.

Considerations based on atmospheric energetics and aqua-planet model simulations link the latitudinal position of the global intertropical convergence zone (ITCZ) to atmospheric cross-equatorial energy transport—a greater southward transport corresponds to a more northerly position of the ITCZ. This study, rather than concentrating of the zonally-averaged ITCZ, focuses on the tropical Pacific and looks separately at precipitation in the northern and southern hemispheres. Using numerical experiments, we show that in the tropical Pacific the response of the fully coupled ocean-atmosphere system to a hemispherically asymmetric thermal forcing, modulating atmospheric cross-equatorial 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 further show that the tropical Pacific precipitation response to thermal forcing is qualitatively different between the aquaplanet (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. The ITCZ/SPCZ seesaw can explain a precipitation dipole pattern observed in paleoclimate without invoking a too strong climate forcing and is relevant to future projections of tropical precipitation.

How to cite: Fedorov, A. and Zhao, B.: The seesaw response of the Intertropical and South Pacific convergence zones to hemispherically asymmetric thermal forcing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1428, https://doi.org/10.5194/egusphere-egu21-1428, 2021.

Global climate models have difficulties to simulate the northward extension of the monsoonal precipitation over north Africa during the mid-Holocene as revealed by proxy data. A common feature of these models is that they usually operate on too coarse grids to explicitly resolve convection, but convection is the most essential mechanism leading to precipitation in the west African monsoon region. Here, we investigate how the representation of tropical deep convection in the ICON climate model affects the meridional distribution of monsoonal precipitation during the mid-Holocene, by comparing regional simulations of the summer monsoon season (July to September, JAS) with parameterized (40km-P) and explicitly resolved convection (5km-E). 
The spatial distribution and intensity of precipitation, are more realistic in the explicitly resolved convection simulations than in the simulations with parameterized convection.
However, in the JAS-mean the 40km-P simulation produces more precipitation and extents further north than the 5km-E simulation, especially between 12° N and 17° N. The higher precipitation rates in the 40km-P simulation are consistent with a stronger monsoonal circulation over land. 
Furthermore, the atmosphere in the 40km-P simulation is less stably stratified and notably moister. The differences in atmospheric water vapor are the result of substantial differences in the probability distribution function of precipitation and its resulting interactions with the land surface. The parametrization of convection produces light and large-scale precipitation, keeping the soils moist and supporting the development of convection. 
In contrast, less frequent but locally intense precipitation events lead to high amounts of runoff in explicitly resolved convection simulations. The stronger runoff inhibits the moistening of the soil during the monsoon season and limits the amount of water available to evaporation. 

How to cite: Jungandreas, L., Hohenegger, C., and Claussen, M.: Influence of the representation of convection on the mid-Holocene West African Monsoon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2345, https://doi.org/10.5194/egusphere-egu21-2345, 2021.

Growing evidence suggests that the oceanic and atmospheric circulation experiences a systematic poleward shift under climate change. However, due to the complexity of climate system, such as, the coupling between the ocean and the atmosphere, natural climate variability and land-sea distribution, the dynamical mechanism of such shift is still not fully understood. Here, using an idealized partially coupled ocean and atmosphere aqua-planet model, we explore the mechanism of the shifting oceanic and atmospheric circulation. We find that, in contrast to the rising GHG concentration, the subtropical ocean warming plays a dominant role in driving the shift in the circulation system. More specifically, due to background ocean dynamics, a relatively faster warming over the subtropical ocean drives a poleward shift in the atmospheric circulation. The shift in the atmospheric circulation in turn drives a shift in the oceanic circulation. Our simulations, despite being idealized, capture the main features of observed climate changes, for example, the enhanced subtropical ocean warming, poleward shift of the patterns of near-surface wind, sea level pressure, cloud, precipitation, storm tracks and large-scale ocean circulation, implying that global warming not only raises the temperature, but also systematically shifts the climate zones.​

How to cite: Yang, H., Lu, J., Shi, X., Wang, Q., and Lohmann, G.: Understanding the dynamic of poleward shifting of atmospheric and oceanic circulation using aqua-planet model simulations​, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2172, https://doi.org/10.5194/egusphere-egu21-2172, 2021.

The principal sources of freshwater in India include precipitation, glaciers, and snowmelt. The former dominates the country’s annual river water contribution, which is important for agriculture and livelihood of the residents, and the latter two sources contribute at a much lower fraction in comparison to precipitation to even meet the minimum requirements. However, there is a large degree of variations in their spatio-temporal distribution throughout the country. India receives a major portion of its annual precipitation during the boreal summer (June – September). The well-known but relatively unexplored contributors to precipitation in India are atmospheric rivers (ARs). This study aims to understand the main climatological and dynamical differences between the Indian summer monsoon (ISM) and ARs in boreal summer. Zonal (‘u’) and meridional (‘v’) wind speeds, integrated water vapor transport (IVT), and integrated water vapor (IWV) are used to identify distinct features in ARs in the Indian sub-continent that can be used to distinguish them from ISM. The major differences between the two synoptic features were found in the increased zonal wind speed and moisture inputs during AR events, which often result in extreme precipitation and floods. Besides understanding them, the identification of ARs in this region and accounting for their existential contribution to moisture during peak rainfall seasons is critical for further hydrological impacts studies.

How to cite: V. Lyngwa, R. and Ahmad Nayak, M.: Quantitative study of atmospheric rivers in the Indian subcontinent, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11986, https://doi.org/10.5194/egusphere-egu21-11986, 2021.