AS1.17
Orals: Mon, 24 Apr | Room 1.85/86
Rossby waveguides constrain the propagation of Rossby waves by ducting eddy activity along paths of enhanced waveguidability in the atmosphere. Conceptually, waveguidability is the property of an eddy-free background state on which waves exist as perturbations. Because eddies are always present in the atmosphere, a procedure is required to separate the waves from the background. The choice of procedure is of practical importance when diagnosing waveguides. For example, a zonal-mean background state is easy to compute from data and often used, but does not allow for longitudinal variation of waveguidability. It has also been shown to exhibit waveguide artifacts in the presence of finite-amplitude eddies.
We introduce a new procedure to obtain an eddy-free background state for the analysis of waveguides in the atmosphere. It utilizes a redistribution (so-called zonalization) of Ertel potential vorticity on isentropes to remove eddies, including those of finite amplitude, while retaining local information. Because the procedure can be applied to instantaneous data without a need for temporal aggregation, it is suitable for causal analyses and can be applied to forecast data without lead time restrictions. Our construction is based on the "slowly evolving background state" by Nakamura and Solomon (2011) and Methven and Berrisford (2015), with additions and approximations to achieve a pragmatic compromise between theoretical grounding, usability and ease of computation.
The effectiveness of the procedure to meaningfully separate waves and the background state is illustrated with reanalysis data. Rossby waveguides are diagnosed from the background-state PV fields with a gradient-based metric. We show that our localized procedure leads to regional differences in the diagnosed waveguidability and discuss the existence of circumglobal waveguides.
How to cite: Polster, C. and Wirth, V.: A new atmospheric background state to diagnose local waveguidability, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5448, https://doi.org/10.5194/egusphere-egu23-5448, 2023.
The propagation and the characteristics of Rossby waves are influenced by the large-scale background flow where they occur: for instance, the role of upper-level jet streams in promoting Rossby wave propagation along preferred directions (so-called “waveguidability”) is a classic problem in climate dynamics. We propose a linear framework to study barotropic Rossby waves over a spherical domain for arbitrary orographic forcing and zonal background flow configurations, including cases with localised single and double jet streams. The approach allows to analytically obtain the steady-state linear flow response to orographic forcing without performing lengthy numerical integrations, together with the flow evolution as a combination of few modes composed by the various eigensolutions of the unforced problem (thus independent of the forcing). The connection between jet strength and waveguidability noticed by previous studies is confirmed. Background flow states featuring a strong jet stream are also prone to barotropic instability.
The eigenvalue analysis reveals the spatial structure of the associated Rossby modes and their growth rates, allowing to detect the presence of instabilities. We notice that, even in presence of a damping term, some background flow configurations allow wave instabilities to exist. According to the linear theory, the flow should diverge from the equilibrium state, since some waves are linearly unstable. Nonlinear simulations are performed to provide insights about the waves evolution in the unstable case. Such simulations reveal two interesting effects: 1) a damping effect operated by the nonlinear terms (i.e., the flow is unstable linearly but stable nonlinearly) for medium jet strengths; 2) a quasi-periodic behaviour around the unstable equilibrium state for the strongest jets, indicating the existence of a limit cycle. The linear analysis was still able to capture the unstable equilibrium state at the center of the limit cycle and to provide insights about the spatial structure of the dominant modes. These results indicate the usefulness of linearized approaches in the development of a reduced-order model to describe the barotropic instability mechanisms driving spherical Rossby waves.
How to cite: Segalini, A., Riboldi, J., Wirth, V., and Messori, G.: A linearized approach to study stability and waveguidability of barotropic Rossby waves, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8210, https://doi.org/10.5194/egusphere-egu23-8210, 2023.
The wintertime extratropical general circulation may be viewed as being primarily governed by interactions between Rossby waves and the background flow. These Rossby waves propagate vertically and meridionally away from their sources and amplify within the core of the tropopause-level jet, which acts as a waveguide. The strength of this waveguide is in part controlled by tropopause sharpness, which itself is a function of the strength of tropopause inversion layer (TIL), a layer of enhanced static stability just above the tropopause. Here, we report a strong relation between interannual-to-multidecadal variations in the strength of the midlatitude TIL and jet latitude in a reanalysis and climate models. Similar relationships hold for the variability across climate models. Experiments with a mechanistic model show that a sharper tropopause promotes an intensified general circulation and an equatorward shifted jet.
Reference: https://doi.org/10.1038/s41612-022-00319-6
How to cite: Birner, T. and Boljka, L.: Potential impact of tropopause sharpness on jet latitude, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14014, https://doi.org/10.5194/egusphere-egu23-14014, 2023.
Atmospheric blocking is known to be one of the most important drivers of large-scale atmospheric variability at mid-high latitudes. Blocking events consist of a disruption and/or deceleration of the mean westerly circumpolar flow, and are generally associated with large-scale high-pressure patterns, which may be connected with the occurrence of climate extremes, such as heat waves and cold spells. Atmospheric dynamics in the Arctic region may be very important in shaping the spatial and temporal patterns of blocking at mid-high latitudes, and consequently the occurrence of associated climate extremes. In particular, Arctic Amplification (AA), namely the recent amplified warming in the Arctic region compared to lower latitudes, has recently been argued to have an impact on blocking patterns and behaviour at mid-high latitudes.
The objective of this study is to investigate the most relevant mechanisms playing a role in the relationship between blocking and Arctic Amplification, by analysing the variability and frequency of the associated spatial patterns at various timescales using variables from the ERA5 reanalysis dataset for the time interval 1959-2022. Blocking events are detected based on geopotential height gradients between mid- and high-latitude regions, while Arctic Amplification is quantified as the difference of 1000hPa temperature between high and mid latitudes.
The climatological number of events per year and their average lifetime, along with the long-term trends and their relationship with the AA are analysed. Furthermore, possible mechanisms linking blocking variability and the AA are explored through the analysis of the jetstream dynamics and teleconnection patterns in the Northern Hemisphere.
How to cite: Cadau, M., Fosser, G., Bordoni, S., Sannino, G., and Gaetani, M.: On the relationship between Atmospheric Blocking and Arctic Amplification, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7466, https://doi.org/10.5194/egusphere-egu23-7466, 2023.
Atmospheric blocking events are persistent tropospheric weather patterns that are associated with extreme events, such as heatwaves. In the summer, they primarily occur at high latitudes, e.g., over northern Eurasia. However, blocking frequency over these regions is underestimated in climate models, and often overestimated over the midlatitudes, while causes for such discrepancies remain elusive. To improve model representation of blocking frequency, it is important to understand different processes that affect it. Here, we explore blocking frequency in a reanalysis and experiments with an atmospheric general circulation model forced with different sea surface temperature (SSTs) profiles. The configurations range from an idealized no SST-front experiment and prescribing idealized SST-front in different regions to prescribing realistic climatological SSTs. Surprisingly, this reveals that more idealized (realistic) experiments lead to more (less) realistic blocking frequency. We find that weaker (less realistic) blocking frequency over northern Eurasia is primarily caused by the circulation changes related to SST gradients over the North Pacific. This suggests an important role of inter-basin interactions between the Atlantic and the Pacific. Additionally, tropical teleconnections can also play a role. This may suggest that models struggle with circulation response to SSTs in boreal summer, especially over the North Pacific. Thus, this work has implications for simulating (future) summer heat (and other) extremes over the high latitudes.
How to cite: Boljka, L., Omrani, N.-E., Cheung, H.-N., Keenlyside, N., Nakamura, H., Spensberger, C., and Ogawa, F.: Potential Role of Inter-Basin Interactions in Eurasian Summer Blocking , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8062, https://doi.org/10.5194/egusphere-egu23-8062, 2023.
We examine regional and seasonal variations of extratropical storm tracks and their maintenance in terms of a column-mean local wave activity budget. Seasonal climatology of wave activity in ERA5 reveals spatial and temporal variations of storm tracks in both hemispheres broadly consistent with previous studies based on other metrics. The seasonal-mean budget consists of horizontal convergence of wave activity fluxes, input from the surface (the upward Eliassen-Palm flux), a small storage, and the residual. When averaged hemispherically, surface injection of wave activity due to baroclinic instability and forced stationary waves is balanced by a negative residual (dissipation) due to mixing and radiative damping.
However, the budget terms show considerable zonal, meridional and seasonal variations, especially in the Northern Hemisphere. Wave activity migrates downstream from a source region to a sink, where the residual is negative and largely balanced by flux convergence. In addition to the surface sources in the regions of strong baroclinicity, the residual term, though negative on average, shows significant positive values where cloud water abounds, suggesting diabatic (and/or nonquasigeostrophic) sources of wave activity.
By reconstructing the budget driven by a fixed transport velocity and damping rate evaluated from the seasonal climatology but suppressing the positive residual values, we estimate the impact of diabatic sources on the mean wave activity. It is found that the diabatic sources contribute to 26% and 20%, respectively, of North Atlantic and North Pacific storm track activities in winter, 28% of wave activity over the Pacific Northwest in summer, and 34% of activity in the Indo-western Pacific sector of the austral storm track in summer.
How to cite: Nakamura, N.: How much does diabatic heating affect storm track activity?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2533, https://doi.org/10.5194/egusphere-egu23-2533, 2023.
When different weather extremes occur at multiple locations at the same time, their aggregated impact can exceed the one of the individual events. Examples can be concomitant summer heatwaves over major breadbasket regions, leading to potential food shortages at the global scale, or the connection between cold spells over North America and windstorms over Europe. These compound events often attract a broad interest by the media and society, as anomalous weather conditions seem to occur “everywhere at the same time”. If it is possible to identify a physical linkage between them, those separate extremes can be considered as parts of a single, spatially compounding weather extreme. Pinpointing a common physical driver is not trivial, however, and it might well be that such extreme events just co-occur by coincidence.
This overview presentation will discuss how the linear and nonlinear dynamics of Rossby waves can help to understand spatially compounding extremes. Examples of linear dynamics involve the propagation of Rossby wave packets across broad portions of the middle latitudes, aided by the presence of upper-level waveguides. The link between extreme weather and atmospheric blocking, on the other hand, can be seen as involving a nonlinear sort of dynamics. Analytical, idealized and data-driven approaches to the study of Rossby waves can shed light on the drivers of spatially compounding extremes, and result in useful tools to study how the drivers of such extremes are being affected by anthropogenic global warming.
How to cite: Riboldi, J.: On the connection between Rossby waves and spatially compounding weather extremes, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8587, https://doi.org/10.5194/egusphere-egu23-8587, 2023.
Extremes in extratropical meridional energy transports in the atmosphere are associated with the dynamics of the atmosphere at multiple spatial scales, from planetary to synoptic. This is related to the nature of amplifying baroclinic waves, that are fundamentally intermittent and sporadic, significantly affecting the net seasonal transport across latitudes. Here, we use the ERA5 reanalysis data to perform a wavenumber decomposition of meridional energy transports in the Northern Hemisphere mid-latitudes during winter and summer. Extreme transport events are linked to atmospheric circulation anomalies and dominant weather regimes, identified by clustering 500 hPa geopotential height fields. Partitioning the extreme events across zonal wavenumber highlights the different role of scales in different seasons and regions. In general, planetary-scale waves determine the strength and meridional position of the synoptic-scale baroclinic activity with their phase and amplitude. During winter, large wavenumbers (k = 2–3) are key drivers of the meridional-energy-transport extremes, and planetary- and synoptic-scale transport extremes virtually never co-occur. In summer, extremes are associated with higher wavenumbers (k = 4–6), identified as synoptic-scale motions. Focusing on recently occurred exceptionally strong summertime heat waves and wintertime cold spells, we notice that regime structures of these events are typical of extremely large poleward meridional energy transports.
How to cite: Lembo, V., Fabiano, F., Galfi, V. M., Graversen, R. G., Lucarini, V., and Messori, G.: Extremes of meridional energy transports in Northern Hemisphere mid-latitudes across zonal wavenumbers and dominant weather regimes, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8331, https://doi.org/10.5194/egusphere-egu23-8331, 2023.
In early 2022, several extreme weather events occurred in the Southern Hemisphere. Devastating floods killed more than 500 people in South Africa (11-12 April) and about 26 people in eastern Australia (24-28 February and 25-31 March), while an unprecedented heatwave broke temperature records in Antarctica (16-22 March). This study presents a multiscale perspective of the atmospheric processes associated with these extreme events from synoptic to planetary scales. Equatorward Rossby wave breaking facilitated the transport of moist air from tropical oceans to the subtropical regions affected by the extreme precipitation events, while poleward Rossby wave breaking forced an intrusion of warm and moist extratropical air masses into the Antarctic Peninsula. Southern hemispheric extratropical wave activity demonstrated relatively normal conditions during February and March, while wave energy reached extremely large values for wave number 5 during April. From a planetary-scale perspective, we investigate how tropical variability, including the El-Nino Southern Oscillation (ENSO; in a La Nina phase) and the Madden-Julian Oscillation (MJO), modulates large-scale atmospheric circulation patterns, extratropical wave activity, and Rossby wave breaking. Overall, this study clarifies the role of regional and remote atmospheric processes in the recent weather extremes in the Southern Hemisphere.
How to cite: De Vries, A. J., Casselman, J. W., Afargan-Gerstman, H., Nangombe, S. S., Pilon, R., Russo, E., Wicker, W., Yadav, P., and Domeisen, D. I. V.: Extreme weather in the Southern Hemisphere in early 2022: from Rossby waves to planetary-scale conditions, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9918, https://doi.org/10.5194/egusphere-egu23-9918, 2023.
The intensity of heat extremes has been increasing in recent decades, with several recent notable heatwaves afflicting highly populated areas. Previous studies have related heatwaves to slow moving amplified Rossby waves, due to the formation of circumglobal teleconnections (i.e., European heatwaves of 2003 and 2010). Other studies have found that there is a statistical link between high amplitude upper-tropospheric transient Rossby wave packets (RWPs) and increased probability of lower-tropospheric temperature extremes. These non-circumglobal RWP amplitudes were found to be better linked to temperature extremes than Fourier amplitudes quantifying circumglobal waviness, including the European heatwaves of 2003 and 2010. In the summer of 2018, several record-breaking and persistent heatwaves occurred simultaneously around the globe and were linked to an amplified hemisphere-wide wavenumber 7 circulation pattern. Here, we investigate the relation of the synoptic RWPs and the circumglobal characteristics of atmospheric circulation characteristics with the heatwaves during 1998-2018, with a focus on the heatwaves that occurred during the summer of 2018. Preliminary results show that the dominant circumglobal pattern for the 2018 summer heat extremes was dominated by a zonal wavenumber 6 circulation pattern and that its amplitude was connected to high amplitude RWPs that occur across the Northern Hemisphere.
How to cite: Pyrina, M. and Domeisen, D.: Heatwaves of 2018: connecting large-scale to synoptic scale circulation, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12319, https://doi.org/10.5194/egusphere-egu23-12319, 2023.
Extratropical Rossby waves intrude over tropical region as well as the Indian region and exert significant influence on the weather features. Over Indian region, the pre-monsoon is a dry summer season. During this season, several studies have identified drivers of heatwaves based on different aspects such as the synoptic-scale systems, regional factors, and large-scale teleconnection patterns around the globe (Perkins 2015). Essentially these drivers identified for the Indian region do not describe the heatwave events as the intensification of some modes. Midlatitude heatwaves, on the other hand, are identified as the extreme phase of Rossby Wave mode amplification. However, over the Indian region studies do not explicitly point out the existence of temperature intraseasonal modes during April-May over the Indian region, and it is not clear if some of the drivers of heatwaves can also explain the April-May temperature variations during heatwaves as derivatives (or amplification) of some subseasonal modes. This study identifies the dominant pair of the intrinsic mode of temperature intraseasonal oscillations (ISO) related to subtropical and extratropical Rossby waves, which can also explain the heatwave spikes.
The ISO modes are derived using the empirical orthogonal function analysis of the detrended surface temperature and further regression analysis demonstrates the dynamical origin of these spatial modes. It is found that both the modes are driven by the mid-latitudinal Rossby waves which propagate towards the Indian region following the ‘preferred teleconnection pathways’ (Ambrizzi and Hoskins 1997). The dominant mode is related to the subtropical westerly jet waveguide, and the second mode is induced by the extratropical to European eddy-driven jet which follows the Europe-Middle East-Indian Ocean pathway. From the different phases of the oscillation obtained from these modes, two phases are favorable for the extreme temperature events and these two phases account for more than 50% of the extreme event occurred over the Indian region.
Global warming is however steering these two inherent modes of ISOs in surface temperature with the first mode having a significant decreasing trend and the second mode showing an increasing trend. The modal difference in trend is likely to be related to the weakening of the subtropical jetstream waveguide and the strengthening of the extratropical jetstream in a warming scenario (Archer and Caldeira 2008). The usefulness of this study is that the ISOs defined in this study could explain the maximum number of extreme temperature events occurring over the Indian region as a projection on two temperature modes. The modal trend could also account for the regional asymmetry of warming over the Indian region in the global warming scenario, and is related to the trend in jetstream waveguide those steers these modes towards the Indian region.
References
Ambrizzi T, Hoskins BJ (1997) Stationary rossby-wave propagation in a baroclinic atmosphere. Q J R Meteorol Soc 123:919–928. https://doi.org/https://doi.org/10.1002/qj.49712354007
Archer CL, Caldeira K (2008) Historical trends in the jet streams. Geophys Res Lett 35:. https://doi.org/https://doi.org/10.1029/2008GL033614
Perkins SE (2015) A review on the scientific understanding of heatwaves—Their measurement, driving mechanisms, and changes at the global scale. Atmos Res 164–165:242–267. https://doi.org/https://doi.org/10.1016/j.atmosres.2015.05.014
How to cite: Saradambal, L. and Chattopadhyay, R.: Propagation of Mid-Latitudinal Rossby waves along the Jetstream waveguides and their Role in Summer Temperature Intraseasonal Oscillations and Extremes over the Indian Region, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1199, https://doi.org/10.5194/egusphere-egu23-1199, 2023.
Heat waves lead to increased mortality due to heat exhaustion and heatstroke, wildfire, reduced agricultural yields, increased energy demand, economic predicaments and other societal issues. Heat wave events over the Middle-East have received far less attention compared to events elsewhere. Here, we provide a comprehensive characterization of heat wave events over Iraq, covering the period 1980-2019.
We use ERA5 reanalysis data for Northern summer (June-July-August) to identify heat waves in daily maximum 2-m temperature (Tmax) data and study them using composite analyses and clustering. We define a heat wave event if the Tmax anomaly exceeds the 90th percentile over three consecutive days, provided this threshold exceedance covers at least 50% of our target area.
The composite-mean evolution of daily Tmax anomalies demonstrates that our heat waves typically strengthen gradually over the week preceding the central day with a sharp decline in strength at positive lags, reaching an average maximum anomaly of ~3.7 K at the central day. We find the heat waves to extend from the Arabian peninsula northward across Iraq toward southwestern Russia. Clustering of all heat wave events reveals two dominant flow anomaly patterns that roughly distinguish early from late summer events.
The first cluster (early summer events) is associated with anomalous anticyclonic flow associated with a quasi-stationary upper-level high pressure system to the north-east of Iraq precisely over Caspian sea. This anomalous anticyclonic flow is embedded in a Rossby wave train that initially propagates along the north Atlantic wave-guide, then further equatorward along the North African-Asian jet just before the central day. Our composite-mean evolution for this first cluster further shows mid-tropospheric subsidence over the Zagros mountains, i.e., upstream of our heat wave target area. Downslope Foehn winds appear to enhance the heat wave over Iraq.
In contrast, the second cluster is primarily composed of late-summer events and shows strong anomalies in the Shamal winds - a pronounced low-level north-westerly jet along the western edge of the Zagros mountains. During these late summer heat wave events the Shamal jet is substantially weakened or even reversed, transporting warm air from the Persian gulf into the target region.
How to cite: Al-Shamarti, H., Birner, T., and Rupp, P.: Circulation aspects associated with heat wave events over Iraq, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-307, https://doi.org/10.5194/egusphere-egu23-307, 2023.
Persistent hot–dry or cold–wet summer weather can have significant impacts on agriculture, health, and the environment. For northwestern Europe, these weather regimes are typically linked to, respectively, blocked or zonal jet stream states. The fundamental dynamics underlying these circulation states are still poorly understood. Edward Lorenz postulated that summer circulation may be either fully or almost intransitive, implying that part of the phase space (capturing circulation variability) cannot be reached within one specific summer. If true, this would have major implications for the predictability of summer weather and our understanding of the drivers of interannual variability of summer weather. Here, we test the two Lorenz hypotheses (i.e., fully or almost intransitive) for European summer circulation, capitalizing on a newly available very large ensemble (2000 years) of present-day climate data in the fully coupled global climate model EC-Earth. Using self-organizing maps, we quantify the phase space of summer circulation and the trajectories through phase space in unprecedented detail. We show that, based on Markov assumptions, the summer circulation is strongly dependent on its initial state in early summer with the atmospheric memory ranging from 28 days up to ~45 days. The memory is particularly long if the initial state is either a blocked or a zonal flow state. Furthermore, we identify two groups of summers that are characterized by distinctly different trajectories through phase space, and that prefer either a blocked or zonal circulation state, respectively. These results suggest that intransitivity is indeed a fundamental property of the atmosphere and an important driver of interannual variability.
How to cite: Sperna Weiland, R., van der Wiel, K., Selten, F., and Coumou, D.: Intransitive Atmosphere Dynamics Leading to Persistent Hot–Dry or Cold–Wet European Summers, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12703, https://doi.org/10.5194/egusphere-egu23-12703, 2023.
Abstract
Extreme precipitation during winter over the western Himalayas (WH) is associated with western disturbances embedded in sub-tropical westerly jet streams, which are potentially linked to planetary wave dynamics. In this study, we explore a possible connection of quasi-resonant amplification (QRA) to precipitation extremes observed over WH using the global high-resolution reanalysis ERA5 during the period 1979-2019. Precipitation extremes have been identified using percentile approach (peak over threshold) where daily precipitation amount from the entire time series of precipitation exceeds the 95th percentile threshold at a particular grid point. Our analysis suggests that substantially magnified, quasi stationary mid-latitude planetary waves with zonal wavenumbers 6 to 8 accompany these extremes, highlighting the influence of QRA phenomenon. Furthermore, we also identified a fingerprint for QRA occurrence in terms of the zonally averaged surface temperature field. Lastly, we classified extreme precipitation intensities and various related key variables using k-means clustering and analyzed the wavenumbers associated with different categories. Our results underpin the significant role of the QRA mechanism in amplification of planetary waves, in turn, favoring western Himalayan precipitation extremes. Detailed results will be discussed.
Keywords: Quasi-resonant amplification, zonal wavenumber, precipitation extremes, western Himalayas
*E-mail of corresponding author: rajuattada@iisermohali.ac.in
How to cite: Sharma, N., Attada, R., and Hunt, K. M. R.: Role of Quasi-resonant Planetary Wave Dynamics in Winter Precipitation Extremes over India’s High Mountain Region, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-16756, https://doi.org/10.5194/egusphere-egu23-16756, 2023.
Clustering extreme rainfall events are successive occurrences over multiple regions. As climate continues to warm up, cluster occurrence is becoming a prevailing feature of extreme weather events and leading to significant socioeconomic impacts. Understanding the associated atmospheric teleconnection patterns and their underlying mechanisms can help quantify their risk, i.e., the probability of occurrence and severity of cluster extremes in the future. In this study, we identified over 400 events of clustering extreme rainfall events over South Asia, East Asia, and North America in the past 42 years. Diagnostic analyses of these events reveal the diversity of teleconnection that paved the road to the events. Three Rossby wave patterns: (1) circum-Pacific Rossby wave, (2) cross-Pacific Rossby wave, and (3) Pacific anticyclone Rossby wave breaking, are the major synoptic-scale dynamics responsible for clustering rainfall events. Specifically, the circum-Pacific Rossby wave dominates in autumn and early winter, while the cross-Pacific Rossby wave pattern prevails during the Indian summer monsoon season. The occurrence frequency of the anticyclone Rossby wave breaking does not show significant seasonal differences.
The key driving mechanisms behind these wave patterns are: 1) The poleward propagation of the circum-Pacific wave can be excited by the heating anomaly originating in the tropics. 2) The mid-latitude cross-Pacific Rossby wave is a portion of the circum-global teleconnection pattern. This recurrent Rossby wave connects Asia and North America, influenced by the Indian summer monsoon. 3) Pacific anticyclone Rossby wave breaking is a quasi-stationary synoptic wave pattern causing persistent extreme weather. The frequency of this pattern increases significantly during La Niña years with a relatively weak subtropical jet. The single or synergistic effects of these three patterns cause the cluster occurrence of extreme rainfall. Findings from this work offer a better understanding of rainfall teleconnection and tropic/midlatitude interaction.
How to cite: Song, Y. and Lu, M.: Cluster occurrence of extreme rainfall events over Indo-Pacific and their associated diverse Rossby wave patterns, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2751, https://doi.org/10.5194/egusphere-egu23-2751, 2023.
Persistent unusual configurations of the North Atlantic jet stream affect the weather and climate over Europe. We focus on winter and on intraseasonal and seasonal time scales, and study persistent jet anomalies through the lens of large deviation theory using CMIP6 simulations of the MPI-ESM-LR model and ERA5 reanalysis data. Our results show that persistent temperature and precipitation extremes over large European regions are anomalously frequent during the unusual, persistent jet configurations we identify. Furthermore, the relative increase in frequency of surface extremes is larger as we consider more intense surface extremes and/or more extreme jet anomalies. The highest extreme event frequencies at the surface are observed in case of precipitation over the Mediterranean and Western Europe during anomalously zonal and/or fast jet events, pointing to these jet anomalies matching rather homogeneous large-scale atmospheric configurations with a clear surface footprint. Additionally, our results emphasise the usefulness of large deviation rate functions to estimate the frequency of occurrence of persistent jet anomalies, and more generally of unusual, persistent atmospheric circulation patterns.
How to cite: Galfi, V. M. and Messori, G.: Persistent anomalies of the North Atlantic jet stream and associated surface extremes over Europe, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3942, https://doi.org/10.5194/egusphere-egu23-3942, 2023.
Since 1980, the U.S. experienced 338 different billion-dollar weather and climate disasters, with greater than 25% (n = 96) of these occurring in the cold season of December–March. These events can have lasting societal and economic impacts that make diagnosing their likelihood of occurrence in the next week, season, or decade an important problem in the context of our changing climate. This analysis will focus on a case study of the predictability and dynamics of the 2021 US cold air outbreak (CAO) and provide a multiscale overview of the event on subseasonal-to-seasonal (S2S) timescales. The analysis of the dynamics focuses on the role of the stratospheric recovery from the January 2021 sudden stratospheric warming and a stratospheric Rossby wave reflection event in the context of the development of high amplitude flow in early February 2021. The second part considers the ability of S2S forecast models to resolve the predecessor events to this CAO. The predictability of this CAO and the role of the stratosphere in the development of CAO are considered by analyzing both high-top and low-top S2S forecast model from the S2S prediction Project Database.
How to cite: Lang, A.: The dynamics and predictability of US winter extremes—a multiscale case study of the 2021 U.S. cold air outbreak, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7975, https://doi.org/10.5194/egusphere-egu23-7975, 2023.
The successful prediction of heatwave onsets on the medium-range forecast time scale (here, 5-12 days) mainly relies on the adequate forecasting of large-scale Rossby wave patterns and their dynamics. In the mid-latitude regions of Europe, lasting heatwaves are often associated with a substantial blocking of the large-scale atmospheric flow due to amplified and/or breaking Rossby waves. To characterize such anomalous flow configurations, which may come in different patterns, we adopt the concept of Euro-Atlantic weather regimes. Based on Empirical Orthogonal Function analysis and subsequent k-means clustering, this widely-used metric reduces the complexity of the atmospheric flow field by projecting it onto the seven main modes of synoptic-scale variability in this domain. In this study, we therefore examine heatwave characteristics in different European regions in relation to Euro-Atlantic weather regimes. A focus is set on the question to which extent the medium-range predictability of heatwave onsets depends on the current or preceding weather regime as well as to flow anomalies further upstream or other potential precursors not directly related to Rossby wave dynamics such as abnormally dry soils.
Heatwaves are objectively diagnosed as a 90th percentile exceedance in 2m maximum temperatures for a minimum of 3 days in both a local and regional context. Using ERA-5 data for the period 1979-present, we find that British and Scandinavian heatwaves are mainly associated with classic blocking regimes (Scandinavian and European blocking), whereas the picture is more diverse for Central Europe where the „no regime“ case is also frequently observed. Remarkably, over the last 20 years, European heatwaves associated with a European blocking seem to be significantly related to pre-existing anomalously dry soils over large parts of Northern America which is, however, not the case for heatwaves related to any other weather regime.
The medium-range predictability of heatwaves is investigated for the period 2001-2018, using hindcast ensembles of two state-of-the art weather forecast models ECMWF-IFS and GEFS-v12, by means of usual metrics such as 500hPa geopotential anomaly correlation coefficients (ACC) and 850hPa temperature mean absolute errors. Preliminary results with a focus on Central Europe suggest that heatwaves in this region seem to be slightly more predictable (roughly one more day until ACC drops below 0.8) when they occur in conjunction with a Scandinavian or European blocking compared to the case with no apparent regime. This may be explained by the overall more transient and phase-error prone nature of the „no regime“-type heatwaves. Interestingly, heatwaves with the worst predictability at 10 days lead time show an intensified jet stream over the Atlantic one week prior and a slight tendency toward wetter than normal soils over North America and Central Europe.
Finally, we also investigate to which extent medium-range forecasts of local maximum temperatures further depend on more local, diabatic processes (soil moisture, cloud cover forecast) and whether there are systematic differences between lead times and weather regimes.
How to cite: Lemburg, A. and Fink, A. H.: Investigating European heatwaves and their medium-range predictability in relation to weather regimes , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8139, https://doi.org/10.5194/egusphere-egu23-8139, 2023.
Posters on site: Mon, 24 Apr, 16:15–18:00 | Hall X5
The jet stream is a circumpolar global band of high wind speeds in the upper troposphere. Meridional meanders of the jet lead to high-impact weather events and may synchronize them over thousands of kilometres. As a Rossby waveguide, the jet influences paths of synoptic-scale eddies, which in turn alter jet dynamics. Coherent regions of enhanced wind speed, so-called jet streaks, typify the jet locally and anomalously strong jet streaks often coincide with extreme precipitation and wind events.
Despite their relevance, process understanding remains limited regarding the formation of jet streaks in interaction with lower-level weather systems. The same is true for the influence of jet streaks on Rossby wave evolution. One way to further the understanding of jet streak dynamics is to study the interaction between adiabatic and diabatic processes during jet streak evolution. However, the relative importance of those processes is difficult to disentangle.
This study utilises a Lagrangian-based PV gradient perspective by applying it to a jet streak relative coordinate system to obtain composites throughout the lifecycles of multiple jet streak events. The theoretical foundation of this approach is the link between the horizontal isentropic PV gradient and wind speed. Local maxima of the normalised isentropic PV gradient are collocated with centers of jet streaks. As PV is conserved under adiabatic and frictionless flow, the PV gradient perspective allows for an investigation of diabatic-adiabatic interaction.
We analyse a convection-resolving 1.1 km COSMO simulation in the eastern North Atlantic in autumn 2016, using PV gradient analysis and online air parcel trajectories to separate diabatic and adiabatic contributions to jet streak development. Cloud processes play an important role in the establishment and maintenance of a strong event, while the dynamics of a weaker jet streak is dominated by effects of adiabatic deformation.
After demonstrating the approach in two case studies, we present results from a composite analysis of multiple jet streak events to achieve a more systematic understanding of diabatic-adiabatic interaction during their evolution.
How to cite: Bukenberger, M., Rüdisühli, S., and Schemm, S.: Jet streaks from a PV gradient perspective: A Lagrangian analysis of diabatic-adiabatic interaction in km-scale simulations, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3115, https://doi.org/10.5194/egusphere-egu23-3115, 2023.
Synoptic-scale filaments of negative potential vorticity (PV) in the northern hemisphere tropopause can form adjacent to the jet stream in the presence of convection and moderate shear (i.e., severe thunderstorm environments). Case-studies have shown that synoptic-scale negative PV can influence in-situ jet stream dynamics. Negative PV arises due to strong vorticity in convective updrafts, driven by the horizontal gradient of diabatic heating (O < 10 km). Its origin from scales not resolvable by contemporary global weather models can thus also impinge on jet stream forecast skill.
Nevertheless, little is still known about the characteristics of synoptic-scale negative PV. How frequently is it observed? And what are its ‘typical’ impacts on the jet stream?
Focusing on North America where severe thunderstorms are frequent, we design an algorithm that tracks the temporal evolution of closed contours of upper-level, negative PV air using ERA5 data. We composites instances in which it is in close-proximity to (‘interacts with’) the jet stream and assess its dynamical response. The role of negative PV on jet evolution and its downstream response over the Atlantic is facilitated through a combination of lagged composite analysis and K-means clustering.
Our composite results in combination with preliminary high-resolution model simulations highlight that elongated bands of negative PV frequently interact with the jet stream, intensify jet wind maxima and may serve as an amplification source for Rossby waves.
How to cite: Lojko, A., Winters, A., Jablonowski, C., and Payne, A.: The Role of North American Convective Storms on Jet Stream Dynamics: A Negative Potential Vorticity Perspective, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3912, https://doi.org/10.5194/egusphere-egu23-3912, 2023.
Regional weather variability and the occurrence of extreme weather events are highly connected to the position of jet streams. Climate models generally project a poleward shift of the jets under the influence of anthropogenic warming. However, ERA5 reanalysis data show that the North Atlantic jet stream in winter has roughly remained in place. We investigate the mechanisms that lead to this behavior. The analysis reveals that upper-level temperature trends produce a reduction in stability that leads to an increase in baroclinicity. Furthermore, momentum convergence is also intensified across the jet core, producing an acceleration of the jet and not a weakening as suggested by arguments based solely on the Artic amplification.
Numerical simulations from an ensemble of fully coupled climate simulations run with the Community Earth System Model under the SSP3.7 scenario are also analyzed along with idealized warming experiments in an aquaplanet setup with a zonal asymmetry in sea surface temperature. The climate simulations exhibit a large spread during the historical period and only a few ensemble members reproduce the observed trends, suggesting that trends only based on ensemble means could lead to misleading projections. Additionally, the aquaplanet runs display high sensitivity to the location of the asymmetry. This provides a supplementary argument in support of inspecting all individual climate projections as small variations in the original jet position can lead to large disparities in the projected trends.
How to cite: Hermoso, A. and Schemm, S.: Winter North Atlantic jet variability under global warming: Past trends and future projections , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5969, https://doi.org/10.5194/egusphere-egu23-5969, 2023.
The tropical circulation is typically not well represented in idealized models used to study jet dynamics.
We implement a convective relaxation algorithm into a dry dynamical core model following Schneider and Walker (2006) to improve the representation of the driving mechanism behind the subtropical jet: the tropical meridional overturning.
We study the dependence of the general circulation on the vertical stratification set by a convective relaxation scheme.
Varying tropospheric lapse rates produces two jet regimes that are characterized by the distance between the subtropical jet and the eddy-driven jet.
The separated jet state features distances of more than 12° latitude between subtropical and eddy-driven jet and is dominant in simulations with higher tropospheric static stability.
Both jets approximately coincide in the joined jet regime, which is dominant in lower stability simulations.
In addition to a steady state analysis, transitions from one regime to the other are induced by changes in convective lapse rate.
Regime changes are also observed as events produced by natural variability in some of the model runs.
This time-dependent perspective shows that the structure of net Rossby wave dissipation in the upper troposphere, measured by the Eliassen-Palm flux divergence, is crucial in order to understand the regimes.
Jet merge and split events are mediated by upper tropospheric momentum flux variability.
They are preceded by heat flux variability and are tied to variations in the hemispheric eddy kinetic energy.
The results are also interpreted through the concept of criticality, relating meridional and vertical gradients in potential temperature.
The analysis highlights the importance of static stability and its changes to mid-latitude jet dynamics.
How to cite: Conrat Fuentes, P., Birner, T., and Garny, H.: Jet Regimes Induced by Stratification Changes in a Dry Dynamical Core Model , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8399, https://doi.org/10.5194/egusphere-egu23-8399, 2023.
The synoptic scale upper level Rossby wave breaking (RWB) has a great influence on the weather pattern on the underlying regions. The RWB events have been studied extensively in the mid-latitude regions as it is prone to such events which often lead to extreme weather conditions. However, studies are elusive especially over the Indian sub-continent except few cases. RWB climatology and variability on monthly, interannual as well as on decadal scales is still poorly understood over this region. To address these shortcomings, in this study, we have used the reanalysis data and implemented a contour searching algorithm to identify RWB events over the period 1979-2021. Using the implemented algorithm, we have detected 513 RWB events for the study period which we further use to examine RWB climatology and variability over the subcontinent (5-40oN, 55-105oE). Our results suggest a significant increase in the number of RWB events per year during the last two decades, as well as an increase in the intensity over the northwest region of the Indian subcontinent. We note that the RWB frequently affects the northwest region in winter, which later shows a shift in peak number of occurrences of RWB towards central India at the end of winter. This shift is linked to seasonal changes in the background zonal wind in the upper-troposphere. Also, the monthly climatology of vertical intrusions of the PV streamers indicate that intrusions are stronger during winter than other months. In addition, the role of RWB on the dynamical changes of the atmosphere such as anomalies of wind circulation patterns and moisture content is analyzed using composite analysis. The variability of the RWB events and its linkages with global sea surface temperature particularly conditions in the Pacific Ocean have also been studied.
How to cite: Thomas, B., Kunchala, R. K., Singh, B. B., and Kondapalli, N. K.: Long-term variability of Rossby Wave Breaking events over the Indian subcontinent, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8465, https://doi.org/10.5194/egusphere-egu23-8465, 2023.
The dynamics of the mid-latitude atmospheric jet is an important component of internal variability in real and modelled climate. The variability may also potentially affect the response to deterministic external forcing, with implications for seasonal prediction including the ‘signal-to-noise’ paradox. Recent research has used ad hoc probabilistic approaches to investigate the paradox but has given little dynamical insight into the behaviour observed in models. This motivates further dynamical study of the factors determining variability and response to forcing. We use a simple stochastically forced barotropic model containing the essential mechanisms for beta-plane jet variability to conduct a range of numerical experiments. We consider first the dependence of the behaviour on the damping time scale and on the amplitude and latitudinal width of stochastic forcing that is statistically homogeneous in longitude. We consider leading empirical orthogonal functions of the zonal mean wind velocity, use these as quantifiers of jet behaviour, and analyse the amplitude, latitudinal structure, and autocorrelation time scale of the simulated variability. We move on to examine cases where there is imposed longitudinal variation. An appropriate decorrelation time scale of the zonal jet stream could be displayed by the model, partly depending on the damping time scale. We conduct experiments with applied forcing to determine whether the basic prediction of the fluctuation-dissipation theorem, that response to forcing is proportional to the autocorrelation time scale for natural variability, holds in this system. This simple dynamical model has good implications for a physical understanding of jet persistence and the signal-to-noise paradox.
How to cite: Wang, W. and Haynes, P.: Variability of beta-plane zonal jets, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8620, https://doi.org/10.5194/egusphere-egu23-8620, 2023.
It is evident that the jet streams are becoming more erratic and unstable in a changing climate. We investigate changes both in position and speed of the midlatitude jet streams at 300 hPa in 31 Atmospheric Model Intercomparison Project (AMIP) runs and the ERA5 reanalysis dataset investigating the ability of the thermal wind concept to explain changes in place and the regime of the jet streams, which are disturbed by Arctic amplification, is the core of this work. All data covers the period 1979-2014.
It is revealed that the changes in jet stream magnitude and position in the multi-model mean (MMM) can largely be explained by the thermal wind. We also discovered that the AMIP models reproduce trends in jet position and strength seen in ERA5. Yet it is a must to state that when inspecting individual models, we find that some models can reproduce ERA5 trends in NH. The large variance in modelled trends, however, leads to a poorly represented MMM.
In the end, the jet stream plays a significant role in shaping global weather patterns and is affected by changing climate as becoming more wobbly and unstable. The potential impact of Arctic warming on the jet stream and how it may lead to more extreme weather events in the mid-latitudes is taken under inspection from the window of thermal wind concept with this study.
How to cite: Gözlet, M. S., Kjellsson, J., and Latif, M.: A Thermal Wind Perspective of Driving Changes in Jet Stream Patterns, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9353, https://doi.org/10.5194/egusphere-egu23-9353, 2023.
Global-scale af-/reforestation (A/R) and deforestation substantially changes the Earth’s energy and water fluxes, thereby affecting the large-scale atmospheric circulation and thus have significant impacts on weather systems. During summer, A/R and deforestation induced changes in the soil moisture are shown to have an impact on the planetary wave response through the jet stream. Such changes might lead to high-amplitude, quasi-stationary circumglobal Rossby waves that have been associated with extreme summer heatwaves and persistent high-impact extremes. In this study we investigate how idealized global land use and land management changes can alter the boreal summer circulation with a focus on the response of the jet stream. For the analysis we conducted model experiments with three fully coupled Earth System Models (EC-EARTH, MPI-ESM and CESM). Each scenario run for 160 years from which we analyze the final 30 years. A control run with constant current land use and land management is compared to a global A/R and a global deforestation (global cropland expansion) simulation. In order to assess clean land-atmosphere interactions, all simulations are kept with constant present-day atmospheric forcings (year 2014). We investigate the potential changes in the amplitude of the waves, the likelihood of quasi-stationary wave activity, and of summer blockings within the three different simulations, and the weather consequences that such changes lead to.
How to cite: Manola, I., Coumou, D., Luo, F., Guo, S., Havermann, F., De Hertog, S., Lejeune, Q., Menke, I., Pongratz, J., Schleussner, C., Seneviratne, S., and Thiery, W.: Summer jet stream response to global af-/reforestation and deforestation , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14529, https://doi.org/10.5194/egusphere-egu23-14529, 2023.
How to cite: Oliveira Guimarães, S., E. Mann, M., Rahmstorf, S., Petri, S., Kornhuber, K., Coumou, D., A. Steinman, B., Brouillette, D., and Christiansen, S.: Projected changes on quasi-resonant amplification by CMIP5 and CMIP6 toward the persistence in extreme summer weather events, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15986, https://doi.org/10.5194/egusphere-egu23-15986, 2023.
Case studies of mid-latitude summer heatwaves commonly regard stationary synoptic-scale Rossby waves as the primary dynamical forcing. Whether this relationship between upper-tropospheric Rossby wave phase speed and persistent temperature extremes can be generalized is less clear. Here, we evaluate interannual and intra-seasonal variability of Rossby wave phase speed in reanalysis datasets employing circumglobal spectral analysis and investigate episodes with a low or a high zonal phase speed, respectively. Locally, we find evidence of Rossby wave phase preferences during episodes with a low phase speed, where preferred locations of ridges coincide with regions of increased heatwave frequency, but globally, there is no indication of an increased heatwave frequency. Unexpectedly, the finding of Rossby wave phase preferences and increased heatwave frequency also hold for episodes with a high phase speed, although in different areas of the summer hemisphere mid-latitudes. These findings, in particular about episodes with a zonal phase speed, will improve our mechanistic understanding of the dynamical drivers of heatwaves.
How to cite: Wicker, W. and Domeisen, D.: On the role of Rossby wave phase speed for persistent temperature extremes, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-16490, https://doi.org/10.5194/egusphere-egu23-16490, 2023.
We examine the tropical meridional overturning circulation in an aquaplanet GCM with fixed orbital parameters and uniform insolation angle. The atmosphere is forced by an imposed non-interactive sea surface temperature (SST) distribution which is varied between present-day Earth-like to a latitudinally uniform profile. A conventional Hadley Cell (HC) -like flow is observed in all experiments along with the poleward transport of energy and momentum. In simulations forced by a non-zero SST gradient, latent heat released from organized convection near the equator sets up a deep tropical cell. Rossby wave activity generated near the extratropical surface propagates upward and turns equatorward on reaching the tropopause. These waves break on the edge of the HC, fluxing heat and momentum poleward and reinforcing a thermally direct cell in the same sense as the HC. When the SST distribution becomes globally uniform, the traditional midlatitude Rossby waves are trapped near the surface as the mean flow inhibits their upward propagation. But, near the tropopause, baroclinicity generates waves that ride on a sharp upper tropospheric potential vorticity gradient. These waves propagate downwards towards the lower equatorial troposphere and transport angular momentum out of the tropics. Together with a dominant MJO-like mode, which facilitates near-equatorial convergence, this leads to a conventional tropical overturning circulation. As the SST gradient weakens, the HC moves from a regime intermediate to thermally and eddy-driven to one that's strongly influenced by eddies. Moreover, the thermal structure of the troposphere becomes uniform with weak gradients, and for flat SSTs, the tropopause in the midlatitudes is also set by convection. A Transformed Eulerian Mean perspective is consistent with this view and highlights the diabatic nature of the midlatitude circulation in the limit of flat sea surface temperatures.
How to cite: Thakur, A. B. S., Sukhatme, J., and Harnik, N.: Investigating the Hadley Cell and eddies with varying sea surface temperature gradients, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-459, https://doi.org/10.5194/egusphere-egu23-459, 2023.
Posters virtual: Mon, 24 Apr, 16:15–18:00 | vHall AS
Wind-driven dust emission from dry, exposed land surfaces plays an important role in the climate system, and also contributes to severe weather and public health hazards around the world. In the past several years, the Northern Hemisphere midlatitude region was stuck by several extreme dust storms with severe socioeconomic and environment consequences within and beyond the dryland source areas. For instance, the 26-27 May 2018 salt storm from the dried-up Aral Sea was considered a first-of-its-kind ecological catastrophe over Central Asia. In March 2021, northern China was hit by the worst sand storm in a decade. Later in November, Uzbekistan recorded the worst dust storm through the country’s meteorological record. Currently, significant knowledge and methodological gaps exist in characterizing the multivariate compound dust events. This study is a first attempt to develop a multivariate approach and ground-based climatology to improve our knowledge of the historical variations, spatial distributions, and governing factors of extreme dust outbreaks over the drylands of Central and East Asia. Detailed case studies will also be conducted to elucidate the role of tropic Pacific and Arctic warming and Rossby wave activities in triggering recent extreme dust events.
How to cite: Xi, X.: A first look at the multivariate extreme dust outbreak over Northern Hemisphere midlatitudes, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10425, https://doi.org/10.5194/egusphere-egu23-10425, 2023.
