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, 23–28 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, 23–28 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, 23–28 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, 23–28 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, 23–28 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, 23–28 Apr 2023, EGU23-8587, https://doi.org/10.5194/egusphere-egu23-8587, 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, 23–28 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, 23–28 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, 23–28 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, 23–28 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, 23–28 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 95^th 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, 23–28 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, 23–28 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, 23–28 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, 23–28 Apr 2023, EGU23-7975, https://doi.org/10.5194/egusphere-egu23-7975, 2023.