Stationary Rossby waves can counteract the eastward drift of westerly wind and persist over a region for extended periods. This prolonged influence over a fixed region makes it highly conducive to the occurrence of extreme event. The propagation of stationary Rossby wave is significantly influenced by the configuration of mean flow. The jet stream, characterized by a narrow region in the atmosphere with high zonal wind speed, is particularly favorable for the propagation of stationary Rossby wave. The jet stream affects the propagation of stationary Rossby wave primarily through the strong lateral wind shear, so can be seen as a barotropic waveguide. In a three-dimensional basic state, a waveguide can also form without the presence of strong lateral wind shear. Instead, it can arise due to significant variations in stratification, forming a baroclinic waveguide. The baroclinic waveguide is particularly pronounced during summertime over northern Eurasia, resulting in nearly twice as much stationary Rossby wave activity at high latitudes compared to that at middle latitudes. These stationary Rossby waves are believed to be directly responsible for the occurrence of extreme heatwaves in Europe, such as those experienced in 2010 and 2019, as well as the Northern China heatwave in 2023. In this talk, I will introduce the characteristics and dynamics of these stationary Rossby waves in the baroclinic waveguide. In addition, I will also discuss the predictability of these stationary Rossby waves in current operational numerical models. It will be shown that these stationary Rossby waves exhibit a zonally-oriented spatial structure along the baroclinic waveguide. The evolution of these waves is primarily determined by the nonlinear interactions with transient eddies. This strong nonlinearity poses a significant challenge for current operational numerical models in accurately predicting them.
How to cite: Xu, P.: Stationary Rossby waves in a baroclinic atmospheric waveguide , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5000, https://doi.org/10.5194/egusphere-egu25-5000, 2025.
Various regions in the Northern Hemispheric midlatitudes have seen pronounced trends in upper-atmosphere summer circulation and surface temperature extremes over recent decades. Several of these regional trends lie outside the range of historic CMIP6 model simulations, and they might constitute a joined dynamic response that is missed by climate models. Here, we examine if the regional trends in circulation are indeed part of a coherent circumglobal wave pattern. Using ERA5 reanalysis data and CMIP6 historical simulations, we find that the observed upper-atmospheric circulation trends consist of at least two separate regional signatures: a US-Atlantic and a Eurasian trend pattern. The circulation trend in these two regions can explain up to 30% of the observed regional temperature trends. The circulation trend in the CMIP6 multi-model-mean does not resemble the observed trend pattern and is much weaker overall. Some individual CMIP6 models do show a resemblance to the observed pattern in ERA5, although still weak. We show that the regional wave patterns in ERA5 resemble known teleconnection patterns, while CMIP6 models appear to lack these teleconnections. Our findings highlight the limitations of CMIP6 models in reproducing teleconnections and their associated regional imprint, creating uncertainty for regional climate projections on decadal to multi-decadal timescales.
How to cite: Happé, T., van Straaten, C., Hamed, R., D'Andrea, F., and Coumou, D.: Observed circulation trends in boreal summer linked to two spatially distinct teleconnection patterns, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15844, https://doi.org/10.5194/egusphere-egu25-15844, 2025.
With increasing global temperatures, there has been an observed increase in the quantity and intensity of extreme weather events, particularly heat extremes in the midlatitude regions. Some recent studies have attributed this increase at least partially to an amplification of upper tropospheric jet stream waves. Whilst there is significant scientific uncertainty over causes of recent trends in jet stream waviness, the impact atmospheric waves have on extreme events is clear. Therefore it is key to quantify whether the relative importance of jet stream waviness on the formation of extreme temperature events changes in the future.
We achieve this by studying the probability ratio between co-occurring high magnitude geopotential height anomalies at 500 hPa, and coincident surface temperature extremes. We calculate this for the historical period (1980-2015) and the future (2065-2100), and compare how this probability ratio - the association between atmospheric circulation and surface temperature extremes - changes between these two periods. To understand the changes seen, we also look at projected changes in the frequency of high magnitude geopotential height anomalies.
Results from three large ensembles show that cold extremes in boreal winter (December-February) exhibit a clear decrease in association between the historical to the future period, indicating that cold extremes at the end of the 21st century become less associated with strong atmospheric circulation anomalies compared to the current historical period. Conversely, hot extremes in boreal summer (June-August) exhibit small regional changes in association for the future period but hemispherically show no clear trend. We further explore the boreal winter trend in CMIP6 models, and explore mechanisms for this trend by comparing across different models with different changes.
How to cite: Roocroft, E. and White, R.: Future changes in association between atmospheric circulation anomalies and extreme temperature events, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14079, https://doi.org/10.5194/egusphere-egu25-14079, 2025.
How to cite: Dorrington, J.: A systematic exploration of the relationship between synoptic dynamics and Euro-Mediterranean extreme rainfall in a changing climate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21020, https://doi.org/10.5194/egusphere-egu25-21020, 2025.
A moist, idealized model is used to examine the driving influence of deep tropical convection on the wintertime subtropical jet. The model is run with fixed zonally symmetric sea surface temperatures under perpetual solstice conditions. To focus on the strongest convective activity, the daily data is re-centered around the longitude of maximum tropical convection. The qualitative picture that emerges suggests that deep tropical convection in the summer hemisphere drives an anomalous localized Hadley cell that crosses into the winter hemisphere and drives a locally strengthened subtropical jet downstream via advection of planetary angular momentum. Momentum fluxes associated with both the divergent overturning circulation and rotational eddies drive a local longitudinal minimum of angular momentum where the localized Hadley cell crosses the equator, thus highlighting the complexity in interpreting the angular-momentum budget due to the inherent zonally asymmetric nature of tropical convection. The results are compared with the circulation in a dry model, where a single jet inside the Ferrel cell dominates the zonal mean flow. The role of moisture in driving a subtropical jet is discussed.
How to cite: Lachmy, O., White, I., and Harnik, N.: Driving of the subtropical jet by tropical convection, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10085, https://doi.org/10.5194/egusphere-egu25-10085, 2025.
Heat extremes are exacerbated by ongoing climate change and have severe consequences on humans and the environment. Climate change also leads to changes in atmospheric circulation that affect the jet stream. One configuration of the jet stream is the double jet, where the jet splits into two branches, potentially triggering persistent weather patterns and prolonged heat extremes. We conducted a comprehensive assessment of double jet stream states over the Northern Hemisphere and their connection to heatwaves in the extended summer period May to October for 1979 to 2023, using ERA5 data. The results show an increase in double jet frequency over North America, as well as in persistence over Asia and North America. More persistent double jets are associated with higher heatwave cumulative intensity. We identified Europe as a double jet stream hotspot region, with the most pronounced connection to heatwaves. 40–80% of heatwaves co-occur with double jet events in Europe, 30–60% in Asia, and 15–50% in North America, particularly in northern regions. Northern Europe, particularly areas north of 50°N, such as Scandinavia, the British Isles, the Baltic region, and western Russia, exhibit a pronounced and statistically significant connection between double jet stream occurrences and heatwaves. We also found a significant relationship between double jet events and heatwaves in some regions of Asia, particularly between 60°N to 80°N, as well as in central China. The pronounced connection in areas northward of 50°N broadly aligns with the position of the double jet stream wind minimum, where persistent weather conditions tend to prevail. Overall, our results reveal a significant connection between double jet events and land heat extremes in the Northern Hemisphere and the shift towards more persistent double jet events, underpinning their importance for extreme weather.
How to cite: Steiner, A. K., Kriegl, M., and Pichler, M.: Double jet streams and their connection to heatwaves in the Northern Hemisphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15789, https://doi.org/10.5194/egusphere-egu25-15789, 2025.
The atmospheric circulation is often decomposed into high- and low- frequency variability. For example, the low-frequency variability in the North-Atlantic includes slowly varying weather regimes such as the North Atlantic Oscillation, with timescales of weeks. The high frequency variability includes the synoptic weather systems, which shape our daily weather fluctuations. The interaction among these timescales is often mediated by Rossby Wave Breaking (RWB) events, which involve the irreversible breaking and dissipation of the baroclinic waves. To investigate this interaction, a simple RWB recipe is derived by exploring which processes contribute to a meridional overturning of high-frequency PV contours. A picture emerges in which the slowly-varying weather regimes influence the tracks of high-frequency systems, which in turn, depending on the position relative to the low-frequency flow, determines whether the frequency of RWB (cyclonic or anticyclonic) is enhanced or suppressed. The recurrence of same-type RWB in a similar position then shapes the overall mean structure of the weather regime.
How to cite: Tamarin-Brodsky, T. and Harnik, N.: On storm tracks, weather regimes, and a wave breaking recipe, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13583, https://doi.org/10.5194/egusphere-egu25-13583, 2025.
The frequencies of thermal coral bleaching events (CBEs) over the Great Barrier Reef (GBR) continue to increase with five mass CBEs reported since 2016. While changes in the local meteorology, such as reduced wind speeds and decreased cloud cover, are known to heat the shallow reef waters, little consideration has been given to the overriding synoptic meteorology. The 2022 CBE, occurring under La Niña conditions, saw ocean temperatures at Davies Reef increase 1.9℃ over 19-days and subsequently cool 2.1℃ back to seasonal norms over eight days.
This event was found to be triggered by repeated Rossby wave breaking disrupting the local trade winds. As the trades broke down, calm winds and clear skies covered the reef, thus inhibiting the latent heat flux, allowing for the build-up of ocean heat to at least 18m. Following the re-establishment of the trade winds via coastal ridging, latent heat fluxes, the primary driver of the event, tripled allowing the ocean to rapidly cool.
Concurrent to the mass bleaching event, these are the same Rossby wave breaking events found to cause the historic Lismore flooding located hundreds of kilometres south of the GBR. This case study notes the first reported linkage between mass coral bleaching and a severe flooding event.
How to cite: Richards, L., Siems, S., Huang, Y., Zhao, W., Harrison, D., Manton, M., and Reeder, M.: The role of Rossby wave breaking in coral bleaching on the Great Barrier Reef, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1363, https://doi.org/10.5194/egusphere-egu25-1363, 2025.
2024 has been recorded as the hottest year in human history. Antarctica, already an ‘extreme’ environment, has been continuously witnessing unprecedented records in heat extremes in recent years. A record-shattering Antarctic heatwave in March 2022 highlights the importance of large-scale atmospheric circulations in the Southern Hemisphere (SH), such as atmospheric blocking. However, the presence of blocking patterns over the Antarctic continent is rarely discussed, often dismissed as 'too far south' to warrant attention.
This study examines the influence of SH blocking patterns on Antarctic heat extremes. We assess the ability of Community Earth System Model 2 (CESM2) to represent this complex relationship. Our findings suggest that SH blocking patterns exhibit elevated occurrences over the Antarctic Peninsula and East Antarctica. The future projections suggest a decline of SH blocking by 30% under SSP370 scenario by the end of the 21st century. We explored the evolving relationship between blocking and heat extremes under a warming climate and found that the future decline in blocking occurrences is disproportionate to the corresponding changes in this relationship. This underscores that the role of blocking in future heat extremes will remain significant, especially due to the poleward expansion of the subtropics.
How to cite: Shelke, P. and Renwick, J.: Does ‘Blocking’ Shape the Future of Antarctic Heat Extremes?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14224, https://doi.org/10.5194/egusphere-egu25-14224, 2025.
The most serious heavy precipitation in the past 43 years (1979-2021) occurred over Fujian-Jiangxi region from late May to early June of 2006, causing significant economic losses. Using the daily precipitation collected at 2479 surface meteorological stations in China and ERA5 reanalysis database, the present study investigates the relationship between the heavy precipitation over Fujian-Jiangxi region in late spring-early summer of 2006 and baroclinic Rossby wave packets in the upper troposphere. Information flow between the two systems has been diagnosed. Results indicate that the disturbance source for this heavy precipitation originated from areas near the Syrian Desert to the north of the Arabian Peninsula and propagated along the northwest-southeast direction, reaching Fujian-Jiangxi region four days later. This kind of baroclinic Rossby wave packets provide the necessary energy for the occurrence and persistence of heavy precipitation. Analysis of wave activity flux vectors indicates that during the heavy precipitation period, disturbance energy was transported from the upstream westerly belt to Fujian-Jiangxi region almost every day. Obviously, there existed information transfer between the two regions, re-confirming that the upstream Rossby wave packets affect the Fujian-Jiangxi precipitation. The above results provide helpful hints for a better understanding of the mechanisms for heavy precipitation in this region and will be helpful for its effective prediction.
How to cite: Ye, D., Guan, Z., Sun, S., and Jin, D.: The connection between baroclinic Rossby wave packets in the upper troposphere and regional-scale heavy precipitation over Fujian-Jiangxi region in the late spring-early summer of 2006, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-226, https://doi.org/10.5194/egusphere-egu25-226, 2025.
Since global warming, the Warm Arctic-Cold Eurasia (WACE) has experienced significant interdecadal variabilities, and its interdecadal variability has increased significantly after Arctic amplification. Temperature changes over Barents-Kara Seas region play a leading role in the interdecadal variability of WACE. Before Arctic amplification, the circulation influencing WACE was primarily characterized by the meridional circulation of the Arctic-Eurasian Dipole. After Arctic amplification, however, the circulation is mainly represented by the north-south Rossby wave trains over the Eurasian continent. Before Arctic amplification, the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO) changed in phase, stimulating the eastward propagation of Rossby wave trains along the mid-latitudes. The PDO-induced Arctic-Eurasian Dipole circulation played a leading role over Eurasian, while the AMO weakened the PDO signal in the key North Atlantic and Arctic regions. While after Arctic amplification, the AMO and PDO change in an out-of-phase relationship, with the eastward propagation of Rossby wave trains still occurring along the mid-latitudes. In this phase, the south-north Rossby wave trains excited by the AMO dominate over Eurasian, with the PDO weakening the AMO signal in the North Atlantic and enhancing the AMO signal in the critical Arctic region. Since the interdecadal variability of WACE is primarily driven by temperature changes in the key regions of Arctic, both the PDO and AMO play crucial roles in modulating the interdecadal changes in WACE before and after Arctic amplification. The two exhibit an antagonistic relationship before Arctic amplification, while their relationship becomes primarily synergistic after amplification.
How to cite: Luo, Y., Lohmann, G., Ionita, M., An, X., Sun, Y., and Li, C.: AMO and PDO modulate the multidecadal variability of WACE, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2523, https://doi.org/10.5194/egusphere-egu25-2523, 2025.
Previous studies have indicated that increased probability of extreme events in many regions of mid-latitude is linked to amplified waviness and slow upper-atmosphere circulation. However, this linkage appears to be dependent on region, and details regarding the waviness or slowness required to promote extreme events locally remain unclear. The objective of this study is to examine the upper atmospheric circulation and the linkage between the occurrence of cold extremes in different regions of the Northern Hemisphere in winter. We examine this link using the fifth-generation ECMWF reanalysis data (ERA5).
The upper atmospheric waviness—both in the vertical and the meridional direction—is computed based on geopotential height at 300 hPa. At each latitude, the vertical waviness is estimated as the circumglobal amplitude of the first five zonal wave numbers based on a Fourier decomposition. For the meridional waviness, the amplitude of each specific ridge and trough is defined as the latitudinal deviation of the isoheight taken as the zonal geopotential height mean over the region of the ridges and troughs. The speed of planetary wave zonal propagation is computed through the utilization of a top-ridge and bottom-trough tracking algorithm.
In most of the studied regions, there is a significant slowdown in the upstream ridge and downstream trough during a cold spell, confirming earlier results. For the North American cold spells, this pattern is mainly observed at high latitudes, particularly between 60° and 75° N. During cold spells over the British Isles, Europe, and Nordic countries, the speed of the ridges and troughs decreases at mid-latitude yet continues moving eastward. For cold spells over Central Asia, the ridges and troughs become significantly slower at high latitudes (60°N–80°N) but faster at lower latitudes (35°N–45°N). Contrary to our expectations, the circumglobal vertical amplitude over mid-latitudes for most regions’ cold spells exhibits less waviness. However, each local meridional wave amplitude associated with upstream ridges and downstream troughs in the vicinity of the cold spell’s location becomes significantly larger. Hence, the waviness and slow upper-atmosphere circulation associated with each region's cold extremes occur more locally than globally. Our results also indicate that amplified local meridional wave amplitude always precedes cold spells, but ridges and troughs become slower—depending on the locations of the cold spells—before or during cold spells.
How to cite: Babaei, M., Grand Graversen, R., and Patrick Stoll, J.: Linking Northern Hemisphere extreme cold weather events to upper atmospheric circulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2544, https://doi.org/10.5194/egusphere-egu25-2544, 2025.
Extremes of temperature or precipitation on intra-seasonal time scales are commonly structured in a circumglobal wave with a synoptic-scale zonal wavenumber. The mechanism that determines this wavenumber and the relationship with processes on shorter timescales, such as Rossby wave packets, are not fully understood. We employ a simple kinematic model to demonstrate how a transient Rossby wave packet produces significant temporal mean circulation anomalies with a wavelength that is larger than the wavelength of the instantaneous wave packet itself. This demonstration is verified by comparison with a linear, quasi-geostrophic channel model where we can assess sensitivities to the latitude and the meridional extent of the wave packet. These two highly idealized models help us to better understand the role of Rossby wave packets in concurrent and compound weather extremes.
How to cite: Wicker, W. and Domeisen, D.: The temporal mean of transient Rossby wave packets, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4152, https://doi.org/10.5194/egusphere-egu25-4152, 2025.
Mesoscale heating can influence the mid-latitude large-scale flow by redistributing potential
vorticity (PV) along the tropopause, impacting Rossby wave evolution. This study explores how
mesoscale heating not only perturbs the mid-latitude circulation but may also catalyze Rossby
wave breaking along the jet stream.
A forecast bust of a Rossby wave breaking event over the North Atlantic in April 2020 is
examined. The case involves a warm conveyor belt (WCB) with embedded convection and
upstream thunderstorms over North America. The case is evaluated using archived forecast data
from various weather centers and reforecasts conducted with the Model for Prediction Across
Scales (MPAS) at horizontal resolutions from 60 to 3.75 km. Potential vorticity tendencies (i.e.,
microphysics, convection, radiation) are output to diagnose multi-scale interactions.
Key findings show mesoscale heating on the jet stream's equatorward side is critical for Rossby
wave predictability. Higher-resolution simulations capture a more persistent WCB and stronger
PV reduction along the tropopause due to microphysics, amplifying the Rossby wave. In
contrast, coarser simulations failed to sustain WCB persistence, favoring cyclonic wave breaking
regardless of initial conditions.
Ensemble members with persistent mesoscale convective systems over North America were
associated with slowed Rossby wave packet progression, leading to anticyclonic wave breaking
over the Atlantic, and the most accurate forecasts. This outcome was sensitive to initial
conditions and to the persistence of the adjacent WCB.
The presented findings highlight the importance of faithfully simulating mesoscale heating
across a RWP to successfully forecast an individual Rossby wave breaking event. Implications of
these results for ongoing global convection-resolving MPAS simulations at the National Center
for Atmospheric Research are also discussed.
How to cite: Lojko, A.: The Importance of Mesoscale Heating Along a Rossby Wave Packet for the Predictability of a Rossby Wave Breaking Event, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4590, https://doi.org/10.5194/egusphere-egu25-4590, 2025.
The jet stream — the hemispheric-wide band of westerly winds that circles the mid-latitudes and shapes day-to-day weather by guiding large-scale flow features — has been a long-standing area of interest in atmospheric dynamics. Within this stream, jet streaks are regions of enhanced wind speed. These important features of atmospheric flow are frequently accompanied by clear-air-turbulence, affecting air travel flight times, comfort, and safety.
Moreover, upper level divergence in the equatorward entrance and poleward exit regions couples jet streaks to surface weather via vertical motion. This links jet streaks to rapid cyclogenesis, intense precipitation, and extreme wind events. Extreme jet streaks are also often linked to poor performance of numerical weather prediction (NWP). Understanding the dynamics of (extreme) jet streaks is hence important to further the mechanistic understanding of extreme weather events as well as error busts, and ultimately improving forecast quality.
Traditional tools, like the PV gradient and PV frontogenesis frameworks, have shed light on the dry dynamics of jet streaks. Similarly, the classical four-quadrant model explains their influence on surface weather. However, diabatic processes have been shown to play an important role in jet streak development. They are also key for the (mis)representation of jet streak in NWPs, warranting systematic and quantitative analysis.
In this study, we explore the impact of diabatic processes on jet streak evolution using composite analysis and a Lagrangian PV-gradient diagnostic. It is based on ERA5 data from North Atlantic winters (DJF) spanning 1979–2023. We begin by characterizing the life cycle of jet streaks and extreme jet streaks and their relationship with Rossby waves and Rossby wave breaking.
Our findings show that stronger jet streaks tend to last longer, with their maximum wind speeds scaling with the PV gradient at their core. Extreme jet streaks frequently coincide with intense low-pressure systems, heavy precipitation, and upstream warm conveyor belts, indicating a heightened role of diabatic processes in their evolution. Using the Lagrangian PV gradient framework, we quantify the influence of diabatic processes, comparing extreme and non-extreme jet streaks. The results reveal a clear upward interaction between surface weather and jet streak development, with diabatic effects more pronounced in extreme cases.
Our findings underscore the need for further research into individual diabatic processes driving jet streak evolution. They also add to the growing evidence that extreme jet streak events may become more frequent in a warming climate.
How to cite: Bukenberger, M., Schemm, S., Fasnacht, L., and Rüdisühli, S.: The influence of diabatic processes on North Atlantic winter jet streaks and their extremes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4858, https://doi.org/10.5194/egusphere-egu25-4858, 2025.
North American cold spells are frequently associated with the amplification of a large-scale atmospheric circulation pattern known as the Alaskan Ridge. This pattern is characterised by a persistent high over Alaska and two low-pressure centres over the Pacific Ocean and Hudson Bay. While the Alaskan Ridge (particularly the high over Alaska) has been widely discussed in the literature, a comprehensive understanding of the multiple drivers behind its amplification remains elusive. Here, we consider the dynamical drivers of the intensified high over Alaska leading to cold spells in central North America. First, we separate the cold spells based on whether they are associated with stratospheric wave reflection. This separation reveals two distinct atmospheric states resulting in upper-tropospheric high formation. Second, we employ a wave decomposition technique based on normal-mode functions to understand the role of tropospheric dynamics on different scales in favouring the Alaskan Ridge amplification. This methodology's advantage is the ability to separate Rossby and inertia-gravity regimes as opposed to the widely utilised Fourier decomposition. The focus is on planetary (zonal wavenumbers 1-3) and synoptic (zonal wavenumbers 4-8) scales. The results show enhanced synoptic-scale Rossby wave activity prior to the Alaskan Ridge amplification. Based on the shape and location of the synoptic anomalies, we attribute the enhancement to an extratropical-midlatitude interaction, a driver previously proposed in the literature. Our approach enables a holistic picture of the atmospheric evolution leading to the central North American cold spells, supporting a dynamical understanding of their origin.
How to cite: Strigunova, I. and Messori, G.: Scale-Dependent Dynamics of Alaskan Ridge Amplification: A Holistic View of Circulation Driving North American Cold Spells, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7089, https://doi.org/10.5194/egusphere-egu25-7089, 2025.
The boreal summer Circum-global teleconnection (CGT) pattern is identified as a stationary Rossby wave train propagating along the subtropical jet at interannual timescale. The variation of CGT is closely linked to the occurrence of heat extremes over mid-latitude regions. How CGT would change under global warming and the associated climatic effect remains unclear. Here, based on 34 models from Coupled Model Intercomparison Project phase 6 (CMIP6), we show evidence that the amplitude of CGT wave train will reduce robustly by 2100, with multi-model ensemble mean decrease of 31.8%. The reduction of CGT amplitude is reflected in the decrease of Rossby wave source (RWS) anomalies, with upstream signal located at jet entrance over eastern Mediterranean. The weakening of eastern Mediterranean RWS anomalies is further resulted from decreased circulation anomalies. The weakened CGT further alters the pattern of associated heat extreme events. Heat extreme days during each positive CGT event significantly increase from 5.5 days to 7.0 days at four hotspot regions across mid-latitudes: eastern Europe, Tibetan Plateau, northeastern Asia and southern Great Plains. Our findings highlight the aggravation of heat extremes induced by change of atmospheric teleconnection under global warming and additional burden on food security and ecosystems for policymakers to consider.
How to cite: Yu, H.: Weakened Circum-global Teleconnection Pattern under global warming would exacerbate heat extremes across Eurasia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8255, https://doi.org/10.5194/egusphere-egu25-8255, 2025.
Extreme extratropcial cyclones (ETC) are associated with heavy precipitation, and strong winds causing damage to infrastructure, or diverse economic losses. They can be characterised by a set of variables or diagnosis named intensity measures. Based on them, meteorologists are able to study the intricate relationship between dynamical features and impacts of ETCs. However, considerable additional research is required to improve our understanding of the relationship between ETCs' intensity and the the background state they develop in. Our baroclinic wave simulation setup implemented in OpenIFS 43r3 has shown the possibility to create stable and flexible background states able to run with moisture and full physics. Moreover, 7 parameters can be easily varied to produce a vast array of different background states. By varying these parameters, an ensemble of 6,500 baroclinic waves are simulated using OpenIFS@home, a version of OpenIFS that runs within a volunteer computing framework. In these cases, the developing ETCs are physically realistic with poleward motion, upstream and downstream development and sensible minimum mean sea level pressure.
This study proposes a Machine Learning based and systematic approach to link the 6,500 background states with their developing ETCs. Each ETC is isolated and tracked. A total of 75 features are extracted from tracked ETCs for each case. A Random Forest Regressor (RFR) is use to predict each 13 intensity measures with 5 background features. One of the properties of the RFR is its ability to rank its input features during the training. As a result, this embedded feature selection allow to quantify the strength of relationship - called feature importance. For example, the feature importance between the initial average temperature with the resulting accumulated precipitation, or the horizontal temperature gradient at 300hPa with the maximum relative vorticity at 850hPa can be estimated. The proposed methodology is able to (1) predict 13 intensity measure, (2) link them to 5 background features, and (3) reduce the training dataset by filtering the ETCs to the most intense. To stabilise the feature selection, a bootstrapping-based approach has been implemented. Using the distributed nature of the workflow, the whole ensemble of 6,500 baroclinic waves is processed within 1.5 days on 40 cores and the computational time reduces linearly with the number of cores.
With the exception of the storm severity index, the RFR is able to predict the intensity measures with a coefficient of determination between 0.65 and 0.92. Moreover, this study demonstrates an increase feature importance of the upper-troposphere baroclinicity as the training dataset is reduced to the most intense ETCs. Concurrently, the importance of the lower-tropospheric baroclinicity decreases. The feature importance of friction, initial relative humidity, initial laps rate and average surface virtual temperature stays constant. Future work will include the use of Deep Learning Regressor and wrapped feature selection in order to validate and extend the main result of this study.
How to cite: Bouvier, C., Cornér, J., Bowery, A., Carver, G., Sparrow, S., Wallom, D., and Sinclair, V.: Quantifying the relationship between extratropical cyclones' intensity measures and their background state: systematic exploration of a baroclinic wave simulation ensemble, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8345, https://doi.org/10.5194/egusphere-egu25-8345, 2025.
Record-breaking rainfall occurred coherently over subtropical West Asia (WA) and East Asia (EA) in April 2024, causing catastrophic damages around the Persian Gulf and South China. Strong barotropic cyclones are directly responsible for the long-lasting extreme rainfall over WA and EA. Based on observational analyses and numerical simulations by a linear baroclinic model (LBM), here we show evidences that these two rainfall extremes are tele-connected and are tied to the record-breaking latent heat release over tropical Indian Ocean (TIO). The record-breaking latent heat release over TIO triggers a stationary Rossby wave train propagating northward, with a barotropic anticyclone over Northern Indian Ocean and a barotropic cyclone over WA, leading to extreme WA rainfall. The intense latent heat release associated with the extreme rainfall over WA triggers another stationary Rossby wave train along the Asian subtropical jet (ASJ), with a wavelength of about 50~55 degrees in longitude. This wave train anchors a downstream barotropic cyclone anomaly on the eastern periphery of Tibetan Plateau with southerly flow from South China Sea to Eastern China, in favor of excessive rainfall over the EA region.
The above mechanism not only explains why rainfall extremes in WA and EA are located at a same latitude (20°-30°N) along the ASJ, but also clarifies why the intense rainfall over WA and EA occurred in April 2024 rather than other seasons. Spring 2024 was associated with a rapid decay of an El Niño event, and convection over TIO was suppressed by descending branch of Walker circulation before April. Along with the decay of warm SST anomaly over equatorial Pacific, TIO became warmer than Pacific in April, giving rise to intense convection over TIO which triggered the stationary Rossby waves. Although record-strong latent heating anomaly over TIO persisted from April into May in 2024, the substantially northward shifted ASJ in May cannot anchor the stationary Rossby waves in response to TIO heating, since subtropical circulation response to tropical heating is strongly dependent on the basic state flow. This work highlights the importance of both basic state and tropical heating anomaly in shaping tele-connected Asian climate extremes during the decaying phase of El Niño.
References: He C, Kucharski F (2025) Tele-connected rainfall extremes over West and East Asia in April 2024 tied to Indian Ocean heating. Accepted by Clim Dynam.
How to cite: He, C. and Kucharski, F.: Tele-connected rainfall extremes over West and East Asia in April 2024 tied to Indian Ocean heating, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8636, https://doi.org/10.5194/egusphere-egu25-8636, 2025.
This study investigates the impact of Arctic Amplification (AA) on midlatitude temperature extremes using aquaplanet simulations within the ISCA intermediate complexity modeling framework. We use a mixed-layer ocean as boundary condition and grey radiation. Simulations are run with two setups: a zonally symmetric control and a zonally asymmetric experiment. In the asymmetric experiment a localized oceanic heating is prescribed in the midlatitudes to mimic a western boundary current and generate a eddy transient kinetic energy maximum (or storm track). For each setup, we run simulations without and with AA, whereby anomalous heating is imposed in the northern polar region.
We investigate the autocorrelation of local wave activity (LWA) in our experiments, as this allows us to focus on persistent LWA regimes, which can be linked with temperature extremes such as heatwaves and cold spells. We find that the autocorrelation maxima in the asymmetric configuration correlate with well known atmospheric patterns such as atmospheric blocking, demonstrating that our model setup, despite its simplicity, can reproduce realistic features of Earth’s atmospheric circulation. Early results show how the LWA autocorrelation slightly increases with AA in the zonally symmetric setup, and decreases with AA in the zonally asymmetric setup, indicating that the sign of the change depends on the zonal symmetry. This suggests that the LWA climatology, highly sensitive to the presence of a storm track region, plays a crucial role in the atmospheric response to AA.
How to cite: Filippucci, M., Thomson, S., Lewis, N., and Bordoni, S.: The circulation response to Arctic Amplification in zonally symmetric and asymmetric aquaplanets, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11835, https://doi.org/10.5194/egusphere-egu25-11835, 2025.
Summer Greenland blocking, a persistent anticyclonic pattern, has high impacts on local and regional weather and climate, especially on triggering huge melt over the Greenland ice sheet. The phenomenon is observed to increase in intensity (based on Greenland blocking index, GB2) in recent decades. This increase is highly correlated with the negative phase of the dominant climate oscillation in the North Atlantic, namely the NAO. However, summer NAO shows different behavior in June in comparison with high summer months, i.e., July and August. In this study, we analyse the spatial patterns of Greenland blocking events in these individual months to evaluate how different they are. We use different approaches including a clustering analysis with Self-Organising Map (SOM) to evaluate individual blocking days, and an event-based analysis to assess the development of blocking events over the course of 9 days centred at the day when GB2 reach maximum value during that event. The results from both analyses show that Greenland blocking patterns are more alike between July and August, while those of June are different.
How to cite: Luu, L. N., Hanna, E., and Fettweis, X.: Intra-seasonal differences in summer blocking patterns over Greenland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13852, https://doi.org/10.5194/egusphere-egu25-13852, 2025.
In recent decades, Antarctica has experienced significant climate change, with previous studies predominantly focusing on the impact of oceanic multiscale variability on Antarctica, especially West Antarctica. However, our research reveals that Indian summer monsoon (ISM) rainfall significantly affects the austral winter (June–August) Antarctic climate and sea ice through atmospheric teleconnection. The diabatic heating of ISM rainfall causes northward movement of the Hadley cell, triggering a Rossby wave train that propagates from the southern Indian Ocean into Antarctica, which changes sea level pressure and introduces warm advection to East and West Antarctica, causing widespread warming across the Antarctic continent. Under the influence of surface wind stress and temperature advection, the sea ice in the Ross Sea-Amundsen Sea exhibits a dipole distribution, characterized by an increase in the Ross Sea and a decrease in the Amundsen Sea. Our findings have significant implications for climate change research in Antarctica, particularly East Antarctica.
How to cite: Song, Q., Wang, C., Zhang, L., and Fan, H.: Impacts of the Indian summer monsoon on the Antarctic climate and sea ice, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16688, https://doi.org/10.5194/egusphere-egu25-16688, 2025.
Two intense cold wave events impacted Japan between late December 2020 and early January 2021, motivating us to conduct a case study for the predictability of such high-impact weather. This study utilizes operational forecasts, a hybrid-machine learning weather model and ensemble adjoint sensitivity analysis to investigate the synoptic-scale mechanisms leading to these cold air outbreaks. We find that both events were preceded by distinctive cross-polar flows, which originated from cyclogenesis south of Greenland. These cyclonic systems generated cross-polar flows in addition to Rossby wave trains along the subpolar jet, efficiently transporting Arctic air masses towards the Far East. The second cold wave, occurring on January 8th, demonstrated a shorter predictability window, likely due to the weaker intensity and more compact spatial scale of the precursor storm than those of Storm Bella, highlighting the influence of storm characteristics on cold wave development and predictability. Both operational and machine learning models fail to predict from the state initialized on 28 December, implying an existence of predictability limit. Adjoint sensitivity analysis for the latter case reveals a coherent European (EU)-like pattern and a geopotential height anomaly off the east coast of Greenland two to four days prior to the spell. This study underscores the interconnectedness of storm track activity in the North Atlantic and North Pacific via the Arctic, demonstrating the influence of this trans-basin pathway on high-impact weather in East Asia. Our findings emphasize the crucial role of accurately representing these large-scale interactions for improving the predictability of extreme weather events.
How to cite: Enomoto, T., Nomura, S., and Fukushima, M.: Trans-Arctic Influence on Far East Cold Waves: A Case Study of the 2020-2021 Events, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16723, https://doi.org/10.5194/egusphere-egu25-16723, 2025.
Recent studies have highlighted the link between upper-level jet dynamics, especially the persistence of certain configurations, and extreme summer weather in Europe. Using our recently published toolbox for jet dynamics characterization, we use the various persistence metrics developed therein to find the most persistent episodes in recent data, as well as in a large ensemble containing future scenarios. We study precursors to these persistent episodes with potential for added predictability, as well as the surface weather extremes that can co-occur with these episodes.
First, we apply a jet axis detection and tracking algorithm in order to extract individual jets and classify them in the canonical categories of polar and subtropical jets. This allows us to measure the jets' instantaneous advection speed, as well as their lifetime, until they have weakened and cannot be extracted from the background wind anymore, or until they are advected out of the domain. These two metrics, advection speed and lifetime, provide measures of object persistence for each of the jets, that are, respectively, local and non-local in time.
Second, we apply the self-organizing map (SOM) clustering algorithm to the same data to create a distance-preserving, discrete, 2D phase space. The dynamics can then be described by the time series of visited SOM nodes, in which a long stay in a given node relates to a persistent state and a rapid transition between nodes that are far apart relates to a sudden dramatic shift in the configuration of upper-level flow. This allows us to quantify state persistence using the average length of a stay on a given SOM node.
Under these different views of persistence in the Euro-Atlantic sector, we establish certain jet properties as precursors for persistent episodes, study the role of Rossby wave breaking before and during these episodes, and explore the impacts of a persistent upper-level flow on surface weather and weather extremes in Europe.
How to cite: Banderier, H., Tuel, A., Woollings, T., and Martius, O.: Aspects of North Atlantic jet stream persistence and impacts on the surface weather in Europe, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19170, https://doi.org/10.5194/egusphere-egu25-19170, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 5
EGU25-16295 | ECS | Posters virtual | VPS2
Composition and Regimes of Advection Driving the Temperature Anomaly Lifecycle in Northwest India: A Machine Learning Based FrameworkTue, 29 Apr, 14:00–15:45 (CEST) | vP5.32
The PDFs of daily mean 2m temperature (T2m) in observational data have been characterized using the first three moments. For identifying the role of dynamical processes, studies focussed on the midlatitudes have analyzed temperature variability at 850 hPa, which represents the free troposphere. The observed skew could not be reproduced by linear theory of advection ([1]), but was owed to the covariance between anomalous winds and anomalous temperature ([2], [3]). Recently, frameworks have also been developed for studying the roles of different processes in driving temperature tendencies in different percentiles of temperature ([3]). However, most of the studies involving advection consider a purely meridional mixing process. Given the dynamical links between meridional and vertical advection, it is unclear if this is sufficient.
We turn focus to T2m, and consider 3D advection. We use the ERA5 reanalysis dataset to study the drivers of variability of T2m anomaly over the northwest Indian heatwave hotspot region during March and April, 1980-2022. We characterize the dry static energy (DSE) fluxes into this region, and develop a framework to identify quasilinear (QL) and nonlinear (NL) advective contributions to the temperature anomaly lifecycle.
Daily change in T2m was highly correlated with daily advection of DSE into a 600-900 hPa box over the region. Leveraging the decision tree framework to identify the dominant weather patterns explaining different terciles of advected DSE, we found that the zonal mean flow and anomalous vertical flow ([1], [2]) acted to reverse the effect of the anomalous meridional flow. Using regression, we established that an additive combination of QL terms involving these flow components served as the dominant mechanism acting throughout the distribution of net advection, with r2 > 0.65. The rest of the variability was almost entirely explained by the sum of NL terms. We saw that the NL sum acts to saturate the growth of the QL sum in its tails, supporting the observations made by [2]. Net advection peaked before the peak of the QL sum due to such a relationship, restricting the growth of net advection.
Furthermore, we study the patterns of advection in a phase space generated by the NL and QL terms. Regimes of advection were readily identified by identifying the NL terms dominating a particular region of the phase space.
We show how interpretable machine learning algorithms, like decision tree and regression, can be used to identify dominant circulation patterns and provide a mapping between magnitude of advection and eddy configurations with respect to the region of interest.
References
[1] Schneider, T., T. Bischoff, and H. P lotka, 2015: Physics of Changes in Synoptic Midlatitude Temperature Variability J. Climate, 28, 2312–2331.
[2] Garfinkel, C. I., and N. Harnik, 2017: The Non-Gaussianity and Spatial Asymmetry of Temperature Extremes Relative to the Storm Track: The Role of Horizontal Advection. J. Climate, 30, 445–464.
[3] Tamarin-Brodsky, T., K. Hodges, B. J. Hoskins, and T. G. Shepherd, 2019: A Dynamical Perspective on Atmospheric Temperature Variability and Its Response to Climate Change. J. Climate, 32, 1707–1724.
How to cite: Shah, H. and Monteiro, J.: Composition and Regimes of Advection Driving the Temperature Anomaly Lifecycle in Northwest India: A Machine Learning Based Framework , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16295, https://doi.org/10.5194/egusphere-egu25-16295, 2025.
