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.

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.