Recent extreme weather and climate episodes, like heavy rainfall events leading to flash floods, recurrent and concurrent summer heatwaves or unforeseen winter cold spells, highlight the need to improve our understanding of jet streams and of the associated linear and non-linear, planetary and synoptic-scale Rossby wave dynamics in the atmosphere to better constrain the impacts of Rossby waves and of atmospheric blocking on extreme weather and climate events.
Abstracts are invited on a wide range of topics, with a focus on, but not limited to, the following areas:
(1) Theoretical developments in the dry and moist dynamics of Rossby waves, wave breaking, atmospheric blocking, and of jet streams acting as atmospheric Rossby waveguides. This includes the role of local and remote drivers (e.g., the tropics, Arctic, or stratosphere) in affecting Rossby wave evolution.
(2) Linkages between extreme weather/climate events and the jet stream, as well as the associated linear and non-linear Rossby wave evolution during such events, including wave breaking, cut-off formation and re-absorption, and atmospheric blocking.
(3) Application of cutting-edge methods to study the multi-scale interaction of Rossby waves from the convective scale to the large-scale dynamics, and its representation in existing weather and climate models (e.g. hierarchical and/or high-resolution modelling, machine learning/AI-based approaches).
(4) Exploring the effect of Rossby wave packets on predictability at lead times from medium range (~2 weeks) to seasonal time-scales. This includes the potential role of blocking and of teleconnections involving Rossby wave propagation.
(5) Projected future changes in planetary or synoptic-scale Rossby waves, or in their future connection to weather and climate events.
The mid-latitude jet streams play a defining role in shaping regional weather and climate, making it crucial to understand their current state as well as future changes under anthropogenic forcing. While model uncertainties have reduced over time, significant spread in projections still exists. The problem is exacerbated by a multitude of different jet stream drivers whose influence varies with season and region. This talk will discuss some work in trying to constrain future jet projections and give an overview of regional and seasonal characteristics of jet streams and their drivers. It will further discuss potential new avenues for establishing meaningful physical relationships within the high-dimensional frameworks of jet streams and drivers to better understand regional impacts.
How to cite: Breul, P.: Seasonal and regional jet stream changes, their drivers, and how to connect them., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6800, https://doi.org/10.5194/egusphere-egu26-6800, 2026.
The subtropical jet dominates over specific longitudinal sectors during both winters. The major source of this zonal asymmetry is localized tropical convection. In particular, during austral winter, the wide and powerful convection over the Asian monsoon region and Maritime Continent drives a subtropical jet over the Indian Ocean, Australia and the west and central Pacific. Further downstream in the east Pacific the jet tilts poleward, gradually shifting towards eddy-driven jet characteristics, while in the Atlantic sector only an eddy-driven jet prevails.
In this study, we show that the upper tropospheric circulation pattern over the whole Southern Hemisphere during winter is similar to that in an idealized model simulation, where the only zonal asymmetry source is localized tropical convection in the summer hemisphere. A similar momentum budget is found for the observations and model simulation. The first-order momentum balance is the geostrophic balance associated with a stationary Rossby wave driven by tropical convection. The upstream part of the subtropical jet (the Indian Ocean jet) is associated with a high equatorward of it, and the downstream part (the Pacific jet) is associated with a low poleward of it. This demonstrates that the subtropical jet zonally asymmetric component is a manifestation of a stationary Rossby wave in the upper troposphere. The second-order momentum balance is associated with approximate absolute angular momentum conservation in the localized Hadley cell, as is the dominant balance in zonally symmetric models. The third-order momentum balance is between meridional advection of absolute angular momentum and zonal momentum advection. Transient eddy momentum fluxes are negligible in the maintenance of the subtropical jet zonal structure.
How to cite: Lachmy, O. and White, I.: The maintenance of a zonally asymmetric subtropical jet, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2944, https://doi.org/10.5194/egusphere-egu26-2944, 2026.
Synoptic systems are understood to organize heat and momentum transport along jet streams, yet the diagnostics used to identify jets remain fundamentally Eulerian in nature. This creates conceptual tension: if the eddy-driven jet can be meaningfully separated from the synoptic eddies that maintain it, then it must be a persistent flow that Eulerian diagnostics are not designed to isolate. An alternative Lagrangian perspective on jet streams (JetLag) was recently developed and identifies jets not as maxima of wind speed (or derivative variables), but as maxima of isentropic displacement. In this view, jets become persistent features that remain identifiable over synoptic timescales. This definition recovers well-known features of the atmospheric circulation, with some systematic differences relative to Eulerian diagnostics. Here we adopt the Lagrangian definition to revisit jets and their variability using a hierarchy of models, ranging from idealized configurations to reanalyses. We explore the connections between synoptic systems and jets, and those between the upper troposphere and the surface.
How to cite: Rivoire, L., Kaspi, Y., Tamarin-Brodsky, T., and Hadas, O.: A Lagrangian perspective on jet streams, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14802, https://doi.org/10.5194/egusphere-egu26-14802, 2026.
This study investigates the propagation of negative potential vorticity (PV) anomalies in idealized shallow-water simulations, with particular emphasis on how their evolution is governed by the structure and latitude of the jet. The initial conditions of the experiments constitute a zonally symmetric midlatitude jet representing a Rossby waveguide, and an isolated, axisymmetric negative PV vortex representing upper-level ridges and diabatically generated outflows associated with warm conveyor belts (WCBs).
The experiments provide a first systematic demonstration that vortex propagation is governed by the combined effects of intrinsic Rossby-wave propagation and advection by the jet, with the relative importance of these processes determined by the latitude of vortex initialization relative to the jet. Importantly, the resulting propagation behavior is not symmetric about the position of the vortex relative to the jet axis.
These results also provide a direct dynamical analogue for the behavior of WCB outflows across different interaction types with the Rossby waveguide in the real atmosphere. In particular, vortices initiated close to the jet core or slightly equatorward correspond to no-interaction WCB outflows, which exhibit rapid advection and equatorward displacement. The ridge-interaction outflows, characterized by relatively weaker advection, are represented by vortices initialized on the poleward flank of the jet. In contrast, anomalies initialized farther poleward of the jet, with minimal direct influence from the westerlies and quasi-stationary behavior, correspond to blocking and cutoff interactions of WCB outflows.
The structure of the jet is equally important: variations in jet strength in the idealized simulations modulate the degree of eastward advection of the vortices, while changes in jet width and latitude primarily shift the spatial extent of the jet’s influence; in all cases, vortex behavior is governed by its relative position with respect to the Rossby waveguide.
How to cite: Selvakumar, V., Sprenger, M., Joos, H., and Wernli, H.: Idealized shallow-water simulations of potential vorticity perturbations in zonal jet-waveguides and links to observed dynamical processes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7081, https://doi.org/10.5194/egusphere-egu26-7081, 2026.
We investigate the development of ensemble forecast uncertianty associated with jet stream perturbations and dynamics. We partition uncertainty growth into diabatic and dynamic processes. A case study focusses on the recent Fujiwara-style interaction of Hurricanes Humberto and Imelda , and their subsequent interactions with the jet stream. These are seen to be able to perturb the jet and inject considerable uncertainty via diabatic processes. Later, dynamical processes along the jet (such as the development of cut-of features) act to further magnify uncertainty. The result for Europe was Storm Amy, which caused significant damage and some loss of life, but which was not well predicted. Through further experimentation, we try to understand the key diabatic and dynamical processes, how they combine to govern operational predictive skill, and their sensitivity to model resolution.
How to cite: Rodwell, M., Tsiringakis, A., Gray, S., Methven, J., and Wood, D.: Perturbation and uncertainty growth along the jet stream: the role of tropical cyclones, jet stream dynamics, and sensitivity to resolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19201, https://doi.org/10.5194/egusphere-egu26-19201, 2026.
Atmospheric Rossby waves are a fundamental component of large-scale circulation and low-frequency atmospheric variability. In classical theory, quasi-stationary planetary waves are characterized by infinite periods and are typically regarded as slowly varying background disturbances, which limits their ability to explain the widespread intraseasonal oscillations (ISOs) observed in the atmosphere. Given that ISOs share comparable spatial and temporal scales with planetary waves, a nonstationary Rossby waves framework provides a promising theoretical basis for interpreting their propagation characteristics.
In this study, we develop a theoretical framework for nonstationary horizontally propagating Rossby waves embedded in a prescribed background flow. We systematically derive the necessary conditions for the existence of three propagating solution branches, expressed equivalently in terms of the supremum and infimum of phase speed and wave period. Both the phase-speed and period supremum and infimum are determined by the background wind field, while the supremum and infimum of the period additionally depend on the zonal wavenumber. Two distinct regimes of admissible phase-speed and period ranges emerge, reflecting different background-flow configurations.
By combining these theoretical constraints with atmospheric reanalysis data, we diagnose the climatological supremum and infimum of nonstationary Rossby wave speriods in both the upper and lower troposphere over key tropical regions. The results reveal pronounced seasonal and regional variations in the theoretical period ranges due to differences in background circulation between tropospheric layers. In the upper troposphere, the equatorial Indian–western Pacific region does not support eastward-propagating solutions, whereas in the lower troposphere, eastward-propagating nonstationary waves with intraseasonal periods become possible under monsoonal flow conditions, consistent with monsoon ISO characteristics. During boreal winter and spring, the theoretical period supremum and infimum of lower-tropospheric nonstationary waves over the equatorial Indian–western Pacific exhibit Madden–Julian Oscillation (MJO)-like features. Over the equatorial Atlantic, vertically asymmetric background flows lead to distinct propagation characteristics between the upper and lower troposphere, consistent with observed ISO structures.
This work extends the classical theory of Rossby waves propagation by incorporating nonstationary waves and provides a unified theoretical interpretation linking nonstationary planetary waves to tropical intraseasonal variability.
How to cite: Liu, Y. and Li, J.: The theory and climatological characteristics of nonstationary horizontally Rossby waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4697, https://doi.org/10.5194/egusphere-egu26-4697, 2026.
The climatological quasi-stationary waves (QSW) amplitude has a distinct spatial pattern, with clear zonal asymmetries, particularly in the Northern Hemisphere; those asymmetries must be impacted by stationary forcings such as land, topography, and sea surface temperatures (SSTs). To investigate the effects of stationary forcings on QSW characteristics, including their duration and spatial distribution, we conducted eight CAM6 simulations with prescribed SSTs, spanning realistic, semi-realistic, and fully idealized configurations. Stationary forcings tend to extend the duration of QSWs and strongly impact their zonal asymmetric distribution. QSWs are primarily influenced by both the local stationary wavenumber Ks, which depends on jet speed and its second-order meridional gradient, and by the strength of transient eddies. However, the covariation between transient eddies and QSWs varies across different types of stationary forcings. For example, in experiment pairs showing the impact of zonal SST patterns, the correlation between changes in QSW strength and transient eddies is stronger, while the correlation with stationary wavenumber is of similar magnitude across all experiments. In some cases, QSW strength is also associated with the strength of the stationary waves. When the timescale of the QSWs is changed, the relative contributions from different mechanisms changes, but stationary wavenumber Ks and transient eddies strength are important in all time scales for experiments with realistic land. This work suggests that transient Rossby waves with given wavenumbers can become stationary under background conditions with the corresponding stationary wavenumbers.
How to cite: Fei, C. and White, R.: The Role of Topography, Land and Sea Surface Temperatures on Quasi-Stationary Waves in Northern Hemisphere Winter: Insights from CAM6 Simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16052, https://doi.org/10.5194/egusphere-egu26-16052, 2026.
Rossby wave packets (RWPs) organize large-scale energy transport in the atmosphere. The significance of this energy transport for atmospheric predictability and teleconnections has long been recognized. We here focus on RWPs along the midlatitude jet, which have received much attention as predictable precursors to high-impact weather events. RWPs are frequently considered as physical entities identified by the Rossby-wave envelope. From this perspective, RWPs appear as features on a scale larger than that of the underlying troughs and ridges. In particular, a long-standing hypothesis by Lee and Held (1993) states that "the packet envelope should be more predictable than the individual weather systems, because the packet can remain coherent despite chaotic internal dynamics". Testing this hypothesis with ERA5 re-forecasts, we find that the RWP envelope does not exhibit this hypothesized higher predictability, at least when compared to the pattern of the underlying Rossby waves themselves, and until the end of the available lead time range of 10 days. This statistical result is substantiated by the examination of the underlying error-growth mechanisms. We will further provide a dynamics-based explanation of the counterintuitive result that the (seemingly) larger-scale envelope feature does not exhibit higher predictability. We conclude the presentation with a discussion of the role of the envelope perspective for predictability questions beyond the medium range.
How to cite: Riemer, M. and Gölz, L.: Do Rossby wave packet envelopes exhibit enhanced predictability?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19251, https://doi.org/10.5194/egusphere-egu26-19251, 2026.
The occurrence and magnitude of extreme events have been linked to quasi-stationary waves (QSW). However, the response of QSWs to climate change is uncertain. Here, we gain insight into the forced QSW response by looking at internal variability in QSW activity. The Rossby wave spectra is highly influenced by the location and strength of the background jet stream. It is known that the poleward shift of the jets in response to external forcing resembles internal variability in the jet such as the Southern Annular Mode. Although open questions remain on the driving mechanisms of these jet responses, we can identify common changes in the Rossby wave spectra within internal variability and the climate change response.
Using the daily meridional velocity from the Community Earth System Model 2 Large Ensemble, we calculate a space-time spectral decomposition over the midlatitudes, revealing changes in the wavenumber-phase speed structure of synoptic Rossby waves. We investigate the climate change response of the spectra and use maximum covariance analysis between the spectra and the vertically integrated zonal wind to find co-varying patterns of internal variability. Under the SSP3-7.0 scenario in the Southern Hemisphere, we observe a polewards shift of the jet, faster jet speeds, and a corresponding shift of the spectra perpendicular to the barotropic Rossby wave dispersion relationship. This results in a decrease in power in higher wavenumbers and an increase in lower wavenumbers across all phase speeds, including quasi-stationary ones, corresponding to a decrease in stationarity (i.e. wave power with near-zero phase speed). We find this relationship holds on monthly timescales and in response to climate change. The response in the Northern Hemisphere is more complex and differs between the Atlantic and Pacific basin. Our results provide a simple explanation for the wavenumber-dependent changes in Rossby waves and the reduced stationarity of QSWs in response to climate change, which have implications for future changes in weather extremes.
How to cite: Xuan, Z., Riboldi, J., and Jnglin Wills, R.: Linking jet stream and Rossby wave spectra changes within internal variability and climate change responses, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20138, https://doi.org/10.5194/egusphere-egu26-20138, 2026.
Building upon the established Rossby wave ray tracing framework, we introduce a phase tracing approach, derived from two-dimensional spherical Rossby wave theory on a horizontally non-uniform basic flow, to explicitly diagnose the evolution of wave crests and troughs along stationary Rossby wave rays.
The method is first applied to a series of idealized basic flows and validated against forced solutions from a barotropic model, with a particular emphasis on contrasting flows with and without a mean meridional wind. The theoretical phase tracing accurately reproduces both the ray pathways and the spatial structure of the simulated responses, in agreement with the theoretical prediction that local zonal and meridional wave scales are primarily controlled by the background flow rather than by the forcing scale. Importantly, the inclusion of a mean meridional flow emerges as a key dynamical ingredient: it not only permits one-way propagation of stationary Rossby waves across tropical easterlies, but also substantially enlarges both zonal and meridional wave scales, with the zonal scale becoming dominant, thereby shaping zonally elongated wave-train structures.
The framework is further applied to climatological summertime flows to investigate the structure of the Pacific–Japan (PJ) teleconnection. In the lower troposphere, northward-propagating Rossby waves embedded in the monsoonal southwesterly exhibit a characteristic ‘− / + / −’ phase pattern, while in the upper troposphere the phase evolution of southeastward- and southwestward-propagating Rossby waves displays a complementary ‘+ / − / +’ structure. The phase transition points along the rays are found to coincide closely with the centers of positive and negative vorticity anomalies, providing a clear dynamical explanation for the formation of the zonally elongated tripolar structure of the PJ teleconnection.
In addition, the Li–Yang wave ray flux (WRF) is employed to quantify the intensity of wave propagation along the diagnosed ray pathways, offering a complementary measure of wave activity during propagation.
Together, the phase tracing framework and wave ray flux diagnostics enable a precise and physically constrained diagnosis of atmospheric teleconnection patterns, and hold broad applicability for understanding the structure and variability of Rossby wave–mediated teleconnections in a realistic, non-uniform background flow.
How to cite: Zhao, S., Yang, Y., and Li, J.: Rossby wave phase tracing and its application to the structure of the Pacific–Japan teleconnection, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8490, https://doi.org/10.5194/egusphere-egu26-8490, 2026.
Pacific-origin atmospheric teleconnections play a central role in shaping Northern Hemisphere summer circulation, yet their downstream expression over the North Atlantic–European sector varies substantially across models. Here, we assess the robustness, structure, and background-state dependence of these teleconnections using CMIP6 large ensembles together with idealized SST-perturbation experiments from the Decadal Climate Prediction Project (DCPP-C). The study focuses on Rossby Wave Sources (RWS) over the northeastern Pacific and the resulting wavetrain that propagates across North America, the Atlantic, and Eurasia during boreal summer.
All large ensembles reproduce a coherent circumglobal Rossby wave train associated with enhanced RWS in the northeastern Pacific. However, the degree of agreement deteriorates downstream, with the largest spread occurring over the North Atlantic and Europe. Model differences in upper-tropospheric jet strength and meridional position strongly modulate the phasing and amplitude of the wave train in this region. Models with small jet biases compared to the ERA5 reanalysis maintain a realistic sequence of alternating geopotential height anomalies, while stronger or latitudinally displaced jets distort or shift the European node of the teleconnection.
Idealized DCPP-C experiments reveal that the Pacific-Atlantic interaction is strongly state-dependent. Simulations with intensified RWS (negative IPV phase) produce a PDO-like surface cooling pattern in the northeastern Pacific and a robust cooling response in the North Atlantic, confirming a direct trans-basin link. Atlantic SST anomalies further modulate the downstream atmospheric response: a warm Atlantic suppresses the Pacific–Europe teleconnection, while a cold Atlantic allows for a strengthened and more coherent wave train. Additional experiments combining AMV and IPV phases demonstrate that the Pacific signal can be either reinforced or damped depending on the Atlantic background state.
These results highlight the joint role of northeastern Pacific RWS variability, upper-level jet biases, and Atlantic SST state in shaping the structure and persistence of Pacific-to-Europe summer teleconnections. Improving the representation of these elements is essential to reduce inter-model spread and enhance confidence in simulated boreal-summer circulation patterns.
How to cite: Fuentes-Franco, R., Lockwood, J. F., Dunstone, N., Scaife, A., and Koenigk, T.: Dynamical Controls on Pacific-Origin Rossby Wave Propagation Across the North Atlantic–European Sector, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10101, https://doi.org/10.5194/egusphere-egu26-10101, 2026.
How to cite: Barnes, M. A., Reeder, M. J., and Ndarana, T.: The evolution of cyclonic and anticyclonic Rossby wave breaking morphologies and their importance in extremes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2487, https://doi.org/10.5194/egusphere-egu26-2487, 2026.
Forecast busts over Europe—periods of abnormally low predictive skill—are often associated with extreme weather events and linked to misrepresented upper-level dynamics, including latent heating from mesoscale convective systems (MCSs), Rossby wave breaking, and warm conveyor belt (WCB) outflow. This study investigates how explicitly resolving mesoscale processes affects the simulation of these key mechanisms in global ICON ensemble forecasts at grid spacings ranging from 40 km down to 2.5 km. As a test case, we analyze a forecast bust from ECMWF’s Integrated Forecasting System (IFS) related to the development of Storm Dennis (February 2020), the second-most intense North Atlantic winter storm of the past 150 years, and compare ICON with IFS.
We find a systematic improvement in forecast skill with finer grid spacing. Coarse-resolution simulations reproduce the forecast bust and fail to capture the correct trough–ridge pattern, while convection-permitting simulations more accurately represent upper-level potential vorticity anomalies, WCB structure, and cyclone development.
Our analysis reveals a multi-stage chain of error growth arising from several interacting factors. Large initial-condition uncertainties over the North Pacific provide a background sensitivity, but the strongest early error growth occurs over the central United States, coinciding with a period of deep convection from MCSs. Convection-permitting simulations produce stronger and more coherent MCSs, leading to enhanced negative PV injection near 250 hPa and substantially reduced Rossby wave activity errors. In contrast, coarser-resolution simulations exhibit weaker or misplaced MCSs, resulting in larger errors in the upper-tropospheric flow. These midlatitude convective differences subsequently modulate the intensity and orientation of downstream WCBs over the North Atlantic. The WCB then amplifies the pre-existing errors, linking the central-U.S. convective phase to the eventual European forecast bust.
Overall, our results demonstrate that mesoscale processes over North America—especially MCS-driven PV perturbations—play a key role in setting the predictability of the North Atlantic flow regime during Storm Dennis. Convection-permitting global simulations improve the representation of these processes and offer a physically consistent pathway toward reducing forecast busts in high-impact weather situations. To assess the robustness and generality of these findings, additional case studies are currently being analyzed.
How to cite: Rixen, M., Prein, A., Pothapakula, P., Sprenger, M., and Zeman, C.: Resolution Sensitivity of Rossby Wave Breaking and Warm Conveyor Belts in Global ICON Simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2689, https://doi.org/10.5194/egusphere-egu26-2689, 2026.
Atmospheric blocking, conventionally studied as a quasi-stationary phenomenon, often exhibits zonal movement under the influence of factors like the background flow and retrograding Rossby waves. However, the impact of this mobility on cold extremes remains under-investigated. This study classifies atmospheric blocking events during the winters of 1979/80–2020/21 into westward-moving, eastward-moving, and quasi-stationary types to analyze their distinct impacts on surface air temperature by region.
Our results show that westward-moving blocks occurred most frequently over the western North Pacific, whereas quasi-stationary blocks were dominant in most other regions. In terms of duration, westward-moving blocks consistently persisted longer than the other types across all regions. Notably, these long-lasting, westward-moving events were closely associated with inducing strong cold waves in downstream areas during their dissipation phase. This is attributed to the enhanced advection of cold Arctic air by blocking-induced low-level wind anomalies. These characteristics were successfully reproduced in CESM1-LENS simulations, suggesting that a better understanding of blocking mobility can contribute to improving extreme cold surge prediction.
How to cite: Kim, S.-H. and Kim, B.-M.: Characterizing Blocking Mobility and Its Role in Northern Hemisphere Cold Extremes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2300, https://doi.org/10.5194/egusphere-egu26-2300, 2026.
Despite the profound influence of Eurasian blocking on the East Asian winter monsoon, its objective detection remains a challenge due to a systematic under-detection in standard algorithms. The widely adopted Hybrid method (HYB) applies a hemispheric constant threshold for anomaly detection prior to the flow reversal criterion. This constrained design neglects the lower geopotential height variability characteristic of the Eurasian continent, resulting in the premature filtering of meteorologically significant events. Here, we propose the Regional Hybrid method (RHYB), a refined framework that incorporates anomaly thresholds tailored to local geopotential height variance. By reconciling detection criteria with regional physical characteristics, RHYB explicitly captures "reversal-dominated" systems—events with clear flow disruption but modest amplitude—that were previously obscured. Using ERA5 reanalysis, we demonstrate that these newly identified events are robust drivers of severe wintertime cold surges over East Asia, indicating that their prior omission has led to a significant underestimation of regional climate risks. These results underscore that RHYB is an essential tool for accurately diagnosing midlatitude extremes and their evolving dynamics in a warming world.
How to cite: Kim, B.-M., Noh, H., Ku, H.-Y., and Sung, M.-K.: Uncovering Missing Eurasian Blocking Events and Their Robust Role in East Asian Winter Extremes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6324, https://doi.org/10.5194/egusphere-egu26-6324, 2026.
Atmospheric Rossby waves exert a strong control on the emerging pattern of summer heat and winter cold over the Arabian Peninsula, yet their regional impacts remain poorly quantified. This study uses 25 years (2000–2024) of reanalysis and observational data to assess how upper-tropospheric Rossby wave activity modulates seasonal 2 m temperature extremes over Saudi Arabia and how these responses are embedded in large-scale teleconnections linked to ENSO and Indo-Pacific variability. The analysis focuses on the evolution of warm-core structures in summer, the spatial spread of winter cold anomalies, and two recent extreme years, 2017 and 2023, that reveal the sensitivity of the Peninsula to Rossby wave regime shifts.
Results show a progressive amplification and spatial expansion of August near-surface temperatures across Saudi Arabia, with the 37–38 °C isotherms migrating northward and westward after 2010 to form a quasi-continuous warm core spanning the eastern lowlands, Rub al Khali, and central plateau. The fraction of land exceeding 39 °C in August increased from isolated spots in the early 2000s to over 20% after 2015, signifying a step-like intensification of summertime heat. Composite analyses indicate that these hot cores coincide with upper-level anticyclonic ridges and subsidence maxima, consistent with Rossby wave–induced adiabatic warming and suppressed convection.
Within this long-term warming context, 2017 stands out as a dynamical outlier. Amplified and breaking Rossby waves over the Middle East generated a quasi-stationary ridge over the Peninsula, producing exceptionally broad August heat with mean temperatures above 38 °C across central and northeastern regions. In winter 2017, enhanced wave activity drove deep trough intrusions and widespread sub‑16 °C anomalies, yielding an unusual combination of extreme summer heat and pronounced winter cooling within one year. A renewed Rossby forcing episode in 2023 accompanied one of the hottest summers on record, when the southeastern warm core intensified and spread northwestward while winter again featured strong meridional temperature gradients and broad cold coverage.
Wave activity flux diagnostics and teleconnection analyses reveal that both 2017 and 2023 extremes arose from Indo-Pacific–Eurasian Rossby wave trains. In 2017, La Niña–like conditions and a positive Indian Ocean Dipole excited a Eurasian wave train that channelled energy along the subtropical jet, reinforcing anticyclonic ridging in summer and deep winter troughs. In 2023, an ENSO phase transition under neutral IOD conditions triggered renewed Rossby dispersion from the tropical western Pacific into the Asian jet, again focusing anomalous ridging and subsidence over the Peninsula.
These results suggest that modest upstream anomalies now yield amplified regional thermal responses, implying increased dynamical gain due to background warming and altered land–atmosphere coupling. The findings point to a Rossby wave–dominated regime shift since 2017, wherein upper-level wave geometry and teleconnections increasingly control the extent of summer heat and winter cold. Saudi Arabia thus emerges as a dynamically sensitive node in the global Rossby waveguide system.
How to cite: Albert, J., Navaz, M. F., Saleem, A. A., Chaitanya Akurathi, V. S., Lateef, S., Shafeeque, M., and Alhems, L.: Seasonal Rossby Wave Dynamics Driving Winter and Summer Temperature Extremes in the Arabian Peninsula, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2241, https://doi.org/10.5194/egusphere-egu26-2241, 2026.
There is increasing interest within the community in the mechanisms behind the development of concurrent heatwaves, i.e., heatwaves that occur simultaneously in geographically remote regions. This interest is motivated by their socio-economic implications and by the fact that they are occurring more frequently with global warming.
While the large-scale atmospheric dynamical drivers of concurrent heatwaves have often been emphasized, with a focus on quasi-stationary wave patterns favoring the formation of blockings, particularly in Summer, the thermodynamic drivers have so far received less attention, despite the recognized role of moisture and latent heat transport for the development of blockings, especially in Winter.
Here, we relate extremes in hemispheric meridional heat transport (MHT) to occurrences of hemispheric land-surface temperature (LST) warm and cold extremes. We find that the combination of extremely weak MHT and extremely warm hemispheric LST days occurs significantly more often than other combinations, and that these events are associated with a substantial amount of concurrent heatwaves in the Northern Hemisphere mid-latitudes, both in boreal Winter and Summer. We highlight that, in Summer, the phase and amplitude of high-latitude blockings associated with these occurrences lead to vanishing, and sometimes even equatorward, overall MHT, together with an intensification of the Pacific branch of the jet stream. In Winter, MHT is largely suppressed by an excessively zonal flow, bringing mild and moist air towards continental regions, both in Eurasia and North America. The reversal or suppression of zonal wavenumber-2 and -3 contributions to MHT is found to be related to these MHT extremes, pointing towards the predominant role of ultra-long planetary-scale waves.
How to cite: Lembo, V., Messori, G., Faranda, D., Galfi, V. M., Graversen, R. G., and Pons, F. E.: Concurrent heat waves and their linkage to large-scale meridional heat transports through planetary-scale waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19340, https://doi.org/10.5194/egusphere-egu26-19340, 2026.
Since 2010, European heatwaves have dramatically escalated in both duration and severity. The cumulative intensity of European heatwaves has surged by over 50% in the recent decade. Recent studies have reported accelerating Arctic warming and associated mid-latitude circulation changes. However, its summer impacts remain uncertain. Here we provide evidence that the recent summer changes in the Arctic play a critical role in the escalation of European heatwaves. The Arctic has experienced unprecedented regional changes with substantial sea-ice loss since 2010. The Barents-Kara Seas have warmed by 2.3 °C per decade, while western Greenland has cooled by 0.6 °C per decade. The temperature changes in these two regions influenced European weather through two different pathways: 1) Barents-Kara Sea warming weakened daily weather activities over western Eurasia, thereby promoting persistently hot weather; 2) Greenland cooling shifted the North Atlantic jet stream, which allowed easy invasion of warm flows from the subtropics and Sahara. These pathways have intensified concurrently since 2010, which likely exacerbates heatwave risks in Europe.
How to cite: Noh, E., Kim, J., Kosaka, Y., Yeh, S.-W., Son, S.-W., Jun, S.-Y., and Moon, W.: European Heatwave Exacerbated by Summer Arctic Changes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20715, https://doi.org/10.5194/egusphere-egu26-20715, 2026.
During the last years, the statistical analysis of compound extremes has gained increasing interest among the scientific community due to the multiple threats posed by such events to society, economy, and the environment. In many situations, this analysis is based on bivariate extreme value theory and measures provided by this framework. Such methods may however not properly address two relevant aspects: the non-zero duration of extreme events (which can be rather persistent, e.g. in the case of droughts or heatwaves, heavily violating the independence assumption of classical extreme value theory) and the fact that not all events of practical relevance can actually be described as cases falling into the tails of the continuous distribution of some observable of interest.
A versatile approach addressing the non-extremeness aspect is event coincidence analysis (ECA), which quantifies the empirical frequency of co-occurring events of arbitrary types and allows its comparison with the values for certain random null models like independent Poisson processes with prescribed event rates. While standard ECA builds upon the concept of temporal point processes and hence may be criticized for not applying to persistent events, a new methodological variant called interval coverage analysis (InCA) provides a straightforward generalization specifically addressing co-occurrence properties of persistent events. To highlight the broad range of potential applications of ECA and InCA in the context of compound event studies, we study two examples of co-occurrences between specific atmospheric circulation configurations and different types of surface extremes.
Example 1 highlights the instantaneous as well as time-lagged co-occurrence between boreal summer Northern hemispheric jet stream configurations with two distinct zonal wind maxima (“double jet”) and atmospheric heat waves. The presented results demonstrate that double jet conditions over certain sectors are closely linked with a statistically significant enhancement or suppression of heatwave activity in distinct regions, resembling the spatial patterns of atmospheric wave trains. These patterns provide a useful starting point for further targeted research to reveal the underlying atmospheric circulation mechanisms and their association with other spatially compounding extreme events and impacts.
Example 2 subsequently addresses the co-occurrence of subtropical ridges and atmospheric blockings with precipitation patterns in the Southern hemisphere. The obtained results indicate that the presence of ridges in specific sectors is commonly accompanied by a suppression of precipitation within these sectors, while surrounding regions may exhibit characteristic spatial clusters of significantly elevated probability of precipitation.
This work has been partially supported via the JPI Climate/JPI Oceans NextG-Climate Science project ROADMAP and the bilateral German-Portuguese science exchange project EXCECIF (jointly funded by DAAD and FCT).
How to cite: Diedrich, D., Lima, M., Trigo, R., Russo, A., Di Capua, G., Bishnoi, G., and Donner, R. V.: A simple statistical approach for establishing dynamical linkages between specific atmospheric circulation patterns and spatially compounding persistent extremes and impacts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16728, https://doi.org/10.5194/egusphere-egu26-16728, 2026.
Cold air outbreaks (CAOs), characterized by the southward intrusion of high-latitude cold air into the midlatitudes, often cause severe weather phenomena such as extreme cold waves and heavy snowfall during winter months. This study investigates the critical role of a CAO in a record-breaking heavy snowfall event over the Korean peninsula in November 2024. During the event, the accumulated snowfall was recorded over 43 cm across the central region of the Korean Peninsula for about 3 days, causing severe socioeconomic disasters.
Two days prior to the heavy snowfall event, an upper-level cut-off low generated over eastern Siberia propagated southward, inducing an extreme CAO over the northern Peninsula. The cut-off low enhanced an upper-level frontogenesis with tropopause folding, which transported cold and dry air downward and formed a barotropic cold dome over the region. Concurrently, the Yellow Sea located west of the Korean Peninsula exhibited anomalous high sea surface temperatures, which created an intense air-sea temperature contrast exceeding 17°C. The resulting sensible and latent heat fluxes triggered meso-scale convection, which persistently intruded into the central region of the Korean Peninsula along the southern boundary of the cold dome. It is known that CAO is often accompanied by atmospheric blocking linked to upper-level Rossby wave breaking. In this event, Kamchatka blocking prevented the upper-level cut-off low from propagating eastward and maintained it in a quasi-stationary state during about 3 days. Consequently, the unexpected CAO enhanced by quasi-stationary cut-off low and the persistent snowstorms by lake-effect resulted in the record-breaking heavy snowfall over the Korean Peninsula during early winter.
Our findings demonstrate that upper-level atmospheric circulation patterns, which have received little attention in previous studies, can play a crucial role in heavy snowfall events over the Korean Peninsula.
Key words: Heavy snowfall, Cold air outbreak, cut-off low, air-sea contrast, blocking
This work was funded by the Korea Meteorological Administration Research and Development Program under Grant (RS-2023-00240346)
How to cite: Oh, Y., Baek, E., and Kim, J.: A role of cold air outbreak in an early winter heavy snowfall event over the Korean Peninsula, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16164, https://doi.org/10.5194/egusphere-egu26-16164, 2026.
Atmospheric waveguides can affect the propagation of Rossby waves, and have been hypothesized to be associated with amplified quasi-stationary waves and thus to extreme weather events in the mid-latitudes. Here, we compare different methods of calculating temporally and spatially varying waveguides, including different ways of separating the waveguides (background flow) from waves, and show that upstream PV waveguides are often present in the days prior to heatwaves. We compare waveguides from potential vorticity (PV) gradients (“PV waveguides”) with barotropic waveguides based on what is known as the stationary wavenumber, or KS (“KS waveguides”). Composites of days with high waveguide strength over particular regions show distinct differences between the two waveguide definitions. Strong KS waveguides in many regions are associated with a double-jet structure, consistent with previous research; this structure is rarely present for strong PV waveguides. The presence of high geopotential heights occurs with the double-jet anomaly, consistent with atmospheric blocking creating the KS waveguide conditions through the influence on local zonal winds, highlighting that this methodology does not sufficiently separate non-linear perturbations (i.e. blocking) from the waveguides, or background flow. Significant positive correlations exist between local waveguide strength and the amplitude of quasi-stationary waves; these correlations are stronger and more widespread for PV waveguides than for KS waveguides, and they are strongest when the rolling-zonalization background flow method is used. We caution against using KS waveguides on temporally and/or zonally varying scales and recommend rolling-zonalization PV waveguides for the study of waveguides and their connections to quasi-stationary atmospheric waves. Using PV waveguides, we find strong connections with heatwaves, with enhanced waveguides upstream from 1-6 days prior to heatwave days.
How to cite: White, R. and Admasu, L. M.: Atmospheric waveguides, quasi-stationary waves, and temperature extremes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-23268, https://doi.org/10.5194/egusphere-egu26-23268, 2026.
Winter precipitation over the Tibetan Plateau (TP) and the European Alps exhibits pronounced interannual to decadal variability, yet the stability of their large-scale linkage and the associated dynamical and moisture-related processes remain incompletely understood. Using multiple observational datasets and ERA5 reanalysis for the period 1940–2018, this study examines the decadal evolution of the TP–Alps winter precipitation relationship and its connections with atmospheric circulation and moisture transport.
The results indicate that the relationship between winter precipitation over the two regions undergoes a marked decadal transition, with contrasting behavior before and after the late twentieth century. During the earlier period, precipitation variability over the TP and the Alps displays a coherent out-of-phase structure, whereas this relationship becomes substantially weaker in subsequent decades.
Further analyses suggest that these changes are associated with variability in large-scale climate modes linked to tropical sea surface temperature anomalies and midlatitude atmospheric circulation. Regression analyses of upper-tropospheric circulation reveal organized Rossby wave responses over Eurasia, while the corresponding wave activity flux pathways exhibit pronounced decadal dependence, indicating changes in the background circulation structure. Consistent with these circulation variations, regressions of whole-column integrated vapor transport (IVT) show notable decadal differences in the strength and pathways of moisture transport toward the TP and the Alps, with implications for regional moisture convergence.
Overall, this study highlights the importance of large-scale circulation variability and moisture transport in shaping the decadal evolution of winter precipitation linkages over Eurasia, providing a broader context for understanding long-term hydroclimate variability across distant mountainous regions.
How to cite: Qie, J. and Wang, Y.: Decadal changes in the teleconnection of winter precipitation across Eurasian mountainous regions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11470, https://doi.org/10.5194/egusphere-egu26-11470, 2026.
Extreme midlatitude weather is often associated with pronounced Rossby waves. This has motivated interest in how the ‘waviness’ of the atmosphere is changing as Earth warms. Multiple summary metrics have been used to assess midlatitude waviness, which include both descriptions of the magnitudes of associated anomalies in geopotential height, and geometric measures of deviations of the jet from a more zonal state.
Recent work illustrated that a) these metrics can respond differently to warming, and that the same metric can respond differently to warming applied in different ways (Geen et al. 2023), and b) that different metrics can link to rather different patterns of extreme temperature (Roocroft et al. 2025). It remains unclear what specific types of characteristic jet structures these various metrics capture, and how these dynamically link to surface weather extremes.
Here, we first explore how different metrics relate to extreme winter weather events (cold, rain and wind) over Europe and North America, and how these relationships compare to known modes of climate variability such as the NAO. Next, to explore underlying jet structures driving these extremes, we apply a Self Organising Maps analysis to 500-hPa geopotential height anomalies. This allows us to map the values taken by different metrics and the likelihoods of extreme events for different jet configurations in a reduced dimensionality space.
References
Geen, R., Thomson, S. I., Screen, J. A., Blackport, R., Lewis, N. T., Mudhar, R., ... & Vallis, G. K. (2023). An explanation for the metric dependence of the midlatitude jet‐waviness change in response to polar warming. Geophysical Research Letters, 50(21), e2023GL105132.
Roocroft, E., White, R. H., & Radić, V. (2025). Linking atmospheric waviness to extreme temperatures across the Northern Hemisphere: Comparison of different waviness metrics. Journal of Geophysical Research: Atmospheres, 130(20), e2024JD042631.
How to cite: Geen, R., Jones, M., Riggs, R., and Cao, Y.: Jet regimes, waviness metrics, and links to extreme weather, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19586, https://doi.org/10.5194/egusphere-egu26-19586, 2026.
Pressure vertical velocity (ω = Dp/Dt) is commonly approximated from the geometric vertical velocity (w = Dz/Dt) as ω ≈ -ρgw, which invokes the hydrostatic relation ∂p/∂z ≈ -ρg together with the additional assumption that local pressure tendency and horizontal pressure advection term are negligible at planetary and synoptic scales. Using global nonhydrostatic simulations with the ICON model, we show that the horizontal pressure advection term can be relatively large compared with the vertical pressure advection term at planetary-to-synoptic scales in regions of strong jets such as in the winter stratosphere, contradicting the conventional assumption ω ≈ -ρgw. We further show that the horizontal and vertical pressure advection terms exhibit a predominantly out-of-phase structure and that their comparable amplitudes lead to substantial cancellation. As a consequence, ω can be suppressed or amplified at large scales relative to the -ρgw diagnostic, despite the validity of the hydrostatic balance. Scale diagnostics indicate that the large-scale enhancement of the horizontal pressure advection arises from interactions between the mean flow and eddies. From an energetic perspective, these advection terms correspond to compensating contributions of pressure-gradient work in different directions. Consequently, ω behaves more like the net pressure gradient work, rather than a direct measure of vertical motion.
How to cite: Chen, J., Vasylkevych, S., Žagar, N., and Hohenegger, C.: On the interpretation of the pressure vertical velocity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2413, https://doi.org/10.5194/egusphere-egu26-2413, 2026.
In December 14 to 16, 2023, East Asia experienced a severe cold wave, with record-breaking low temperatures and consequently severe natural disasters over broad areas. Results suggest that anomalous downward potential vorticity circulation (PVC) forcing across the tropopause played a critical role in triggering and amplifying this event. The results indicated that in early December, a strong positive potential vorticity substance (PVS) reservoir accompanied by an anomalous downward PVC persisted in the lower stratosphere over Siberia, whereas two distinct upper tropospheric fronts (UTFs) were located over East Asia. By December 12, as the downward PVC penetrated the tropopause into the troposphere, enhancing the northern UTF and triggering a perturbation trough at its western end. This northern trough propagated faster eastward along the UTF than its southern counterpart, and its PVS was intensified by the descending northerly flow. As the two UTFs merged on the eastern Tibetan Plateau, the northern trough was phase-locked with the southern trough, forming a deep East Asian trough with a well-developed PVS. The prominent cold descending northerly flow dominated the troposphere behind the trough, generating extremely high surface pressure and abnormal cold temperature advection below. Consequently, a severe cold wave swept over East Asia. This study improves upon previous work by directly linking tropopause PVC forcing to trough phase-locking, a previously overlooked pathway for cold wave amplification.
How to cite: Li, Y., Wu, G., Liu, Y., He, B., Mao, J., and Sheng, C.: The Influence of Tropopause Potential Vorticity Circulation Forcing on the Development of the East Asian Cold Wave in December 2023, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2492, https://doi.org/10.5194/egusphere-egu26-2492, 2026.
Arctic amplification has been linked to significant changes in mid-latitude weather patterns, including the increasing frequency and persistence of extreme weather events. This study investigates the influence of Barents-Kara (BK) sea-ice variability on wintertime Ural blocking and its role in Eurasian cold temperature anomalies. Using ERA5 reanalysis data, we analyse Ural blocking frequency and persistence based on two commonly used blocking indices (an absolute geopotential height reversal index and an anomaly-based index method). The relationships between BK sea ice, Ural blocking, and Eurasian surface temperature are examined within a causal network framework, accounting for ENSO as a potential common driver by including it as a covariate and by stratifying the analysis by ENSO phase. We find that Ural blocking events occur more frequently and persist longer during winters with reduced BK sea ice. Although, results are sensitive to blocking index but remain qualitatively consistent and robust across indices. Composite analyses show a characteristic warm-Arctic/cold-Eurasia temperature pattern during Ural blocking events, which is amplified during winters with low BK sea ice and La Niña conditions. To assess whether Ural blocking is influenced by specific Arctic background conditions, we further classify winters into Deep and Shallow Arctic warming regimes over the Barents-Kara region. We find that Ural blocking occurs more frequently and is more persistent under Deep Arctic warming states, leading to a stronger cold-Eurasia temperature response compared to Shallow warming regimes. By statistically quantifying the relationships between Arctic sea ice, Ural blocking, and Eurasian temperature variability, this work advances the understanding of Arctic-midlatitude interactions.
Keywords: Arctic Amplification, Ural blocking, Barents-Kara sea ice, ENSO, Blocking indices, Blocking frequency and persistence, Eurasian cold winters.
How to cite: Agyemang-Oko, E. and Kretschmer, M.: Quantifying the influence of Barents-Kara sea ice loss on Ural blocking, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3038, https://doi.org/10.5194/egusphere-egu26-3038, 2026.
Rossby wave breaking (RWB) is a key process through which synoptic-scale eddies reorganize the extratropical circulation, interacting with jet shifts, storm track variability, and the persistence of weather regimes. Despite extensive evidence that diabatic heating strongly influences synoptic eddies and supports blocking, its influence on when and how Rossby waves break remains largely unexplored. This gap limits our physical understanding of how moist processes reshape the potential vorticity structure that governs RWB and, in turn, the large-scale circulation.
We investigate the influence of diabatic processes on RWB using aquaplanet simulations at 100, 20, and 2.5 km horizontal resolution, which systematically alter the representation of diabatic heating. By comparing RWB frequency, geometry, and life cycles across resolutions, we isolate how the resolution-dependent representation of diabatic heating shapes RWB and the RWB-mediated circulation response, including jet latitude and storm track position. These idealized results are complemented by an observational analysis of RWB events and associated warm conveyor belts in ERA5 reanalyses.
Together, these analyses provide new physical insight into how diabatic processes modulate RWB and thereby shape the extratropical circulation, with implications for the interpretation of resolution-dependent circulation biases and the representation of moist processes in weather and climate models.
How to cite: Federer, M., Bukenberger, M., and Tamarin-Brodsky, T.: The role of diabatic heating in Rossby wave breaking, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5587, https://doi.org/10.5194/egusphere-egu26-5587, 2026.
Atmospheric blocking is a quasi-stationary high-pressure circulation pattern that disrupts the midlatitude westerlies and is closely linked to high-impact weather extremes. Blocking detection, however, is highly method-dependent, often producing divergent blocking climatologies. This uncertainty also affects future projections, because climate-models frequently underestimate blocking relative to observations, limiting reliable assessments of blocking-related extremes. To address these challenges, we propose an objective deep learning–based framework for blocking detection that can be applied consistently across reanalysis datasets and climate model simulations.
We frame blocking detection as identifying spatial patterns in 2D atmospheric fields, analogous to semantic image segmentation, and employ a U-Net architecture to produce daily blocking masks. A two-stage training strategy is adopted: the network is first pre-trained using labels from the standard Hybrid Index (HYB; Dunn-Sigouin et al. 2013) across all seasons and then fine-tuned with a regionally modified variant, the Regional Hybrid Index (RHYB), using boreal-winter data. This strategy allows the model to incorporate regional dependence in background variability while retraining the broad blocking characteristics learned from HYB.
Although fine-tuning is restricted to boreal winter, the trained model generalizes to boreal summer and detect additional blocking events relative to HYB. When applied to the CESM2 Large Ensemble (LESN2), the framework mitigates the tendency of traditional indices to under-detect blocking frequency. Overall, this approach offers a more objective and transferable detection method that may improve the consistency of blocking diagnostics and support more reliable evaluations of blocking-related extremes in climate-model simulations.
How to cite: Noh, H., Park, H.-J., Kim, J.-H., Kim, B.-M., Kang, D., and Sung, M.-K.: U-Net-based Objective Detection of Atmospheric Blocking , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8842, https://doi.org/10.5194/egusphere-egu26-8842, 2026.
Atmospheric blockings are among the most frequently studied weather patterns. They not only cause extreme weather events and associated losses but also significantly influence general weather variability. A deeper understanding and more reliable prediction of these phenomena would therefore be of great value to both the scientific community and the public.
However, various definitions and identification methods for atmospheric blockings are currently applied, which can lead to inconsistent results and confusion. While all approaches are valid and justified, the precise differences between these definitions and their implications often remain unclear.
This study examines two widely used blocking algorithms: the Anomaly Index, which is based on vertically integrated potential vorticity (PV) anomalies (see Schwierz et al., 2004), and the Absolute Index, which identifies blockings through the reversal of the 500 hPa geopotential height gradient (see Davini et al., 2012).
The two indices differ substantially already with regard to climatological blocking frequencies: the Anomaly Index primarily detects blockings south of Greenland/Iceland, whereas the Absolute Index identifies a local maximum over southern Scandinavia. Our analyses have not indicated any systematic longitudinal, latitudinal, or temporal offset between the events captured by the two indices. A synoptic investigation suggests that the algorithms detect different types of blockings: the Absolute Index requires a Rossby wave breaking for identification, while the Anomaly Index considers an extended ridge sufficient.
Further research aims to clarify the differences in dynamical and synoptic conditions between these and other algorithms.
How to cite: Ruff, L. and Pfahl, S.: Comparison of Different Blocking Indices and Analysis of Underlying Dynamics and Synoptic Situations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12607, https://doi.org/10.5194/egusphere-egu26-12607, 2026.
This study investigates atmospheric blocking from the perspective of the instantaneous stationarity of the jet stream. The framework of the quasi-stationary state (QS) dynamical theory is applied to characterize the behavior of ensemble prediction members. Using the Japanese Reanalysis for Three Quarters of a Century (JRA-3Q), we classified atmospheric conditions over the Northern Hemisphere into states characterized by small and large temporal variability in jet stream tendency, referred to as QS and Non-QS respectively, and examined the relationship between the former and blocking patterns.
During QS conditions, the westerlies exhibited significant meandering, and blocking occurred regardless of the blocking type (Omega or Dipole). These results are consistent with blocking defined by potential vorticity reversal at the dynamical tropopause and its persistence.
Based on linearized equations, a relationship is identified between QS and the non-stationary minimum point (MP), where at least one of its eigenvalues is zero. Analysis of forecast data from JMA's Global Ensemble Prediction System (GEPS) revealed that ensemble spread tends to increase with forecast time when the initial state is QS. This result is consistent with the proposed dynamics. Conversely, under a Non-QS initial state, initial uncertainty persists throughout forecast evolution.
These findings suggest that atmospheric blocking is a manifestation of the instantaneous stationarity of the jet stream, indicating that this theoretical framework is valuable for examining the predictability of blocking and interpreting ensemble forecasts.
How to cite: Nomura, S. and Enomoto, T.: Dynamical linkage between blocking predictability and jet stream quasi-stationary states, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20078, https://doi.org/10.5194/egusphere-egu26-20078, 2026.
Severe heatwaves have become increasingly frequent over the Indian subcontinent in recent decades. This study found that the increase in extreme heatwaves is related to a significant decadal change in surface temperatures over the Indian subcontinent, and revealed that the increase in convective activity in the Philippine Sea plays a crucial role in this decadal change in surface temperature. Specifically, the surface temperature over the Indian subcontinent in spring has increased significantly by approximately 0.64 ◦C in recent years (1998–2022: post-1998) compared to the past (1959–1997: pre-1998), leading to more intense and frequent heatwaves, particularly in March and April. The difference in atmospheric changes between these two periods shows that the enhancement of convective activity over the Philippine Sea drives an anomalous elongated anticyclonic circulation over the Indian subcontinent. This circulation pattern, marked by clearer skies and increased incident solar radiation, significantly contributes to the heat extremes in the Indian subcontinent. Additionally, stationary wave model experiments demonstrate that local diabatic heating over the Philippine Sea is significantly linked to robust spring Indian heatwaves through the Matsuno–Gill response.
How to cite: Ok, J., Song, E.-J., Yang, S., Kim, B.-M., and Kim, K.-Y.: Interdecadal changes and the role of Philippine Sea convection in the intensification of Indian spring heatwaves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4663, https://doi.org/10.5194/egusphere-egu26-4663, 2026.
Over the last decade, North American cold extreme events have exhibited a notable shift in timing, occurring more frequently in February rather than earlier in winter. This delayed-season tendency suggests a strong influence from intraseasonal climate variability. In addition we identify a pronounced warming trend in sea surface temperature (SST) over the equatorial Pacific warm pool region, with the warming signal becoming particularly distinct during the most recent decade. We examine a dynamical linkage between the Madden-Julian Oscillation (MJO) and cold extremes over the North America in late-winter. As the equatorial Pacific warm pool region shows a warming trend, the eastward propagation speed of the MJO tends to slow, resulting in increased residence time and a higher occurrence frequency of MJO phase 7 during February for a recent decade. Under these conditions, persistent convection over the equatorial western Pacific enhances diabatic heating and strengthens tropical thermal forcing. This sustained forcing excites Rossby wave responses, facilitating downstream wave propagation into the central North America region. The resulting MJO teleconnections favor the development of large-scale flow patterns conducive to cold extremes over North America, thereby increasing the likelihood of February cold waves.
How to cite: Kim, M., Kim, H., and Sung, M.: MJO modulation on the cold extreme over the North America in a recent decade, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6203, https://doi.org/10.5194/egusphere-egu26-6203, 2026.
During April-May 2024, South China experienced an unprecedented extreme precipitation event, leading to substantial socioeconomic losses and human casualties. The primary driver of this event was an exceptionally strong moisture convergence linked to a local low-level horizontal trough. This trough was passively induced by two meridionally-oriented anomalous anticyclones located over the tropical western North Pacific and Northeast Asia. The tropical anticyclone facilitated the advection of abundant moisture towards southern China, while the Northeast Asian anticyclone impeded northward moisture export, jointly resulting in the observed extreme precipitation. The tropical anticyclone represents a typical Kelvin wave response to convection anomalies over the tropical Indian Ocean, which were forced by localized positive sea surface temperature (SST) anomalies. In contrast, the Northeast Asian anticyclone was a node of a mid-to-high latitude barotropic Rossby wave train. This Rossby wave train, initiated by the tropical Atlantic convection, was guided towards Northeast Asia by a transient eddy-driven polar front jet. Although the European Centre for Medium-Range Weather Forecasts showed high skill in predicting tropical Atlantic and Indian Ocean SST and associated convection anomalies, its ability to predict the April-May 2024 South China precipitation extreme was limited, primarily owing to difficulties in accurately predicting the strength of polar front jet. Overall, this study highlights the critical role of extratropical mean flow in modulating climate extremes that are responsive to tropical forcing.
How to cite: Liu, X. and Zhu, Z.: A manipulator of the extreme precipitation in South China behind the tropical sea surface temperature: the polar front jet, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6369, https://doi.org/10.5194/egusphere-egu26-6369, 2026.
The occurrence of extreme weather has recently been associated with the mechanism of Rossby wave resonance along a circumglobal jet. Resonance is possible to the extent that the jet acts as a zonal waveguide. Recently, a method was introduced to diagnose this mechanism in the framework of the linear barotropic model through numerically solving a judiciously designed model configuration. In that method, any wave activity leaving the jet region is dissipated in sponges and, hence, discarded from further consideration.
The present work goes a step further by explicitly accounting for polar and singular waveguides, which occur through wave reflection off the pole or off a critical level. In the absence of damping, these reflective boundaries generate additional resonant cavities and allow higher meridional modes to participate in the resonance. These higher meridional modes imply resonance at multiple zonal wavenumbers, in stark contrast with the earlier results. However, when a small amount of damping is included, any wave activity is strongly dissipated before these reflecting surfaces are encountered. Consequently, the impact of the polar and the singular waveguides vanishes, and the resonant behavior reduces to that from the original diagnostic. It is concluded that the impact of reflecting surfaces beyond the jet region proper is unlikely to be of practical importance for diagnosing the Rossby wave resonance along a circumglobal midlatitude jet.
How to cite: Hempel, T. and Wirth, V.: The importance of polar and singular waveguides for the occurrence of Rossby wave resonance, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3695, https://doi.org/10.5194/egusphere-egu26-3695, 2026.
Atmospheric blocking is a key driver of persistent circulation anomalies and associated extreme events in the Southern Hemisphere, yet its characteristics around Antarctica remain poorly understood due to methodological diversity and the absence of a consolidated, long-term dataset. This contribution investigates how methodological choices in blocking detection influence the inferred characteristics of Antarctic blocking and discusses the implications for large-scale circulation variability and climate extremes. Using ERA5 reanalysis for the period 1979 to 2024, we apply several established blocking diagnostics based on geopotential height and potential vorticity within a unified spatiotemporal framework. By standardising filtering, event identification, tracking, and aggregation procedures, we isolate differences that arise specifically from the diagnostic formulation rather than from implementation details. The comparison reveals substantial method dependent variability in blocking frequency, spatial extent, persistence, and intensity, particularly at high southern latitudes where circulation regimes differ from classical midlatitude blocking. Geopotential height based diagnostics identify a broader range of quasi stationary anticyclonic anomalies, including events extending toward the Antarctic continent, while potential vorticity based diagnostics isolate fewer and more spatially confined events associated with dynamically coherent upper level disturbances near the polar vortex. These methodological contrasts have direct implications for how blocking related climate extremes are interpreted, including links to temperature anomalies, moisture intrusions, and surface melt episodes. Differences in diagnosed event duration and location can substantially alter the attribution of extreme conditions to blocking regimes. Ongoing work examines how blocking characteristics identified by different diagnostics relate to variability in large scale circulation modes such as the Southern Annular Mode and ENSO, highlighting the importance of methodological awareness when assessing teleconnections and long term variability. Overall, the results demonstrate that Antarctic atmospheric blocking cannot be fully characterised by a single diagnostic perspective and that method dependence must be explicitly considered in studies of polar circulation variability, climate extremes, and future change.
How to cite: Bozkurt, D., Opazo, C., Marín, J. C., Clem, K. R., Pohl, B., Buffet, V., Favier, V., Carrasco-Escaff, T., and Barrett, B. S.: Method dependence of Antarctic atmospheric blocking and implications for large-scale circulation and climate extremes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14616, https://doi.org/10.5194/egusphere-egu26-14616, 2026.
The Yangtze River Basin (YRB) is a critical economic and agricultural center in China, and the large summer precipitation variability here causes severe effects on social and economic. It is well known that the YRB precipitation (YRBP) is affected by multi factors, including anomalous anticyclone over the western North Pacific and local cyclone in the lower troposphere, the meridional displacement of the East Asian jet in the upper troposphere, et al. However, from the perspective of wave dynamics, influences of multi-scale Rossby waves on the intraseasonal variability of Yangtze River Basin precipitation are poorly understood. In this study, the authors used the three-dimensional multivariate circulation decomposition to quantify the multi-scale Rossby wave variability associated with the YRBP. Rossby waves with zonal wavenumber (k) being 1-20 are analyzed and categorized into planetary (k=1-3) and synoptic (k=4-20) scales, with waves of larger wavenumbers excluded due to their negligible amplitudes.
Results indicate that the planetary- and synoptic- scale Rossby waves associated with the YRBP are favorable to the precipitation by different physical processes. On the one hand, planetary-scale Rossby waves contribute to the large-scale circulation anomalies, including the anticyclone over the western North Pacific, and the zonal cyclone over East Asia in the upper troposphere, which suggests a southward displacement of the East Asian jet. On the other hand, synoptic-scale Rosby waves are featured by a zonal wave train and contribute to local cyclonic anomalies in the lower troposphere to enhance the YRBP.
Further lead-lag regression analysis is on-going.
How to cite: Chen, P., Lu, R., Wu, L., Žagar, N., and Lunkeit, F.: Influences of planetary- and synoptic-scale Rossby waves on the intraseasonal variability of Yangtze River Basin precipitation in summer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15695, https://doi.org/10.5194/egusphere-egu26-15695, 2026.
Despite the ongoing global warming trend, winter temperature variability, particularly the recurrence of cold extremes across Eurasia and North America, has drawn considerable attention. These widespread anomalies suggest potentially coherent temperature variations between the two continents. Previous studies have identified the Asian–Bering–North American (ABNA) teleconnection as a key contributor to such in-phase winter temperature variations. The ABNA is characterized by a zonally elongated “negative–positive–negative” (or “positive–negative–positive”) geopotential height anomaly pattern extending across northern Asia, eastern Siberia–Alaska, and eastern North America. The ABNA is independent of, and often more dominant than, that of the ENSO-related Pacific–North American (PNA) pattern, explaining a larger portion of winter temperature variability over eastern North America. Our analysis reveals that the ABNA is intrinsically linked to the second leading mode of tropospheric thickness (a proxy for mean tropospheric temperature) variability in the Northern Hemisphere, while the first mode reflects Arctic warming. This finding positions the ABNA as a fundamental mode characterizing Eurasia–North America winter temperature co-variability. Further results show that the ABNA is modulated by both the Arctic stratospheric polar vortex (SPV) and tropical western Pacific SST anomalies. The ABNA pattern is dynamically coupled with a meridionally stretched SPV structure extending toward Eurasia and North America, forming a tropospheric bridge between the stratosphere and surface climate. This stratosphere–troposphere coupling may be initiated by Eurasian snow cover anomalies in the preceding autumn. In addition, tropical western Pacific SST anomalies can excite a poleward-propagating Rossby wave train that reinforces the ABNA pattern, in a manner comparable to but distinct from the ENSO–PNA connection. These findings highlight the ABNA as a critical and underappreciated pathway for winter climate variability and offer new sources of predictability for subseasonal-to-seasonal temperature forecasts across the Northern Hemisphere, particularly in eastern North America.
How to cite: Zhong, W. and Wu, Z.: The Asian–Bering–North American Teleconnection: A Key Mode of Winter Temperature Co-Variability Across Eurasia and North America, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15892, https://doi.org/10.5194/egusphere-egu26-15892, 2026.
The response of upper-tropospheric jet streams to warming effects is a pivotal uncertainty in current climate projections. This study provides a rigorous diagnostic analysis of the spatio-temporal variability and seasonal evolution of jet stream characteristics over North America (NA) and the North Pacific Ocean (NPO) during the four-decade period of 1984-2023. Utilizing high-resolution ERA5 and NCEP/NCAR reanalysis datasets, we analyzed the three-dimensional structure of jet cores and their interaction with localized baroclinic environments.
Our diagnostics reveal two distinct centers of action where jet dynamics are significantly perturbed: the North Pacific Ocean (NPO) and the Eastern portion of North America (EPNA). A systematic poleward migration of the jet axes approximately 10 degrees in latitude is identified across all seasons except summer, concurrent with a persistent altitudinal ascent. Seasonal analysis indicates that trajectory instability reaches its maximum during summer in the NPO, whereas the most pronounced variability in EPNA occurs during the autumn months. Notably, our results establish a significant positive trend in zonal wind speeds, ranging from 0.5 to 1.5 m/s per decade, which is closely coupled with enhanced meridional temperature gradients in the mid-to-upper troposphere.
Furthermore, wavelet power spectrum analysis across multiple pressure levels (100-400 hPa) uncovers dominant multi-annual periodicities of 5, 7, and 10 years, suggesting robust modulation by large-scale climatic oscillations. A critical finding is the divergent altitudinal behavior between the two regions: while NPO jet streams exhibit an upward trend with stabilized flow, winter and autumn jet streams over EPNA demonstrate a significant downward intrusion into the lower troposphere. This vertical shift facilitates intensified moisture advection from the Gulf of Mexico, potentially exacerbating the frequency and magnitude of extreme hydrological events, such as atmospheric rivers, in northeastern Canada. These findings underscore the non-uniform regional response of the global circulation to a warming atmosphere and provide a framework for improving regional climate predictability.
How to cite: Salimi, S. and Ouarda, T. B. M. J.: Decadal Evolution of Mid-latitude Jet Stream Dynamics: Spatio-temporal Trends and Seasonal Oscillations over North America and the North Pacific Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21982, https://doi.org/10.5194/egusphere-egu26-21982, 2026.
