The mechanism behind the early-period connection is a textbook example of an Arctic-to-tropical teleconnection. When Barents Sea ice concentration was high in May, the authors show, the overlying atmosphere responded with enhanced divergence in the upper troposphere. That divergence acted as a Rossby wave source, launching a train of planetary-scale waves that traveled a remarkable distance: across the North Pacific Ocean, over the United States and Mexico, and finally down into the Tropical North Atlantic. Along the way, the wave train modulated geopotential height anomalies, reshaping the pressure field across half the Northern Hemisphere. By the time the signal reached the tropical Atlantic, it had set the stage for changes in sea surface temperature that would ultimately influence rainfall thousands of kilometers from where the story began.

The critical link in this chain is the Tropical North Atlantic. The wave train arriving from the north favored warm sea surface temperature anomalies over that basin, and those warm waters in turn strengthened low-level moisture convergence into West Africa during the summer monsoon season. More moisture converging over the Sahel and Guinea Coast regions translated directly into enhanced monsoonal rainfall. The study thus traces a complete physical pathway from a springtime Arctic sea ice anomaly, through midlatitude atmospheric dynamics, into tropical ocean temperatures, and finally into the convective engine of the West African monsoon. Each step in the pathway is supported by observational datasets and reanalysis products, including sea ice concentration and sea surface temperature records from the Met Office Hadley Centre, atmospheric circulation fields from the NCEP/NCAR reanalysis, and the CHIRPS satellite-gauge precipitation dataset.

After 2009, however, the machinery quietly fell apart. The researchers document that Barents Sea ice variability itself diminished in the post-2009 period, which matters because a less variable forcing produces a weaker and less consistent atmospheric response. The upper-tropospheric wave activity flux that had carried the signal across the hemisphere weakened, the Rossby wave sources over the Arctic became less energetic, and the coupling between Barents sea ice and Tropical North Atlantic sea surface temperatures broke down. Without the tropical Atlantic response, the atmospheric moisture convergence over West Africa no longer received the Arctic imprint, and the influence of Barents sea ice on monsoon rainfall faded to near insignificance. The teleconnection did not simply weaken gradually; the analysis indicates a genuine interdecadal shift in the character of Arctic-tropical interactions.

The timing of the breakdown is telling. The late 2000s mark a period when the Arctic climate system was undergoing profound transformation, with accelerating sea ice decline and rapid regional warming reshaping the background state against which year-to-year anomalies play out. When the mean state changes, the sensitivity of the atmosphere to a given sea ice anomaly can change with it. The study’s results are consistent with a growing body of literature documenting nonstationary relationships between Arctic sea ice and remote climate phenomena, including documented interdecadal shifts in sea ice links to East Asian temperatures, Indian monsoon rainfall, and tropical Pacific variability. Climate teleconnections, the study underscores, are not fixed features of the atmosphere but dynamic relationships that evolve as the planet warms.

To test whether the observed mechanism is physically real rather than a statistical coincidence, the team turned to the NCAR Community Atmosphere Model version 5.3, a state-of-the-art atmospheric general circulation model. The model reasonably reproduced the observed mechanisms linking increased Barents sea ice concentration to West African monsoon variability in both the early and late periods. This agreement between observations and model experiments strengthens confidence that the Rossby wave pathway identified in the reanalysis data is a genuine dynamical pathway rather than an artifact of short observational records or confounding influences. Model confirmation of teleconnection mechanisms is particularly valuable in the Arctic-tropical context, where the signal must survive a long journey through noisy midlatitude circulation before reaching the tropics.

The implications for seasonal prediction are immediate and sobering. Forecast systems and statistical models that were calibrated on the 1981 to 2008 period, and that incorporated Barents sea ice as a predictor for West African summer rainfall, would have seen their skill degrade after 2009 without any obvious warning to their users. In a region where rainfall variability determines agricultural outcomes, water resource management, and food security for vast populations, a silent failure of a forecast predictor is not an academic concern. The authors emphasize that recognizing the dynamic, nonstationary nature of Arctic-tropical interactions is essential for improving the predictability of West African Summer Monsoon rainfall and for informing regional adaptation planning under ongoing climate change.

The study also contributes to a broader scientific debate about how Arctic change propagates into the tropics. Tropical climate responses to projected sea ice loss have been identified in model studies as significant, affecting the Hadley circulation and tropical sea surface temperature patterns, yet the observed pathways remain contested. By documenting a concrete, observationally grounded pathway that operated for decades and then failed, the new work provides a natural experiment in teleconnection fragility. It suggests that the strength of Arctic influence on the tropics depends on the amplitude of sea ice variability itself, on the efficiency of Rossby wave generation in the upper troposphere, and on the integrity of the ocean-atmosphere coupling in the tropical Atlantic. When any of these elements erodes, the entire chain of influence can collapse.

For the people of the Sahel and the wider West African region, the research arrives amid persistent challenges in understanding and predicting monsoon rainfall, which has a history of dramatic multidecadal swings including the devastating droughts of the 1970s and 1980s. The new findings add an Arctic dimension to that story, one that was strong enough to shape rainfall for nearly three decades and weak enough to vanish within a few years. As the Barents Sea continues its trajectory toward a seasonally ice-free future, the study’s central lesson will only grow in relevance: the connections that link the frozen north to the tropical rain belt are real, powerful, and perishable, and the forecasting community must continually re-examine which of them still holds. The work was supported by the National Key Research and Development Program of China and the Postgraduate Research and Practice Innovation Program of Jiangsu Province.

Subject of Research: Interdecadal changes in the teleconnection between May Barents Sea ice concentration and West African summer monsoon rainfall

Article Title: Interdecadal shifts in the May Barents sea ice-West African Summer Monsoon link after the late 2000s

Article References: Datti, A. D., Zeng, G., Chen, C., & Tao, Y. (2026). Interdecadal shifts in the May Barents sea ice-West African Summer Monsoon link after the late 2000s. Climate Dynamics, 64(11), Article 465. https://doi.org/10.1007/s00382-026-08423-6

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08423-6

Keywords: Barents Sea ice, West African monsoon, Rossby wave train, Tropical North Atlantic, teleconnections, sea ice decline, interdecadal variability, seasonal prediction, Sahel rainfall, CAM5.3, climate dynamics, Arctic warming

The team’s breakthrough was made possible by a recently developed 64-year reconstruction of hail days covering 1959 to 2022, built by combining a radar-based hail proxy with environmental predictors drawn from the ERA5 atmospheric reanalysis. Because systematic hail observations in Switzerland only begin around 2002, this reconstruction gave the researchers something they had never had before: a long, statistically homogeneous record of daily hail occurrence for two regions, one north and one south of the Alpine crest. From this record they identified the ten most and ten least active hail seasons in each region, corresponding to roughly the top and bottom fifteen percent of years, and then composited atmospheric, oceanic and land-surface conditions across the entire Northern Hemisphere for those seasons.

The results reveal that active hail seasons are not the product of a single freak weather pattern but of a recurring large-scale circulation regime. North of the Alps, the most active seasons display a zonally oriented Rossby wave train in the mid-troposphere, an alternating ridge-trough-ridge pattern of 500-hectopascal geopotential height anomalies stretching from the Atlantic into Central Europe. Crucially, the study shows using a recurrence metric that this pattern is not a single persistent anomaly. Instead, similar ridge-trough configurations are repeatedly re-established throughout the season, each episode advecting warm, moist air into northern Switzerland while upstream troughs provide the dynamical lift needed for convection.

That circulation works hand in hand with a preconditioned surface state. During active northern seasons, surface temperatures over Central Europe run well above average, specific humidity is enhanced, and the Mediterranean and Bay of Biscay show positive sea surface temperature anomalies. The combination produces boundary layers that are warm and moist but not saturated, driving up convective available potential energy, or CAPE, while leaving a moderate cap of convective inhibition, or CIN. That balance matters: a moderate cap suppresses premature, disorganized storms and allows instability to accumulate until a mesoscale trigger, such as convergence along the Prealpine foothills, releases it in the form of long-lived, organized supercells capable of growing large hailstones.

South of the Alps, the picture changes in a way that underscores how sharply the Alpine divide separates two climatic worlds. Active hail seasons in the south are tied to a meridionally oriented dipole: strong blocking over Greenland and the northwestern Atlantic, a deep trough over the British Isles, and high pressure over the Mediterranean and southern Europe. This configuration drives persistent southwesterly flow across Central Europe, advecting warm, relatively dry air from the Iberian Peninsula aloft over a moist Mediterranean boundary layer. The result is a recurrent elevated dry layer, a capping inversion that delays convection until forced ascent from fronts or the Alps erodes it, unleashing intense storms. Similar elevated mixed layers have been documented for giant-hail events in the Po Valley.

The regional contrast extends to which ingredient limits hail production. When the researchers standardized their anomalies by local variability, the largest signals north of the Alps appeared in surface temperature and sea surface temperature, marking the north as a temperature-limited regime that needs strong seasonal warming to cross the convective threshold. South of the Alps, the same thermodynamic anomalies emerge but with much weaker amplitudes, because the climatological environment there is already close to favorable for convection. Indeed, the reconstruction shows hail occurs on about twelve percent of all days south of the Alps versus roughly ten percent to the north, and modest synoptic perturbations are enough to tip the balance.

One of the study’s most striking findings comes from comparing seasonal background conditions with the environments of individual hail days. In the north, large-scale circulation, sea surface temperature and surface temperature anomalies are already present on non-hail days within active seasons, marking them as genuine seasonal preconditioning. Variables like CAPE, CIN and low-level humidity, by contrast, only become strongly anomalous on hail days themselves, indicating they characterize event-scale environments. Hail days in ordinary seasons look qualitatively like hail days in exceptional seasons; what distinguishes a hail-rich year is not a fundamentally different kind of storm but a higher recurrence of the same favorable conditions, embedded in a warmer, moister background state.

Perhaps most tantalizing are the precursors. Winters preceding active hail seasons in both regions show enhanced Eurasian snow cover, widespread continental cooling across Central Europe, Scandinavia and western Russia, and Pacific sea surface temperature anomalies resembling the positive phase of the Pacific Decadal Oscillation, with a cool eastern and central Pacific and a warm Gulf of Alaska. The authors caution that these signals may reflect co-occurrence rather than causation, and that disentangling driver from response would require targeted numerical sensitivity experiments. Even so, because sea surface temperature anomalies evolve slowly and can be predicted months ahead with reasonable skill, they offer a genuine foothold for seasonal forecasting of a hazard long considered unpredictable beyond a few days.

The study also places Swiss hail within the broader architecture of Northern Hemisphere climate variability. Correlation analysis shows that the East Atlantic and Scandinavian patterns, rather than the often-invoked North Atlantic Oscillation, exhibit the strongest and most consistent relationships with hail frequency in both regions, with generally higher correlations south of the Alps. The weak NAO signal makes physical sense: positive NAO phases suppress large-scale lifting while enhancing moisture transport, so competing dynamical and thermodynamic influences largely cancel out. Hail variability, the authors conclude, is shaped by a combination of teleconnections rather than any single dominant mode.

The implications reach well beyond academic interest. Severe convective storms were the most damaging natural hazard globally in 2023, and projections suggest both the frequency and severity of Swiss hailstorms will rise under climate change, sharpening the stakes for the insurance and agricultural sectors that absorb the losses. By identifying coherent circulation regimes, persistent surface boundary conditions and winter-season precursor signals, the Bern-led team has laid the groundwork for subseasonal to seasonal hail prediction, potentially through machine learning frameworks trained on the patterns documented here. The authors recommend extending the same analysis to other parts of Central Europe to determine which mechanisms are regionally specific and which are universal drivers of hail variability, a step that could transform how a continent braces for its most violent summer storms.

Subject of Research: Interannual variability of hail in Switzerland and its large-scale atmospheric, oceanic and land-surface drivers

Article Title: Towards understanding the interannual variability of hail in Switzerland

Article References: Wilhelm, L., Feldmann, M., Schröer, K., Schwierz, C., & Martius, O. (2026). Towards understanding the interannual variability of hail in Switzerland. Weather and Climate Dynamics, 7(3), 1681-1707. https://doi.org/10.5194/wcd-7-1681-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1681-2026

Keywords: hail, Switzerland, severe convective storms, Rossby wave train, Pacific Decadal Oscillation, Mediterranean sea surface temperature, soil moisture, convective available potential energy, seasonal prediction, ERA5 reanalysis, atmospheric blocking, teleconnections

The research team assembled seasonal hindcasts, retrospective forecasts of past winters, from eight state-of-the-art operational prediction systems. Hindcasting is the standard proving ground for seasonal prediction: models are initialized before known winters and their output is scored against observations, allowing scientists to quantify skill without the benefit of hindsight. The team evaluated how well each system reproduced the dominant mode of East Asian Winter Monsoon variability that is tied to the El Niño–Southern Oscillation, the planet’s most influential year-to-year climate fluctuation, which originates in the tropical Pacific through coupled interactions between sea surface temperatures and atmospheric convection.

The headline number from the assessment is a correlation skill of roughly 0.4 for the best-performing model. In seasonal forecasting, where the atmosphere’s own memory fades within about two weeks, any correlation above zero represents genuine predictive information, and values near 0.4 indicate that a meaningful but limited fraction of monsoon variability is captured. The spread among the eight systems was substantial, and that spread became the central puzzle of the study. If all models see the same tropical sea surface temperatures, why do some translate them into East Asian winter circulation far better than others?

The researchers’ diagnostics point to two distinct teleconnection pathways through which ENSO influences the monsoon, one in the lower troposphere and one in the upper troposphere. The lower-tropospheric pathway is the classical picture: ENSO-related heating anomalies modify the western Pacific subtropical circulation, the Philippine Sea anticyclone and the low-level winds that carry cold air southward. This pathway has long been the focus of model evaluation. The upper-tropospheric pathway, by contrast, operates roughly ten kilometers above the surface and has received far less attention in the seasonal prediction literature, despite the new evidence that it may be the decisive factor separating skillful models from unskillful ones.

The study names this upper-level feature the Southeast Asia–Japan pattern. In observations, tropical convective anomalies clustered around the Maritime Continent, the archipelago-rich region spanning Indonesia, Malaysia and surrounding seas, generate a divergence anomaly in the upper troposphere over the southeastern Tibetan Plateau. That divergence acts as a source of stationary Rossby wave activity, exciting a wave train that propagates northeastward toward Japan. The physics is rooted in the vorticity budget of the tropical upper troposphere: divergent outflow from deep convection generates rotational flow through the well-known mechanism described by Sardeshmukh and Hoskins, and the resulting wave energy travels along the westerly jet, guided by the propagation characteristics of the background flow.

When this wave train arrives over Japan, it establishes a barotropic circulation anomaly, meaning a coherent signal through a deep layer of the atmosphere, which then modulates the East Asian trough in the mid-troposphere and the meridional winds near the surface. The East Asian trough is a keystone of the winter circulation, anchoring the storm track and controlling the southward penetration of cold surges. By shifting and reshaping the trough, the upper-tropospheric wave train ultimately influences surface temperature, snowfall and cold-air outbreak frequency across East Asia. The pathway therefore links tropical convection to East Asian winter climate through a chain that runs from divergence over the Tibetan Plateau, through a northeastward-propagating wave, to midlatitude circulation and finally to surface weather.

The crucial finding is that the Southeast Asia–Japan pattern depends sensitively on the longitudinal position and amplitude of the ENSO-related tropical convective anomalies. Models that place their convective responses in the right location, with realistic amplitude, reproduce the divergence over the southeastern Tibetan Plateau and hence the observed wave train and its downstream impacts. Models in which the convective response is shifted too far eastward, or is too weak, displace the divergence source and consequently fail to reproduce the teleconnection. The wave train is not simply absent in these models; it is mislocated, and a mislocated wave train lands its circulation anomalies in the wrong place, degrading the forecast precisely where users need it most.

This diagnosis reframes the challenge of East Asian winter seasonal prediction. Traditional evaluation has emphasized the lower-tropospheric circulation, such as the subtropical high and low-level monsoon winds, as the primary conduit of ENSO influence. The new multi-model assessment demonstrates that an accurate representation of the upper-tropospheric teleconnection is at least as important, and possibly more so, for determining whether a prediction system can anticipate the monsoon’s ENSO-related mode. For model developers, the practical implication is that improving the simulation of tropical convection over the Maritime Continent, a notoriously difficult problem because convection is parameterized rather than explicitly resolved, should pay dividends far downstream over the midlatitude Pacific rim.

The study also carries a broader message about the limits and promise of seasonal forecasting. The modest skill ceiling, even among eight leading operational systems, underscores the inherent difficulty of predicting the East Asian Winter Monsoon, whose variability is shaped not only by ENSO but also by the Arctic Oscillation, Siberian snow cover, Arctic sea ice and internal atmospheric dynamics that are largely unpredictable at seasonal range. Yet by identifying the specific process, the Maritime Continent convection feeding the Southeast Asia–Japan wave train, where model errors concentrate, the research converts a vague skill deficit into a concrete target. The work was supported by the Japan Society for the Promotion of Science and the Environmental Restoration and Conservation Agency, and its diagnostics draw on established reanalysis products including JRA-3Q and precipitation data from the Global Precipitation Climatology Project. For forecasters bracing for the next severe cold surge, the path to better warnings may run ten kilometers overhead, through the divergent outflow of tropical thunderstorms and the Rossby waves they send racing toward Japan.

Subject of Research: Seasonal prediction of the East Asian Winter Monsoon and the upper- and lower-tropospheric pathways of ENSO influence

Article Title: Multi-model assessment of East Asian Winter Monsoon predictions: upper- and lower-tropospheric pathways of ENSO impacts

Article References: Sakamoto, R., Takaya, Y., Hirahara, S., Naoe, H., & Ueda, H. (2026). Multi-model assessment of East Asian Winter Monsoon predictions: upper- and lower-tropospheric pathways of ENSO impacts. Climate Dynamics, 64(10), Article 437. https://doi.org/10.1007/s00382-026-08367-x

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08367-x

Keywords: East Asian Winter Monsoon, ENSO, seasonal prediction, Rossby wave train, Maritime Continent, tropical convection, upper troposphere, Southeast Asia–Japan pattern, East Asian trough, teleconnections, Climate Dynamics, multi-model assessment

The research, led by Shanshan Fu of Nanjing University of Information Science and Technology and the Dalian Meteorological Observatory, together with Zhiwei Zhu and Xiao Pan of the Ocean University of China and the Max Planck Institute for Meteorology, analyzed observational and reanalysis data spanning 1961 to 2024. The team identified three clearly separated phases in how the tripole relates to springtime surface air temperature across the Eurasian and North African landmass. In the first phase, from 1961 to 1986, a positive tripole, meaning warmer water in the subtropical and subpolar North Atlantic flanking a cooler band in between, produced warming over North Africa, the Middle East and the Indo-China Peninsula, along with cooling over Northern Europe. Those were the days when the tripole could be counted on as a reliable seasonal clue.

Then came the middle chapter. Between 1987 and 2005, the subtropical temperature responses largely collapsed. The warming signals over North Africa and the Middle East faded to statistical insignificance, while the cooling over Northern Europe stubbornly persisted. Forecasters and researchers who had built seasonal expectations on the tripole’s influence suddenly found the connection unreliable over much of the affected domain. In the third phase, from 2006 to 2024, the story shifted again: the warming over North Africa re-emerged, but a new and significant cooling signal appeared over Northeast Asia, a region that had not featured prominently in the tripole’s earlier influence map. The pattern, in other words, did not simply strengthen or weaken. It changed its geography.

What could cause a fixed oceanic pattern to reshuffle its downstream fingerprints so dramatically? The authors point to the propagation pathways and structures of the Rossby wave trains that the tripole excites. When sea surface temperature anomalies alter the heating of the overlying atmosphere, they launch planetary-scale waves that arc across the midlatitudes like ripples guided along a curved channel. Where those waves deposit their energy determines which regions warm and which cool. If the wave train takes a more zonal track hugging the subtropics, North Africa and the Middle East feel the effects. If it arcs poleward into higher latitudes, Europe and Northeast Asia come into play. The tripole provides the push, but the atmosphere decides where the push lands.

The decisive factor, according to the study, is the background mean zonal wind, essentially the prevailing east-west flow of the atmosphere through which the waves must travel. Rossby waves do not propagate through a static medium; their trajectories, group velocities and amplitudes depend on the structure of the ambient flow, particularly the position and strength of the jet streams that act as waveguides. When the background zonal wind shifts, the wave train bends with it, and the regions of wave-activity flux convergence, where the teleconnection’s temperature anomalies are ultimately expressed, migrate accordingly. The three phases identified in the observations correspond to three configurations of this mean flow, each steering the tripole’s atmospheric response along a different route.

The study goes one step further by asking what drives the mean flow changes themselves. The authors identify two contributors: intensified local transient-eddy activity and shifted tropical convection anomalies. Transient eddies, the swirling synoptic-scale disturbances that populate the storm tracks, exert a feedback on the time-mean flow, and changes in their intensity can reinforce or reshape the zonal winds. Meanwhile, shifts in tropical convection, the towering organized thunderstorm systems of the deep tropics, can alter the large-scale circulation remotely, nudging the midlatitude flow into a new configuration. Either pathway, operating in different phases, sufficed to explain the observed changes in the background winds that steered the wave trains.

To test whether these mean-flow changes were truly sufficient, rather than merely correlated, the team turned to a linear baroclinic model, a simplified numerical tool designed to isolate the role of the background flow in wave propagation. By imposing the observed mean-flow conditions from each phase and forcing the model with the tripole’s atmospheric signature, they found that the model reproduced the phase-dependent Rossby wave trajectories seen in the real atmosphere. The observed changes in the mean flow alone were enough to recreate the shifting pathways, without needing to invoke changes in the tripole itself. This result elevates the background mean flow from a plausible suspect to the key mechanism: it is the control knob that determines how the ocean’s influence is distributed across the continents.

The findings carry practical weight for seasonal climate prediction. Spring surface air temperature over Eurasia and North Africa affects agriculture, water resources, snowmelt timing and the likelihood of temperature extremes, and forecasters have long used North Atlantic sea surface temperature anomalies as precursors. A teleconnection relationship that holds in one decade may fail in the next, and this study explains why: the relationship is conditional on the state of the background flow, which evolves on interdecadal timescales under the influence of eddy activity and tropical convection. Skillful use of the tripole as a predictor therefore requires monitoring not just the ocean, but the atmospheric highway that carries its signal eastward.

More broadly, the work adds to a growing recognition that climate teleconnections are not fixed features of the Earth system but dynamic relationships that drift as the background circulation evolves. Similar interdecadal changes have been documented for other ocean-atmosphere links, including the influence of the tripole on the western North Pacific subtropical high and on East Asian rainfall, suggesting that the mean-flow modulation mechanism may be a general property of midlatitude teleconnections. As greenhouse warming continues to alter jets, storm tracks and tropical convection, the map of which regions are teleconnected to which ocean basins may keep redrawn itself. The tripole has not lost its voice; the atmosphere has simply changed the rooms in which it can be heard.

Subject of Research: How changes in background mean flow modulate the influence of the North Atlantic tripole sea surface temperature pattern on springtime surface air temperature over Eurasia and North Africa

Article Title: Changing influence of the North Atlantic tripole SST pattern on surface air temperature in Eurasia and North Africa modulated by the background mean flow

Article References: Fu, S., Zhu, Z., & Pan, X. (2026). Changing influence of the North Atlantic tripole SST pattern on surface air temperature in Eurasia and North Africa modulated by the background mean flow. Climate Dynamics, 64(10), Article 429. https://doi.org/10.1007/s00382-026-08391-x

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08391-x

Keywords: North Atlantic tripole, sea surface temperature, surface air temperature, Rossby wave train, background mean flow, Eurasia, North Africa, teleconnection, Climate Dynamics, jet stream, transient eddies, spring climate

The central puzzle the team confronted is one that has long frustrated the subseasonal-to-seasonal, or S2S, forecasting community. The skill of forecasts targeting East Asian winter cold surges does not vary smoothly or randomly; instead, it clusters. Some initialization dates yield forecasts that capture the timing and intensity of a surge with impressive accuracy, while others—initialized only days apart—miss the event entirely. Historically, the only way to know whether a forecast fell into a high-skill window was to wait several days after issuance, when verifying observations began to arrive. By then, of course, the forecast had already been delivered to energy planners, agricultural agencies, and disaster-preparedness officials, who had no way of knowing whether to lean on it or discount it. The new study asks a deceptively simple question: can those windows be recognized in advance, before the forecast is made, using precursor signals in the ocean?

To answer it, the researchers mined ECMWF hindcast archives covering the winters of 1997 through 2021, a quarter-century record that spans numerous El Niño and La Niña events and a wide range of Arctic sea-ice conditions. From this archive they systematically identified fourteen high-skill windows, encompassing fifty-six individual forecast cases in which week-three cold surge predictions performed exceptionally well. The first, and in some ways most sobering, finding was that the pre-forecast atmospheric circulation offered almost no help. When the team examined the state of the atmosphere in the days leading up to initialization, they could not reliably distinguish upcoming high-skill windows from ordinary periods, leaving a false alarm ratio of roughly nineteen percent. The atmosphere, in other words, does not announce when it is about to become predictable. The answer, it turned out, lay beneath the surface—in the sea.

Regardless of whether the tropical Pacific was in an El Niño or La Niña phase, every one of the fourteen high-skill windows was preceded by the same sea surface temperature pattern in the mid-latitude North Pacific: a warm anomaly over the western basin paired with a cold anomaly over the eastern basin, a configuration the authors describe as a warm-west/cold-east dipole. This dipole, averaged over the seven days before forecast initialization, emerged as a consistent precursor signal across all ENSO backgrounds. Its physical significance is considerable, because mid-latitude SST anomalies of this kind can reshape the baroclinic environment of the North Pacific storm track and excite Rossby wave trains—vast, undulating disturbances in the atmospheric flow that propagate downstream and can lock the wintertime circulation into persistent, high-impact patterns such as the Siberian high and Ural blocking.

Yet the study’s most important conclusion is that the North Pacific dipole, on its own, is not sufficient. A high-skill window materializes only when the dipole is joined by a matching sea surface temperature configuration in the other key ocean basins—and, remarkably, the state of those ancillary regions also determines which of two distinct dynamical pathways the atmosphere will follow. When warm SST anomalies occupy the Barents-Kara Seas, a condition widely regarded as an indicator of regional sea-ice loss, the Rossby wave train excited by the North Pacific dipole is favored to maintain itself and then to sustain the Ural blocking high, the anticyclonic anomaly over western Russia that acts as a gateway for Arctic air to spill into East Asia. In this pathway, the oceanic signal reinforces the blocking ridge, and the cold surge follows a well-teleconnected route from the polar reservoir southward.

The alternative pathway unfolds when cold anomalies instead dominate the Indian Ocean. In that case, the wave energy excited by the North Pacific dipole is confined to the North Pacific–polar sector rather than propagating through the Ural sector, and the circulation response takes the form of a meridional dipole that steers polar air directly southward into East Asia. Two different oceanic backgrounds, two different wave-guide behaviors, two different routes to the same destructive outcome. The practical implication is that forecasters cannot simply check one index or one basin; they must verify that the sea surface temperature anomalies in all the key regions—the mid-latitude North Pacific, the Barents-Kara Seas, the Indian Ocean, and the tropical Pacific—meet their respective thresholds at the same time. Only this simultaneous, basin-wide alignment marks a genuine high-skill window.

When the team applied this multi-region screening criterion to the hindcast archive, the results were dramatic. Forecasts selected by requiring all key-region SST thresholds to be satisfied simultaneously achieved hit rates of one hundred percent under La Niña-A conditions, ninety-five percent under La Niña-B conditions, and ninety percent under El Niño conditions. Those figures stand in stark contrast to the roughly nineteen percent false alarm ratio that prevailed when no such screening was applied. In effect, the oceanic precursor pattern functions as a reliability certificate: if the prescribed SST configuration is present at initialization, the week-three cold surge forecast can be issued with a degree of confidence that S2S prediction has rarely been able to claim, and if it is absent, users know to treat the forecast with caution.

Beyond its operational value, the study carries a deeper scientific message: forecast skill windows are not random accidents of chaos but physically traceable events, anchored in slow, predictable components of the climate system. The ocean evolves on timescales of weeks to months, far slower than the atmosphere, and its anomalies act as a kind of memory that conditions how atmospheric disturbances will amplify, propagate, and persist. By mapping which oceanic configurations render the atmosphere more predictable, the researchers have effectively converted an abstract question about ensemble spread and verification statistics into a concrete, observable checklist. The work also underscores the growing recognition that mid-latitude predictability is jointly governed by the tropics, the Arctic, and the mid-latitudes themselves, with the Barents-Kara Seas linking the story of Arctic sea-ice decline directly to the practical skill of winter forecasts thousands of kilometers away.

For the agencies that must act on winter forecasts—grid operators deciding when to pre-position fuel reserves, transportation authorities planning for ice and snow, and farmers protecting overwintering crops—the ability to know in advance that the coming two to three weeks are likely to be a high-skill period is of immediate practical value. It allows limited confidence to be spent where it is justified and withheld where it is not, sharpening decisions in energy dispatch and agricultural disaster prevention alike. As subseasonal prediction matures from a research frontier into an operational mainstay, studies of this kind suggest that the future of reliable week-three forecasting may depend less on faster supercomputers than on learning to read the ocean’s slow, patient signals before the atmosphere ever makes its move.

Subject of Research: Identifying sea surface temperature precursors that reveal high-skill windows in subseasonal forecasts of East Asian winter cold surges

Article Title: A "warm-west/cold-east" North Pacific sea surface temperature dipole, working in concert with anomalies in other key ocean regions, reveals when subseasonal forecasts of East Asian winter cold surges can be trusted

Article References: A "warm-west/cold-east" North Pacific sea surface temperature dipole, working in concert with anomalies in other key ocean regions, reveals when subseasonal forecasts of East Asian winter cold surges can be trusted. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: subseasonal forecasting, East Asian winter monsoon, cold surges, sea surface temperature, North Pacific dipole, El Niño, La Niña, Barents-Kara Seas, Ural blocking, Rossby wave train, ECMWF hindcasts, forecast skill windows

The research, led by Yina Diao and Jianping Li of the Ocean University of China together with colleagues, approaches the problem through an energy lens. Rather than asking only which winds and pressure patterns aligned over South China, the team examined perturbation potential energy, or PPE, a diagnostic quantity that measures the energy stored in deviations of the atmosphere from its background state. First developed as an extension of Edward Lorenz’s classic concept of available potential energy, PPE tracks how much energy is locked into temperature and pressure anomalies and is available to be converted into the kinetic energy of motion, including the vigorous updrafts that drive heavy rain.

The analysis revealed that the record warmth over Eastern Europe in April 2024 was accompanied by an unusually large reservoir of Eastern Europe PPE, abbreviated EEPPE in the study. That energetic anomaly did not sit still. It acted as a stable wave source, amplifying and sustaining a Rossby wave train that the authors call the Eastern Europe high–West Asia low, or EEWA, pattern. This wave train is a chain of alternating high and low pressure anomalies that stretches from Eastern Europe across West Asia toward East Asia, and it serves as the dynamical bridge connecting the two seemingly unrelated extremes: scorching heat in Europe and flooding in southern China.

According to the study, the EEWA wave train reshaped the thermodynamic environment over Guangdong through two distinct pathways. The first acts high in the atmosphere: the wave train modulated the upper-tropospheric westerly jet that flows south of the West Asian low, and changes in the jet altered the vertical motion over South China. Stronger rising motion is a prerequisite for heavy precipitation, because air that ascends cools, water vapor condenses, and latent heat is released. The second pathway operates closer to the surface: the wave pattern modified local thermal advection over Guangdong, pumping warm air into the region and thereby increasing the local PPE, the stored energy available for conversion into storm-scale motion.

The two pathways converged in a mutually reinforcing sequence. As EEPPE increased, vertical velocity over Guangdong strengthened and the local PPE pool deepened. The enhanced upward motion then facilitated the conversion of that local perturbation potential energy into kinetic energy, intensifying the very convection that produced the torrential rain. In energy terms, the atmosphere over southern China was primed with potential energy by remote heating anomalies, and the large-scale dynamics supplied the trigger that unlocked it. This chain of events establishes, for the first time in such a quantitative framework, a direct dynamical link between heat extremes in Eastern Europe and flooding in Guangdong.

One of the most consequential findings of the study is its implications for early warning. The PPE anomalies over Eastern Europe build up roughly a week before the extreme precipitation peaks over Guangdong, meaning that EEPPE can serve as a one-week precursor signal. In operational terms, an energy diagnostic computed over Eastern Europe could give forecasters in South China valuable lead time, complementing conventional monitoring of tropical influences such as the Madden–Julian Oscillation and sea-surface temperature patterns in the Indo-Pacific, which other studies have also implicated in the April 2024 event.

The researchers were careful to test whether this hot-flood teleconnection was a one-off curiosity of 2024. Their analysis shows that it was not. The connection between Eastern European heat-related PPE and Guangdong rainfall has persisted since the early twenty-first century, suggesting a robust and recurring dynamical pathway. What made April 2024 exceptional, the study concludes, was the prolonged and extreme nature of the European heat anomaly, which sustained the EEWA wave train long enough to drive long-lasting extreme precipitation in Guangdong rather than a brief burst of heavy rain.

The findings arrive at a moment when scientists are increasingly recognizing that climate extremes do not respect continental boundaries. Heat waves and floods are often treated as separate hazards in separate regions, but the atmosphere transports their consequences around the planet through teleconnections. Just as sea-surface temperature anomalies in the tropical Pacific can reshape winters in North America, extreme heating over Eastern Europe can, through wave dynamics and energy conversion, help set the stage for catastrophic rainfall in East Asia. As heat extremes intensify under global warming, the study suggests that the frequency and severity of such remote hot-flood linkages may change as well.

Technically, the work showcases the growing utility of PPE as a diagnostic tool in climate research. Unlike purely statistical correlations between distant weather events, the PPE framework traces the actual energetic pathway: heat anomalies build potential energy in one region, wave trains transport its influence downstream, and energy conversion processes release it as the kinetic energy of storms. Earlier applications of the framework have illuminated the energetics of the South China Sea summer monsoon, the Indian Ocean Dipole, and multidecadal variability in the North Atlantic, and this study extends the method to the attribution of a specific record-breaking disaster.

For Guangdong, home to more than 120 million people and the Pearl River Delta megacity cluster, the practical message is sobering. The province’s pre-flood season, already among the rainiest in East Asia, can be supercharged by heat events unfolding a continent away, and the energy fingerprints of those heat events are visible in advance. If the hot-flood teleconnection holds as warming continues, monitoring atmospheric energy reservoirs over Eastern Europe may become as important for South Chinese flood preparedness as watching the skies overhead. The study, supported by China’s National Key Basic Research Project, the National Natural Science Foundation of China, and Laoshan Laboratory, relied on the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis and rainfall observations from the China Meteorological Administration, providing a data-rich foundation for what may become a new chapter in extreme-event forecasting.

The physics underlying the teleconnection rests on a well-established principle of midlatitude dynamics: stationary Rossby waves. When a large-scale heat source perturbs the atmosphere, it displaces air parcels and generates wave energy that propagates along the westerly jet stream. The theoretical foundations for describing this propagation were laid decades ago, from the classic energy-transfer analysis of Eliassen and Palm to later work by Karoly and Plumb on wave activity in sheared and three-dimensional flows. The EEWA wave train identified in the Guangdong study is a contemporary application of this lineage, demonstrating how a thermal anomaly over Eastern Europe can impose a coherent chain of circulation anomalies across the Eurasian continent within days.

The choice of perturbation potential energy as the central diagnostic is itself notable. Traditional analyses of heavy rainfall in South China have emphasized circulation patterns, moisture transport, and convective instability, often drawing on case studies of the pre-flood season that stretches from April through June. The PPE framework reframes these questions energetically: instead of describing where winds converge, it quantifies how much exploitable energy the atmosphere holds and where that energy is created, stored, and destroyed. Because PPE responds directly to diabatic heating, it is particularly sensitive to the kind of prolonged surface warming that characterized Eastern Europe in spring 2024, making it a natural bridge between heat extremes and downstream storm development.

The data underpinning the analysis also merit attention. The ERA5 reanalysis, produced by the European Centre for Medium-Range Weather Forecasts, assimilates observations into a globally consistent record extending back to 1940, allowing anomalies in April 2024 to be placed in a seven-decade context. Pairing this with station-based rainfall observations from the China Meteorological Administration enabled the researchers to verify that the record April precipitation in Guangdong coincided precisely with the phases of the wave train and the energy conversions diagnosed from reanalysis fields, rather than resting on model simulations alone.

The finding that the hot-flood linkage has persisted since the early twenty-first century raises questions about what may have established it. Prior research has documented interdecadal shifts in the factors governing spring rainfall over southern China, including changes in the influence of Eurasian snow cover and in the behavior of the first rainy season itself. Whether the emergence of the EEPPE–Guangdong connection reflects a genuine reorganization of waveguide dynamics, a consequence of warming-driven increases in heat extremes over Europe, or simply the window of the observational record remains an open question that future work will need to address.

For forecasters, the one-week lead time implied by EEPPE anomalies sits in a valuable gap between short-range weather prediction and seasonal outlooks. Subseasonal forecasting has long struggled with the so-called predictability desert, the span beyond about ten days where deterministic forecasts lose skill but seasonal signals have not yet emerged. An energy-based precursor rooted in a physically understood wave mechanism offers a complementary handle on this range, and the study’s demonstration that the signal is recurrent rather than unique to 2024 strengthens the case for testing it in an operational setting.

More broadly, the work adds to a growing catalog of remote linkages in which extremes in one region amplify hazards in another. As the atmosphere warms, the amplitude of heat-driven energy anomalies is expected to grow, potentially strengthening the wave sources that seed such teleconnections and making energy diagnostics an increasingly important part of hazard monitoring worldwide.

Subject of Research: The dynamical teleconnection linking Eastern European heat extremes to extreme precipitation in Guangdong, China through atmospheric wave trains and perturbation potential energy conversion

Article Title: A hot-flood teleconnection as a key contributor to the extreme April 2024 precipitation in Guangdong

Article References: Diao, Y., Zhang, X., Li, J., Yang, Y., Zhang, Y., Huang, F., & Hou, Z. (2026). A hot-flood teleconnection as a key contributor to the extreme April 2024 precipitation in Guangdong. Climate Dynamics, 64(10), Article 421. https://doi.org/10.1007/s00382-026-08372-0

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08372-0

Keywords: extreme precipitation, perturbation potential energy, hot-flood teleconnection, Guangdong floods, Eastern Europe heat, Rossby wave train, Climate Dynamics, ERA5 reanalysis, atmospheric dynamics, extreme weather, South China rainfall, energy conversion