The research team, led by Imad Abdulridha Jasim Al-Khulaifawi of Southern Federal University in Rostov-on-Don and Aqeel Ghazi Mutar of Mustansiriyah University in Baghdad, anchored their analysis in the observational record of the Khanaqin meteorological station, whose thunderstorm-day records were supplied by the Iraqi General Authority for Meteorology and Seismic Monitoring. To understand what the atmosphere looked like on those days, they turned to reanalysis products: ERA5 from the European Centre for Medium-Range Weather Forecasts, the NCEP/NCAR reanalysis from NOAA, and NASA’s MERRA-2. Reanalysis datasets blend decades of observations with numerical weather models to produce a physically consistent picture of the atmosphere, which is essential in regions like eastern Iraq where radiosonde launches and radar coverage are sparse or absent.

The seasonal signal was unambiguous. Just over half of all thunderstorm days, 51.79 percent, fell in spring, when the region sits in the collision zone between lingering winter air masses and rapidly warming surface air. Summer, by contrast, produced few storms despite searing surface temperatures, because the deep boundary layer bakes moisture out of the lower atmosphere and suppresses organized convection. Winter and autumn contributed the remainder, typically in association with passing Mediterranean and Persian Gulf synoptic systems. This springtime maximum aligns with what has been documented across the broader Middle East, where the transition season offers the most favorable combination of moisture, instability, and lifting mechanisms.

To quantify the thermodynamic environment, the researchers calculated three widely used instability measures: the K-index, which weighs the temperature and humidity profile between roughly 850 and 500 hectopascals; the Total Totals index, a composite of lapse rates and mid-level moisture; and CAPE, the integrated buoyant energy available to a rising air parcel. The results showed a springtime atmosphere characterized by only moderate thermal instability, with CAPE values generally remaining below 1000 joules per kilogram. For comparison, severe convective outbreaks in the central United States or the Indian subcontinent frequently unfold in environments exceeding 2000 or even 3000 joules per kilogram. On paper, then, Khanaqin’s storm environment looks unremarkable, yet the storms themselves were anything but shallow.

That apparent contradiction is the heart of the study. By examining the vertical structure of the convective clouds, including the lifting condensation level where cloud bases form, the equilibrium level where rising parcels lose their buoyancy, and the overall thickness of the storm layer, the researchers documented deep thunderstorms with significant vertical extent despite the modest instability. The explanation lies in the dynamics aloft. Upper-level troughs, elongated pools of cold low pressure in the mid-troposphere, steepen the lapse rate over the region and simultaneously provide large-scale ascent that releases whatever instability exists. In effect, the atmosphere over eastern Iraq does not need an enormous reservoir of buoyant energy if a powerful synoptic-scale pump is actively forcing air upward.

The jet stream emerged as a central character in this story. Using the NCEP/NCAR and MERRA-2 datasets for weather-map analysis, the authors examined how jet streams and upper-level divergence interact with deep convection over the region. The mechanism, well established in midlatitude meteorology since the classic work of Uccellini and Johnson in 1979, involves the coupling of upper and lower jet streaks. Air accelerating through the entrance and exit regions of a jet streak creates divergence in its wake at cruise altitude, which acts like a vacuum cleaner, drawing air upward from below. This upper-level suction lowers surface pressure, enhances low-level convergence and moisture inflow, and can tilt storm updrafts in ways that sustain them far longer than buoyancy alone would allow.

When such jet coupling coincided with an upper trough over the Zagros foothills near Khanaqin, the result was convection that punched well above its thermodynamic weight. The study’s conclusion is therefore that thunderstorms in eastern Iraq are primarily regulated by upper-level synoptic influences rather than solely by convective energy, and that CAPE used as a sole predictor systematically underestimates storm potential in semi-arid environments. This has direct operational consequences. Forecasters relying on threshold-based CAPE criteria, a practice imported from more moisture-rich climates, would routinely miss dangerous storm days in Iraq. An ingredients-based approach, which evaluates moisture, instability, and lifting separately and weighs the dynamic lifting contribution explicitly, is better suited to the region.

The work also fills a conspicuous gap in the global storm climatology. Severe convective storms have been extensively cataloged across Europe and the United States, and increasingly in India, but the Middle East remains one of the planet’s data-poor regions for thunderstorm research. Iraq’s observational infrastructure has been strained by decades of conflict, making reanalysis-based reconstruction one of the few viable paths to a long-term storm record. By validating their instability calculations across three independent reanalysis products and tying them to ground-truth thunderstorm reports from a single station over 30 years, the authors have produced a benchmark dataset for a corner of the world where convection mechanisms were previously poorly documented.

There are broader implications as climate change reshapes the region. Theoretical work by Romps and by Agard and Emanuel suggests that CAPE should scale upward with warming following the Clausius–Clapeyron relation, implying more potential storm energy in a hotter world. Yet the Khanaqin record shows no significant trend in thunderstorm frequency over 1990 to 2019, a reminder that in dynamically driven storm regimes, changes in the jet stream and upper-level circulation patterns may offset or complicate any thermodynamic signal. Disentangling those competing influences is a pressing question for future research, particularly for a country already grappling with extreme water stress and shifting rainfall patterns.

For now, the practical payoff is a scientific foundation for better warnings. The authors frame their results as a basis for developing weather forecasting and severe storm early warning systems in Iraq and neighboring countries, where flash floods from spring thunderstorms can be deadly and destructive. Knowing that the key predictors are upper-level troughs, jet streak positioning, and coupled divergence patterns rather than surface heat alone gives regional forecasters a concrete set of signals to monitor. It is a striking illustration of a general truth in meteorology: the most violent weather sometimes comes not from the most energized atmosphere, but from the right large-scale trigger arriving at exactly the right moment.

Subject of Research: Synoptic and thermodynamic controls on thunderstorm activity in eastern Iraq

Article Title: Synoptic–thermodynamic controls of thunderstorm activity over Khanaqin: A 30-year analysis of instability indices and jet stream dynamics

Article References: Al-Khulaifawi, I. A. J., & Mutar, A. G. (2026). Synoptic–thermodynamic controls of thunderstorm activity over Khanaqin: A 30-year analysis of instability indices and jet stream dynamics. Theoretical and Applied Climatology, 157(11), Article 698. https://doi.org/10.1007/s00704-026-06594-0

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06594-0

Keywords: thunderstorms, Khanaqin, Iraq, CAPE, instability indices, jet stream, ERA5, reanalysis data, convection, synoptic meteorology, climatology, early warning systems

The research, led by Gabriel Pascual, Júlia Soliva, Jordi Mazon and David Pino of the Universitat Politècnica de Catalunya in Barcelona, takes an unusual but well-established approach. Rather than tracking individual turbulence events, which last seconds or minutes and cannot be resolved in monthly data, the team analyzed 75 years of monthly ERA5 reanalysis data from 1950 to 2025 to map the slow evolution of the synoptic-scale environments in which clear-air turbulence forms. The method acts like a temporal low-pass filter: it strips away the stochastic noise of individual encounters and reveals the steady, low-frequency variability of the background flow, much as climate scientists use monthly averages of convective available potential energy and wind shear to chart where severe thunderstorms are likely rather than where any single storm struck.

The study domain stretches from 75 degrees north to 20 degrees north and from 35 degrees west to 50 degrees east, covering the Euro-Mediterranean flight corridor. The researchers extracted data at two pressure levels, 200 and 225 hectopascals, which correspond to standard cruise altitudes for long-haul commercial aviation and coincide with the mid-latitude jet stream core and the oscillating boundary of the tropopause, where vertical wind shear and static stability gradients are most pronounced. From the 21 turbulence diagnostics proposed in earlier literature, seven indices were selected for their consistency across previous studies: Horizontal Divergence, the Richardson Number, the Brown Index, the North Carolina State University Index NCSU1, the two Ellrod Indices, and Potential Vorticity.

Each index captures a different facet of turbulence physics. The Richardson Number expresses the dimensionless ratio of buoyancy to shear generation and is the classical measure of whether a flow is dynamically stable; values below the critical threshold indicate that small disturbances can grow into full turbulence. The Ellrod Indices combine vertical wind shear with horizontal deformation, the kinematic precursors identified by Ellrod and Knapp in 1992, and the second variant, which adds a horizontal convergence term, has been verified against pilot reports with a probability of detection between 70 and 84 percent. The NCSU1 index integrates localized rotation, vertical shear and buoyancy-related instability to identify the flow’s capacity to amplify minor perturbations, particularly near jet streams and mountain waves, while Potential Vorticity serves as a tracer for the tropopause itself.

Before searching for trends, the team first tested how the indices relate to one another when computed from time-averaged fields, using the Spearman rank correlation because the data are not normally distributed. The Ellrod Index 1 emerged as the most robust proxy for general turbulence trends, correlating strongly, above 0.70, with EI2, NCSU1 and the Brown Index, all of which share a dependence on vertical wind shear or the deformation tensor. In contrast, Horizontal Divergence and the Richardson Number showed negligible correlation with the deformation-based suite, and Potential Vorticity displayed a consistent moderate negative correlation, reflecting the inverse relationship between PV-defined stability and turbulent kinetic energy. All correlations were statistically significant at the 0.05 level, with their strength varying between the two pressure levels.

The trend analysis then focused on three non-redundant indices: HDG, the negative Richardson Number and NCSU1. The results were strikingly index-dependent. Horizontal Divergence showed no statistically significant long-term trend at either level, with regression p-values of 0.532 at 200 hectopascals and 0.358 at 225 hectopascals, far above the significance threshold. The negative Richardson Number, by contrast, declined significantly over the 75 years, with a p-value of 2.88 times ten to the minus four, indicating a systematic shift toward reduced static stability and enhanced macroscopic vertical wind shear at cruise altitudes. NCSU1 showed a statistically significant positive trend, with a p-value of precisely 0.05, suggesting a long-term expansion of atmospheric configurations prone to generating clear-air turbulence, though the authors caution that this result sits right at the conventional significance threshold and is accompanied by considerable interannual variance.

The seasonal breakdown added further nuance. Winter emerged as the most turbulent season, with Richardson Number values closest to zero, indicating the strongest mechanical turbulence, and with NCSU1 values in some cases twice as large as the annual averages. Summer and autumn were the most quiescent, and their temporal evolutions resembled each other more than they resembled winter or spring. The seasonal trends also diverged in direction: for the negative Richardson Number, every season except spring showed a descending trend consistent with a year-round shift toward a more unstable upper-tropospheric background flow, while spring alone bucked the pattern with a positive trend and very high interannual variability. For Horizontal Divergence, winters actually became more stable over the period, potentially because rising vertical temperature gradients affect lapse rates.

The most eye-catching finding came from frequency-domain analysis. Applying the Fast Fourier Transform to the annually averaged series, the team found a dominant periodicity of 2.45 years in the Horizontal Divergence and Potential Vorticity indices, with related short-period peaks of about 2.8 years appearing in NCSU1 and EI2. Crucially, no index showed a periodicity corresponding to the familiar 11-year solar cycle. Instead, the 2.45-year signal aligns with historical findings by Clough in 1928, who showed that once the 11-year cycle is removed, variations in solar activity with a most common period of roughly 2.4 years become visible, a periodicity he argued was not accidental because purely stochastic variations would more likely cluster around 1.6 years. When the researchers compared the years in which the 2.45-year periodicity was most prominent with the onset years of the 11-year solar cycles from 1950 to 2025, none coincided, further suggesting the shorter-period solar cycles rather than the primary cycle are at play.

The physical mechanism, the authors propose, runs through the stratosphere. Quasi-biennial oscillations of solar activity have been documented in sunspot records using methods such as Empirical Mode Decomposition and Singular Spectrum Analysis, and the interaction between solar maxima and the easterly phase of the Quasi-Biennial Oscillation is known to precondition the polar vortex for sudden stratospheric warmings. When these warmings occur, the polar vortex breaks down dramatically and disrupts the jet stream, the primary driver of enhanced turbulence over Europe. Recent work by Vokhmyanin and colleagues achieved an 86 percent success rate in predicting winter stratospheric warming occurrences as early as the preceding August by modeling the interplay between solar and geomagnetic activity and the QBO, underscoring that the 2.45-year turbulence cycle is not a random fluctuation but is consistent with solar-driven oscillations modulating the large-scale stratospheric dynamics that dictate upper-tropospheric conditions. The authors are careful to note that a definitive causal link cannot yet be established and that the hypothesis requires further validation with longer time series or higher-resolution solar proxy data.

The study also carries practical weight for aviation. Previous research has shown that a doubling of carbon dioxide concentrations could raise the frequency of moderate-to-severe North Atlantic turbulence by 40 to 170 percent, that severe turbulence exposure there grew by 55 percent between 1979 and 2020, and that anthropogenic warming strengthens vertical wind shear at 250 hectopascals by 27 percent while reducing stratification and the Richardson number by 47 percent. Yet this new analysis shows that over continental Europe the picture is less uniform than over the open ocean: orographic forcing and higher surface roughness disrupt the coherence of vertical wind shear, making regional trends strongly dependent on which diagnostic is used. Distinguishing these macroscopic climate precursors from microscale turbulence events, the authors argue, is essential for aviation safety risk assessments in a changing climate, and the finding that part of Europe’s turbulence-prone environment pulses on a quasi-biennial solar rhythm adds an unexpected, and potentially predictable, dimension to the forecast.

Subject of Research: Long-term climatology of clear-air turbulence-favorable atmospheric conditions over Europe and their modulation by quasi-biennial solar cycles

Article Title: Climatology of large-scale atmospheric conditions favorable to clear-air turbulence over Europe (1950–2025): the role of quasi-biennial solar modulation

Article References: Pascual, G., Soliva, J., Mazon, J., & Pino, D. (2026). Climatology of large-scale atmospheric conditions favorable to clear-air turbulence over Europe (1950–2025): the role of quasi-biennial solar modulation. Climate Dynamics, 64(11), Article 459. https://doi.org/10.1007/s00382-026-08418-3

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08418-3

Keywords: clear-air turbulence, aviation safety, ERA5 reanalysis, Richardson number, Ellrod index, potential vorticity, jet stream, quasi-biennial oscillation, solar activity, stratospheric dynamics, climate change, Europe

The study, published in the journal Weather and Climate Dynamics by Aleksa Stanković, Rodrigo Caballero and Gabriele Messori, tackles a long-standing puzzle in atmospheric science. By most bulk measures, such as zonally averaged eddy kinetic energy or median annual surface winds, the Southern Ocean is the stormier hemisphere. Its vast, unbroken expanse of water, largely free of continental barriers, allows storms to circle the planet with little interference. Yet when it comes to the extremes, the picture flips: the strongest surface winds consistently occur over the Northern Hemisphere oceans, and above all over the North Atlantic. Understanding why the hemisphere with the calmer average winds produces the fiercest storms is essential for improving projections of how wind extremes will change as the climate warms.

To answer the question, the team turned to ERA5, the most advanced atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts, which blends observations with a numerical weather model to create a physically consistent record of the atmosphere. The researchers focused on the winter months, December through February in the Northern Hemisphere and June through August in the Southern Hemisphere, when cyclones in both hemispheres reach their peak intensity. They restricted their analysis to the satellite era beginning in 1979, because satellite observations are crucial for reliably capturing the sparsely monitored Southern Ocean, and extended the record through 2020.

Identifying the top 100 extremes in each basin required a careful, two-step procedure. First, the team defined storm tracks as regions where the annual 98th percentile of daily maximum surface wind speed exceeds 18 metres per second, a criterion that isolates three comparable basins: the North Atlantic, the North Pacific and the Southern Ocean. Areas within 300 kilometres of the coasts of Greenland and Antarctica were excluded, since the ferocious winds there arise mainly from small-scale katabatic and barrier processes rather than the large-scale cyclone dynamics of interest. Then, for every winter day, the researchers computed a wind severity index over contiguous footprints of extreme winds. The index, widely used in insurance loss modelling, scales with the cube of the wind speed relative to the local climatological threshold, a choice with physical grounding because the cube of wind speed represents the flux of kinetic energy. The 100 days with the highest severity in each basin defined the top 100 extremes, and each event was linked to the nearest objectively tracked cyclone centre.

The heart of the analysis is a time-lagged composite technique. Rather than examining storms one by one, the researchers averaged the atmospheric fields of all 100 extreme events in each basin, aligning them by the location of the cyclone centre and the time of maximum surface winds. Because lines of latitude converge toward the poles, the fields were regridded onto radial grids centred on each storm. Statistical significance was assessed by generating 100 random composites for each basin, in which the real cyclone locations were paired with randomly chosen winter dates, yielding 10,000 random samples, with anomalies accepted as significant only after correction for false discovery.

The composites revealed a strikingly consistent large-scale recipe. A few days before the peak winds, a pre-existing cyclone is already present downstream, situated poleward and eastward of the developing extreme storm. The young cyclone begins as a weak, spatially limited pressure anomaly that grows in size and intensity as it approaches its older neighbour. By the time of maximum winds, the two systems merge into a broad area of low sea-level pressure. In fact, roughly half of the top 100 extremes in each basin, 49 in the North Atlantic, 53 in the North Pacific and 56 in the Southern Ocean, were formally classified as multi-centre cyclones by the tracking algorithm around the time of peak winds. The pre-existing storm is not a passive bystander: it advects high potential vorticity air equatorward, sharpening the upper-level potential vorticity gradients and strengthening the jet streak, the narrow band of ferocious winds near the tropopause, within which the new storm develops and crosses the jet.

Where the hemispheres diverge is in the numbers, not the narrative. Northern Hemisphere extremes develop under stronger upper-level potential vorticity gradients, more powerful jet streaks and broader regions of high mid-tropospheric Eady growth rate, a classical measure of how rapidly baroclinic instabilities amplify in a sheared, stratified atmosphere. Higher Eady growth rates signal a greater potential for rapid intensification, and the observations bear this out dramatically. The rate at which eddy kinetic energy rises in the two days before peak winds is far greater in the Northern Hemisphere, and explosively deepening storms, those whose central pressure falls by more than 24 hectopascals in 24 hours, account for 69 percent of North Atlantic and 75 percent of North Pacific extremes but only 38 percent of Southern Ocean ones. Since vertical wind shear below the jet is the main driver of the hemispheric difference in growth rates, the stronger jets of the Northern Hemisphere, reinforced by the pre-existing downstream cyclones, emerge as the key to the hemisphere’s more violent wind extremes.

To test whether these dynamical differences trace back to the underlying geography of each basin, the team ran sensitivity experiments with ISCA, an open-source intermediate-complexity climate model developed at the University of Oxford and collaborators. Encouragingly, even at the coarse T42 resolution used, ISCA reproduced both the hemispheric asymmetry in extreme winds and the correct ranking of basins, with the North Atlantic strongest, followed by the North Pacific and the Southern Ocean. This suggests the asymmetry is driven primarily by large- and planetary-scale processes rather than by fine-scale features the model cannot resolve. The researchers then systematically altered the model’s boundary conditions across 11 experiments, comparing a realistic control configuration with setups in which sea-surface temperatures were smeared into zonal bands, made hemispherically symmetric, and combined with a planet stripped of mountains.

The results were remarkably clean. When sea-surface temperatures were zonalized across the tropics and extratropics, the difference in extreme winds between the North Atlantic and North Pacific nearly vanished, shrinking by 82 percent, as warming spread over the Pacific storm track and cooling weakened the Atlantic jet. Removing the interhemispheric contrast in sea-surface temperatures and flattening the orography reduced the Atlantic–Southern Ocean asymmetry by 83 percent, through a combination of weaker Atlantic extremes and stronger Southern Ocean ones. Across all experiments and basins, the winter 98th percentile of mid-tropospheric Eady growth rate correlated with the 98th percentile of surface wind speed with a Pearson coefficient of 0.97, significant at the 1 percent level. In other words, one number, the potential for storms to grow quickly, largely determines how extreme the winds can become, regardless of which ocean they form over.

The study comes with honest caveats. The focus on large scales leaves out mesoscale and boundary-layer processes that can contribute to surface wind extremes, and the analysis deliberately set aside the detailed mechanics of cyclone merging and storm–storm interaction. ISCA, lacking a realistic representation of clouds, could not reproduce the pre-existing downstream cyclone in the evolution of its own most extreme individual storms, even though it captured the climatological relationship between growth rates and winds. Still, the broader message stands: the recipe for the planet’s most extreme oceanic windstorms is written once and applied everywhere, with a helper storm downstream, a strong jet overhead and a fast-deepening cyclone at the centre. The only thing that varies between hemispheres is how generously the ocean basins supply the ingredients, a conclusion that should help sharpen both weather forecasting and the climate models on which future windstorm projections depend.

Subject of Research: Large-scale dynamics of extreme surface winds in extratropical cyclones across the Northern and Southern Hemisphere storm tracks

Article Title: Interhemispheric perspective on the most extreme surface winds in the storm tracks

Article References: Stanković, A., Caballero, R., & Messori, G. (2026). Interhemispheric perspective on the most extreme surface winds in the storm tracks. Weather and Climate Dynamics, 7(3), 1507-1523. https://doi.org/10.5194/wcd-7-1507-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1507-2026

Keywords: extratropical cyclones, storm tracks, extreme winds, Eady growth rate, ERA5 reanalysis, jet stream, potential vorticity, Southern Ocean, North Atlantic, North Pacific, ISCA climate model, explosive cyclogenesis

The problem the team set out to solve is known in the forecasting community as underdispersion. Ensemble forecasts work by running a model many times with slightly different starting conditions and perturbed physics, producing a cloud of possible futures whose spread is meant to mirror the true uncertainty of the forecast. When the ensemble spread is much smaller than the average forecast error, the ensemble is underdispersive: it is overconfident, presenting a range of outcomes that is too narrow to encompass what actually happens. Sea ice forecasts, the authors note, are systematically and often severely underdispersive — even at the moment they are initialised, before any forecast error has had time to grow. This is likely tied to the large biases that models exhibit in sea ice and to the poor representation of unresolved processes such as ice fracturing, ridging, and the turbulent exchange of heat between ice, ocean, and atmosphere.

The remedy tested in the study is a stochastically perturbed parameterisation, or SPP, scheme applied to SI³, the state-of-the-art sea ice model now coupled to ECMWF’s Integrated Forecast System. Rather than perturbing the ice directly, the scheme perturbs nine physical parameters inside the model — including the ice strength parameter P*, the ice–ocean drag coefficient, and snow-related properties — multiplying each by a log-normally distributed random field that evolves in space and time. The random field is generated by repeatedly applying a Shapiro filter to white noise, producing smooth spatial correlations on scales of roughly 750 kilometres, and it decorrelates over about ten days. Because the perturbations act on parameters rather than on tendencies or energies, the scheme conservatively respects tracers, momentum, and sea ice volume, keeping it physically consistent with the stochastic schemes already used in the atmospheric component of the model since the late 1990s.

To evaluate the scheme, the team ran 50-member ensemble forecasts initialised on 1 November and 1 May for every year from 1993 to 2023, each running six months to cover full winter and summer seasons. The experiments used a prototype of cycle 49R2 of the IFS, the configuration that will underpin the next-generation seasonal forecast system SEAS6 and the ERA6 reanalysis. Two ensembles were compared: a control run and an otherwise identical run with sea ice SPP switched on. The only difference between them was the stochastic noise in the ice, which means any divergence between the two ensembles must ultimately trace back to the sea ice itself.

The headline result is a robust increase of roughly 10 percent in ensemble spread for both sea ice concentration and sea ice thickness, in both summer and winter, bringing the spread–error ratio closer to the ideal value of one that defines a well-calibrated probabilistic forecast. But the noise did something unexpected as well. Despite being mean-zero by construction, the perturbations systematically shifted the mean state: sea ice concentration decreased on average, thickness increased, and in winter the ice was redistributed outward from the central Arctic pack toward the ice edge, with drift changes on the order of 20 percent of baseline velocities. The authors explain this through a simple but powerful conceptual model built on two asymmetries. First, sea ice concentration is bounded between zero and 100 percent, and its distribution is strongly bimodal: perturbations can only push near-total ice cover downward, while gridpoints knocked to zero effectively vanish from the scheme’s view, so the mean tends to fall. Second, in winter the dense ice pack resists convergence — piling ice up costs energy — whereas there is little resistance to moving ice toward the open edge, so random perturbations to advection preferentially push ice outward along the path of least resistance. A toy model based on repeatedly perturbing a truncated bimodal distribution reproduces exactly this signature of a declining mean.

These ice changes did not stay confined to the cryosphere. Because fluctuations in ice concentration and thickness modulate surface heat fluxes and albedo, the perturbations altered air temperature up to at least the 850 hectopascal level, with changes of order half a degree Celsius. In winter, the central Arctic warmed where ice cover thinned, allowing more heat to escape the ocean, while regions along the Greenland, Barents, and Kara sea ice edges cooled where the perturbations increased ice concentration. Circulation anomalies, including a cyclonic feature near the Ural Mountains that advected cold Siberian air over the Barents and Kara seas, reinforced these thermodynamic responses. Ensemble spread in air temperature also increased by around 10 percent, both over the Arctic and to some extent at midlatitudes. In summer, by contrast, the atmospheric response was much smaller, as Arctic surface temperatures are pinned near the freezing point and the ice has little leverage on the air above it.

The most striking finding concerns forecast skill beyond the ice itself. In winter, the anomaly correlation coefficient of 500 hectopascal geopotential height — the standard yardstick for seasonal circulation forecasts — increased substantially over the subpolar North Atlantic and extended into Scandinavia, with the difference peaking near 0.5. Averaged over a broad subpolar North Atlantic box, the correlation rose from about 0.3 in the control to about 0.6 with the perturbations switched on, and the winter North Atlantic Oscillation, the dominant mode of European winter variability, was forecast somewhat better as well. The authors are careful to stress the statistical caveats: with only about 30 winters in the sample, field significance was not achieved, and Monte Carlo resampling suggests a roughly 40 percent chance that the pattern of significant gridpoints could arise by chance. Yet the same improvement appeared in several additional hindcasts with different configurations, and the region of enhanced skill coincides with where the ice was most strongly affected, lending credibility to the signal.

The proposed mechanism is as intriguing as the result. The control model carries a uniform warm bias across the Arctic, which weakens the meridional temperature gradient in the subpolar North Atlantic. By increasing ice concentration along the Greenland, Barents, and Kara sea edges, the stochastic perturbations cool the overlying atmosphere and partially correct that warm bias, potentially strengthening the temperature gradient that steers the North Atlantic jet stream. Bootstrap resampling of the perturbed ensemble revealed a statistically significant correlation of −0.28 between the degree of cooling along the ice edge and the skill of the North Atlantic circulation forecast, consistent with this pathway. The authors caution that this accounts for only about 8 percent of the variability, that causality is difficult to establish in a system this noisy, and that some of the improvement may reflect bias compensation rather than a genuinely better simulation of the coupled system. Whether the benefit transfers to other models will likely depend on their own baseline biases.

The study also uncovered a subtlety with practical consequences for how such schemes should be deployed. In uncoupled ocean-and-ice-only simulations, the perturbations were dramatically more effective at generating spread than in fully coupled forecasts, where the atmosphere dominates the error growth and may already saturate the achievable spread; switching to independent random fields for each parameter boosted spread by up to 60 percent in the uncoupled case but made little difference when coupled. This suggests the scheme could be especially valuable in data assimilation, where enhancing the spread of ocean and ice initial conditions is a recognised priority, and work is already under way to apply a variant of the scheme in that context. The team also found no evidence that the perturbations strengthened the much-debated teleconnection between Barents–Kara sea ice and the North Atlantic Oscillation reported in earlier climate-model work, hinting that in these forecasts the relevant pathway may run instead through Greenland sea ice.

For operational forecasting, the message is that a modest dose of well-designed randomness in the ice can pay dividends far beyond the polar cap. The scheme achieved its primary goal of better-calibrated sea ice probabilities, and it did so while introducing mean state changes small enough — around 4 percent in concentration and 10 centimetres in thickness — that modest retuning could offset them if desired. As ECMWF prepares its next generation of seasonal and sub-seasonal systems, and as climate change continues to make Arctic sea ice both more variable and more consequential for midlatitude weather, the idea that noise, properly structured, is a form of signal may prove one of the more quietly important lessons in modern numerical weather prediction.

Subject of Research: Stochastic sea ice perturbation schemes for improving seasonal ensemble forecasts of sea ice and the midlatitude atmosphere

Article Title: The impact of stochastic sea ice perturbations on seasonal forecasts

Article References: Strommen, K., Mayer, M., Storto, A., Spaeth, J., & Tietsche, S. (2026). The impact of stochastic sea ice perturbations on seasonal forecasts. Weather and Climate Dynamics, 7(3), 1593-1618. https://doi.org/10.5194/wcd-7-1593-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1593-2026

Keywords: sea ice, seasonal forecasting, stochastic parameterisation, ensemble forecasts, ECMWF, Arctic, North Atlantic Oscillation, underdispersion, sea ice concentration, jet stream, weather and climate dynamics, coupled modelling

The team assembled its benchmark sample from an unusual and valuable archive: special warnings issued by the Peruvian Navy between 2008 and 2025. These operational records classify swell episodes by their expected coastal impact, with a very strong swell defined as waves exceeding three times their normal height and a strong swell between two and three times normal. From this record, the researchers identified five very strong events, all arriving during the austral winter months of May through September with pronounced south-westerly directions from the South Pacific, and six strong events originating in the North Pacific, mostly during boreal winter. Because the Navy archive is the only homogeneous operational classification available for the Peruvian coast, the authors treat these eleven cases as a benchmark sample rather than an exhaustive climatology.

A central challenge in studying remote swells is timing. The waves that batter a Peruvian beach were generated days earlier, sometimes on the far side of an entire ocean basin, so the atmospheric conditions that matter occurred long before the coastal impact. The researchers solved this by applying deep-water wave theory, calculating the group velocity of a swell packet from its wave period and the great-circle distance between the storm’s wind region and the coast. Using representative swell periods of 16 to 20 seconds, consistent with long-period swells previously reported for Peru, they estimated travel times of roughly three to four days for Southern Hemisphere events and eight to eleven days for Northern Hemisphere events. These windows then defined the periods over which they examined atmospheric fields from the ERA5 reanalysis.

The composite analysis of the Southern Hemisphere events revealed a strikingly organized pattern. Days before the swell peak, sea-level pressure anomalies form a tripolar structure over the southeast Pacific: a deep extratropical cyclone south of 40 degrees south, flanked by broad anticyclonic anomalies to its southwest and southeast. This arrangement dramatically tightens the meridional pressure gradient along the storm’s equatorward flank, driving a wide corridor of intensified westerly and south-westerly winds between roughly 35 and 55 degrees south. Crucially, the disturbance is barotropic, meaning the pressure anomalies at the surface, at 500 hectopascals, and at 250 hectopascals are nearly vertically aligned, allowing the storm to persist and sustain its winds over the same stretch of ocean. The result is a swath of enhanced significant wave height exceeding one meter across the high-latitude source region, with wave energy radiating directly toward Peru and Chile.

The jet stream diagnostics added a decisive layer to the picture. Applying a multiparametric method that measures not just jet intensity but also sharpness, tilt, and meridional coherence, the team found that Southern Hemisphere events coincide with a polar-front jet that is significantly stronger and sharper than the austral-winter climatology, with its core confined to a narrow latitude band and a modest southwest-to-northeast tilt. This compact, energized jet reinforces the storm-jet coupling that keeps the surface wind corridor locked in place over the swell-generation region. In the Northern Hemisphere, the configuration is different but equally coherent: the subtropical jet intensifies, sharpens, and extends longitudinally across the North Pacific, while the polar-front jet weakens and becomes less continuous. This redistribution of upper-level momentum supports deep, slow-moving cyclones over the central and western North Pacific, whose westerly to north-westerly winds can still fling wave energy across the Equator toward northern Peru.

The hemispheric contrast also explains why Southern Hemisphere events hit harder. Swell from the South Pacific travels a shorter path, arriving in three to four days with limited dispersion, and the tripolar pressure pattern sustains winds over an exceptionally long fetch. Northern Hemisphere swells must cross the entire Pacific and the Equator, a journey of eight to eleven days during which the wave energy spreads and attenuates. That is why, in the Navy’s classification, Southern Hemisphere events reached the very strong category while their Northern counterparts topped out at strong. Recent episodes illustrate the stakes: a May 2023 anomalous southwest swell caused coastal flooding and closed dozens of Peruvian ports, while a late-December 2024 swell from a North Pacific winter storm surprised tourist beaches and small ports on Peru’s northern coast under otherwise fair local weather.

Perhaps the most consequential finding concerns climate change. Because extreme swell records span only a few years, the team turned to a flow-analogue framework, searching the ERA5 record from 1950 to 2025 for days with atmospheric circulations dynamically similar to each event and comparing the surface winds associated with those analogues in past and present climate periods. Because circulation is held fixed across analogues, any difference in wind speed cannot be attributed to changes in the circulation pattern itself. For the Southern Hemisphere events, the result was unambiguous: analogous circulation patterns are now associated with significantly stronger surface winds than in the past, and the corresponding significant wave heights have increased as well. The one exception was event E3, which showed no significant influence of the dominant climate mode but may instead respond to the regional forcing of the Amundsen Sea Low.

The likely culprit behind this Southern Hemisphere intensification is the Southern Annular Mode, the leading mode of extratropical Southern Hemisphere variability. Analogue days in the recent period occurred predominantly under positive SAM phases, which strengthen and shift the westerly wind belt poleward, boosting windiness over the high-latitude South Pacific exactly where the swell-generating storms live. The trend toward more positive SAM conditions has been linked in the scientific literature to stratospheric ozone depletion and rising greenhouse gas concentrations, providing a plausible physical pathway from anthropogenic forcing to stronger swell-producing winds. The authors are careful to note that differences between past and present analogues may also reflect internal climate variability or other external factors, but the coherence of the signal across nearly all Southern Hemisphere events, supported by statistically significant wave-height increases, makes it physically compelling.

The Northern Hemisphere story is far messier. Analogue reconstructions revealed no consistent climate-change signal: the three most recent events showed stronger present-day winds, while the three earlier ones showed weaker winds, and no single mode of Pacific variability, whether the Pacific Decadal Oscillation, ENSO, the Pacific-North American pattern, the Arctic Oscillation, the Western Pacific pattern, or the Interdecadal Pacific Oscillation, could account for the split. The two groups of events appear governed by opposing configurations of large-scale climate modes, and regression analysis found these modes explain only a modest fraction of the wind variability, implying that local synoptic processes and factors such as aerosol changes, Arctic sea-ice variations, and subtropical tropospheric warming may also be at play. The high interannual and decadal variability of the North Pacific, the authors note, is precisely why the IPCC expresses low confidence in attributing observed changes in extratropical cyclones there to human influence.

The practical payoff of this work lies in early warning. Because the precursor patterns, a deep barotropic cyclone aligned with an energized jet, emerge days before coastal impact, monitoring these large-scale configurations could give Peruvian authorities substantially longer lead times than current approaches, allowing ports to close, fishing fleets to stay ashore, and coastal communities to prepare before the waves arrive. The authors caution that their sample is concentrated after 2010 and that extending the analysis further back will require reconstructing pre-2008 events. They also propose applying the flow-analogue framework to large ensembles of climate model simulations under historical and future forcing scenarios, which would allow a formal attribution of the circulation changes and a more robust assessment of how anthropogenic climate change will shape the swell hazards of Peru’s coastline in the decades ahead.

Subject of Research: Meteorological and jet-stream drivers of extreme remote swell events affecting the Peruvian coast

Article Title: Meteorological drivers of extreme swells on the Peruvian coast

Article References: Agurto Barragán, G., Collazo, S., & García-Herrera, R. (2026). Meteorological drivers of extreme swells on the Peruvian coast. Weather and Climate Dynamics, 7(3), 1779-1796. https://doi.org/10.5194/wcd-7-1779-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1779-2026

Keywords: extreme swells, Peru, jet stream, extratropical cyclones, ERA5 reanalysis, Southern Annular Mode, Pacific Decadal Oscillation, coastal flooding, wave climate, flow analogues, climate change attribution, early warning systems

The research team built their simulation by dynamically downscaling a Community Earth System Model run spanning 1821 to 2100 with the Weather Research and Forecasting model, known as WRF. The global model supplied the large-scale circulation, while WRF resolved the regional detail at 20-kilometre horizontal resolution with hourly output, 49 vertical levels and a domain covering the whole Euro-Atlantic area. This approach matters because global climate models, typically running at resolutions of 100 kilometres or more, struggle to capture the small, short-lived and topographically trapped cyclones of the Mediterranean, which are smaller and shallower than their Atlantic and Pacific cousins but capable of devastating impacts nonetheless.

To track the storms, the researchers applied a cyclone detection algorithm to the 850-hectopascal geopotential height field, regridded to one-degree resolution to filter out weak and unphysical lows over the region’s complex terrain. Cyclones had to show a minimum pressure gradient, persist for at least twelve hours and avoid grid cells above 1000 metres of orography. Around a thousand cyclones were identified per region per period. The team then defined extreme cyclones by the 95th percentile of six-hourly accumulated precipitation and 850-hectopascal wind speed within a 500-kilometre radius of each storm centre, ranking every system by how many standard deviations it departed from the mean.

The headline finding concerns frequency. Comparing the winter half-years of 1821 to 1880, a proxy for the pre-industrial climate, with those of 2039 to 2099 under the high-emission RCP8.5 scenario, the simulation shows a decline in mean cyclone frequency across nearly the entire Mediterranean. The drop is largest over the long-recognised hotspot stretching from Italy towards the Levant and over the Anatolian Plateau, where frequencies fall by up to 0.02 cyclones per day, a relative decrease of roughly one-third. Earlier studies with global models attributed this decline to reduced baroclinicity, increased static stability and a positive shift in the North Atlantic Oscillation, and the new regional results confirm that picture at much finer scale.

Frequency, however, is only half the story. The study reveals that precipitation extremes within cyclones peak about six hours before the storm reaches its minimum core pressure, while wind extremes coincide with the pressure minimum itself. In the western Mediterranean, future extreme precipitation cyclones intensify significantly during their most intense phase, with six-hourly rainfall increasing by roughly five millimetres north of the storm centre twelve hours after peak precipitation, a doubling compared with pre-industrial storms. In the eastern Mediterranean, by contrast, cyclone-related precipitation remains roughly unchanged, and the storms themselves shift southward over the warmer sea, a statistically significant migration that concentrates future hazards over open water.

The physical mechanism behind the western intensification lies in the vertical structure of the storms. Vertical cross-sections through the cyclone cores reveal so-called potential vorticity towers, columns of high potential vorticity air that extend through the entire troposphere, fed from above by stratospheric intrusions and from below by diabatically produced potential vorticity generated where condensation releases latent heat. In future western Mediterranean storms, equivalent potential temperature in the warm sector rises by up to nine degrees Celsius, reflecting a warmer and moister atmosphere, and potential vorticity values in the lower part of the tower increase by up to one unit, roughly fifty percent. This enhanced low-level potential vorticity strengthens the cyclonic circulation, allowing the storms to deepen more rapidly and remain more intense through their mature phase.

Wind extremes tell an even more consistent story. Unlike precipitation, wind speed extremes intensify in both the western and eastern Mediterranean. The explanation lies not in the low levels but aloft, in the behaviour of the jet stream. Composite analysis of the 200-hectopascal wind field shows that extreme wind cyclones typically form on the northern edge of the jet and, as they mature, migrate into the left exit region of a jet streak, the zone where upper-air divergence promotes rising motion and favours cyclone development. Under future conditions the subtropical jet strengthens by up to ten metres per second, so mature wind cyclones find themselves positioned beneath a substantially more powerful jet streak, a configuration that fuels their intensification.

The study also examined compound events, cyclones that simultaneously deliver extreme precipitation and extreme wind, whose combined damage can exceed the sum of the individual hazards. These compound storms cluster over the warm waters of the Tyrrhenian and Adriatic Seas and over northern Italy in the west, and over the Ionian and Aegean Seas in the east. Their future behaviour largely mirrors the individual extreme categories: precipitation in compound events follows the west-intensifies, east-unchanged pattern, while their wind response resembles that of pure wind extremes, with one notable exception. In the eastern Mediterranean, the wind speed of compound extreme cyclones actually decreases in the future, a divergence the authors cannot yet fully explain but which highlights how sensitive compound hazards are to regional dynamics.

Seasonality, remarkably, remains stable. Precipitation extremes peak in autumn when the sea is still warm enough to feed moisture into passing storms, wind extremes occur more evenly across the colder months, and compound events favour the early winter half-year. None of these patterns shifts significantly between the pre-industrial and future periods, meaning that societies will face the same seasonal rhythm of risk, only amplified. The authors validated their simulation against the ERA5 reanalysis for 1981 to 2010, finding that WRF captures the main cyclone hotspots and storm structures well, though it overestimates precipitation and wind intensity in the eastern Mediterranean, where it also produces more cyclones than observed.

The implications are sobering for a region already identified as a climate change hotspot. A drier Mediterranean with fewer storms might sound like relief, but the study shows that mean and extreme precipitation are decoupling: while average winter rainfall declines, the rain that does fall increasingly arrives in concentrated, violent bursts delivered by fewer but fiercer cyclones. Stronger winds across the whole basin compound the threat of coastal flooding, which these storms are known to drive. The authors caution that their analysis rests on a single simulation member, and they call for ensemble regional downscaling and kilometre-scale global models to firm up the projections. Even so, the message is clear: for the Mediterranean, the future of storm risk lies not in how often the cyclones come, but in how hard they hit when they do.

Subject of Research: Projected changes in extreme Mediterranean cyclones and their wind, precipitation and compound hazards under climate change

Article Title: Mediterranean cyclones from pre-industrial to future climate: changes in extreme wind, precipitation and compound precipitation–wind events

Article References: Doensen, O., Messmer, M., Dolores-Tesillos, E., & Raible, C. C. (2026). Mediterranean cyclones from pre-industrial to future climate: changes in extreme wind, precipitation and compound precipitation–wind events. Weather and Climate Dynamics, 7(3), 1899-1917. https://doi.org/10.5194/wcd-7-1899-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1899-2026

Keywords: Mediterranean cyclones, climate change, extreme precipitation, extreme wind, compound events, RCP8.5, regional climate modelling, WRF, potential vorticity, jet stream, cyclogenesis, Weather and Climate Dynamics

The motivation for the work lies in one of the most persistent puzzles in climate science. Observations show that since 1979 the Arctic has warmed nearly four times faster than the global average, a rate so extreme that it has reshaped the temperature contrast between the pole and the equator. Because that temperature gradient is the engine that drives the midlatitude jet stream, the fast-moving river of air that steers storms and separates cold polar air from warmer subtropical air, many scientists have suspected that Arctic amplification must be altering the position, strength, and waviness of the jet. Yet the evidence linking the two has remained contested, with some studies pointing to connections with extreme cold outbreaks and prolonged droughts, and others finding the signals too weak or too model-dependent to be meaningful.

Part of the difficulty is that Arctic amplification is not driven by a single cause. Rising sea surface temperatures warm the planet broadly, while the loss of reflective sea ice exposes dark ocean water that absorbs sunlight and warms the Arctic locally. Previous work has shown that sea ice loss plays a central role in polar warming, but disentangling its effects from those of simultaneous ocean warming requires experiments in which each factor is varied in isolation. That is precisely what Saurral and Kucharski set out to do. Using an intermediate-complexity atmospheric model, a class of models that strips away some of the computational complexity of full climate models while retaining the essential physics of the atmosphere, they ran a suite of simulations in which sea surface temperatures and sea ice concentrations were modified separately and together, allowing them to isolate the fingerprint of each forcing.

The results reveal a striking division of labor between the two drivers. Warming sea surface temperatures dominate the temperature response at low and midlatitudes, spreading warmth across the subtropics and temperate zones, while the loss of sea ice is the primary control on how much the Arctic itself warms. In other words, the global ocean sets the background of the warming world, and the cryosphere determines how extreme the polar amplification becomes on top of it. Neither factor alone can reproduce the observed magnitude of polar amplification: when the researchers imposed sea ice loss on its own, the model overestimated Arctic warming, and when they imposed sea surface temperature changes on their own, the model underestimated it. Only when both forcings were applied together did the simulation produce a realistic picture of the amplified Arctic that observations describe.

Perhaps the most consequential finding concerns the jet stream. The two forcings produced circulation responses that push in opposite directions. Changes in sea surface temperatures drove a poleward shift of the atmospheric jet, consistent with the widening of the tropical circulation and the expansion of the Hadley cell that many studies have documented under global warming. Sea ice loss, by contrast, induced changes in the jet that ran counter to that shift. When the two forcings were combined in a single experiment, their interaction produced a reduced and less robust atmospheric response than either forcing alone, suggesting that the competing influences partially cancel each other out. This cancellation helps explain why different modeling studies, which may weight these forcings differently, have reached such different conclusions about how the midlatitude circulation responds to Arctic change.

The physical logic behind the opposing responses is rooted in how each forcing alters the temperature gradient that sustains the jet. Broad ocean warming raises temperatures most strongly in the tropics and subtropics, steepening the equator-to-pole contrast in the upper troposphere and encouraging the storm tracks and the jet to migrate poleward. Sea ice loss does the opposite at the polar end of the gradient: by warming the lower Arctic atmosphere, it weakens the contrast between the pole and the midlatitudes near the surface, a change that tends to pull the circulation in the other direction. The new experiments show that in a medium-complexity model these two effects do not simply add up, and that their interference is a genuine feature of the coupled response rather than noise.

These findings arrive in the middle of a long-running scientific controversy. An influential hypothesis proposed in 2012 suggested that Arctic amplification would make the jet stream wavier, producing more persistent weather patterns and more frequent extremes such as cold winters and heat waves in the midlatitudes. Subsequent research, including large coordinated model intercomparisons under the Polar Amplification Model Intercomparison Project, has generally found the circulation response to sea ice loss to be robust but weak, and several studies have concluded that Arctic amplification has little influence on midlatitude climate variability. The new work does not settle that debate, but it offers a physical framework for interpreting it: the midlatitude response to climate change reflects the combined effect of global processes, carried by ocean warming, and regional processes, carried by sea ice loss, and the two can work against each other.

The study also carries a methodological message. In an era when climate science increasingly relies on enormous high-resolution simulations, Saurral and Kucharski demonstrate the continuing value of medium-complexity models. Because such models are computationally inexpensive, researchers can run large ensembles of targeted experiments, varying one boundary condition at a time, and thereby isolate mechanisms that would be difficult to separate in comprehensive coupled models where ocean, ice, and atmosphere evolve together. The authors argue that this experimental clarity is exactly what is needed to interpret polar amplification as the joint product of global and regional processes, and to understand why the atmospheric response to Arctic change has proven so elusive in more complex systems.

The implications extend to how scientists project the future. If sea surface temperature changes and sea ice loss produce opposing circulation fingerprints, then the net response of the jet stream and the storm tracks will depend on the balance between them, and that balance may shift as the climate continues to warm. Observations have already documented trends in the jet streams that some researchers have linked to tropical warming, and the new results suggest that any such trend represents the residual of competing influences rather than the effect of a single driver. For regions such as the Mediterranean, where earlier work by some of the same authors has connected sea ice loss to drought conditions, understanding this balance could sharpen assessments of regional climate risk.

What the study ultimately provides is a cleaner conceptual map of a tangled problem. Arctic amplification emerges only when global ocean warming and regional sea ice loss act together, the two leave distinct and partly opposing marks on the midlatitude circulation, and their interaction dampens the overall atmospheric response. As the Arctic continues to transform, experiments of this kind, run in deliberately simplified models where every forcing can be switched on and off at will, may prove to be among the most powerful tools for reading the fingerprints that a changing pole is leaving on the weather of the middle latitudes.

Subject of Research: The separate and combined effects of sea surface temperature warming and Arctic sea ice loss on midlatitude atmospheric circulation in an intermediate-complexity global atmospheric model

Article Title: Arctic amplification fingerprints on the midlatitude circulation in a medium-complexity global atmospheric model

Article References: Saurral, R. I., & Kucharski, F. (2026). Arctic amplification fingerprints on the midlatitude circulation in a medium-complexity global atmospheric model. Climate Dynamics, 64(10), Article 415. https://doi.org/10.1007/s00382-026-08373-z

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08373-z

Keywords: Arctic amplification, sea ice loss, sea surface temperatures, jet stream, midlatitude circulation, climate dynamics, medium-complexity model, polar warming, Hadley cell, storm tracks, climate modeling, atmospheric circulation

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 jet stream is a narrow, meandering current of air near the tropopause, the boundary between the troposphere and the stratosphere, typically located several kilometres above the surface and flowing from west to east at speeds that can exceed 50 metres per second in winter. Over the Atlantic–European sector, this current is maintained by the temperature contrast between the warm subtropical ocean and the cold polar region, a contrast that generates pressure gradients at upper levels of the atmosphere. Because the jet acts as a kind of atmospheric conveyor belt, guiding storm systems from the ocean toward the continent, even modest shifts in its strength or position can reshape winter weather across Britain, Scandinavia, central Europe and the Mediterranean.

Understanding how the jet responds to greenhouse warming has been one of the more contested questions in climate dynamics. One influential line of argument holds that rapid Arctic amplification—the fact that the high northern latitudes are warming several times faster than the globe as a whole—reduces the equator-to-pole temperature gradient near the surface. A weaker gradient, in this view, should sap the energy available to the jet, producing weaker winds, larger meanders and a greater tendency toward persistent blocking patterns that lock cold air over Europe for weeks at a time. Another line of argument emphasizes that the upper troposphere warms more strongly than the surface in the tropics and mid-latitudes, which increases the vertical shear of the winds and can actually strengthen the jet aloft while shifting it poleward.

The new analysis addresses this debate by looking not only at the average behaviour of the Atlantic–European jet in winter but at the full distribution of its variability, including the extreme tail. Using observational and reanalysis datasets—long records of atmospheric conditions reconstructed from weather observations and satellite data—the researchers tracked jet strength over recent decades and examined how frequently the jet reached its most intense values. The central result is a nuanced one: the typical winter jet has become stronger, consistent with the expectation that upper-level warming and changes in the temperature gradient enhance the mean flow, but the extremes of jet strength have not kept pace. In relative terms, the jet is becoming a steadier current rather than a more violent one.

This distinction between the mean and the extremes matters for how scientists and society interpret jet stream change. A stronger mean jet is generally associated with a more vigorous storm track, which can bring more frequent passages of Atlantic low-pressure systems and the mild, wet, windy weather they deliver to northwestern Europe. Extreme jet episodes, by contrast, are often linked to exceptional weather: an unusually intense jet can coincide with explosive cyclogenesis, in which storms deepen rapidly and cause damaging windstorms, while an unusually weak or displaced jet can coincide with prolonged cold spells or drought. If the mean strengthens while the extremes moderate, the net effect on hazardous winter weather may be less dramatic than either a simple strengthening or a simple weakening story would suggest.

The physical reasoning behind such a divergence can be traced to how different components of the climate system respond to warming. The mean jet strength is governed largely by broad-scale temperature gradients and by the vertical structure of tropospheric warming, both of which evolve smoothly and predictably with rising greenhouse gas concentrations. Extreme jet episodes, however, often depend on transient processes: the phasing of planetary-scale Rossby waves, the life cycles of individual baroclinic storms, and episodic coupling with the stratospheric polar vortex. Some of these transient drivers may weaken or become less variable under warming, damping the upper tail of the jet distribution even as the background flow intensifies. The study’s results are consistent with such a mechanism, in which variability about the mean contracts even as the mean itself rises.

The findings also speak to a long-running scientific controversy about Arctic influence on mid-latitude weather. Over the past decade, a vigorous debate has played out in the journals over whether sea-ice loss and Arctic warming make European winters more prone to blocking and severe cold. Proponents of this view point to episodes such as the cold winters of recent decades and to modelling experiments in which reduced sea ice favours a wavier jet. Critics counter that the observational record is short, that internal variability is large, and that model projections more often show a strengthened, poleward-shifted jet with reduced waviness in winter. By documenting a strengthening yet less extreme jet, the new work lends weight to the second camp, while underscoring that the answer may differ by season, by region and by which aspect of the jet is measured.

Methodologically, the study illustrates the value of going beyond simple averages when assessing climate change signals. Jet stream behaviour is notoriously noisy: individual winters can differ enormously, and the Atlantic–European sector in particular is influenced by modes of variability such as the North Atlantic Oscillation, which swings between phases that strengthen or weaken the westerlies on timescales from weeks to decades. Separating a forced climate signal from this internal noise requires careful statistical treatment, long datasets and, in many cases, large ensembles of climate model simulations in which many parallel realizations of the same warming scenario are run to isolate the common response. The researchers’ focus on the distribution of jet strength, rather than a single metric of average speed, allows a more complete picture of how the flow is evolving and reduces the risk of drawing conclusions from a few memorable extreme winters.

The implications extend to practical forecasting and adaptation. Winter storm risk assessment for Europe depends on assumptions about how often severe windstorms strike, and those assumptions are typically based on historical statistics. If the character of the jet is changing—stronger on average but with moderated extremes—then the historical record may be an imperfect guide to the coming decades. Insurers, infrastructure planners and emergency services all have an interest in knowing whether the tail risks of winter weather are growing or shrinking. The new results suggest that for jet-driven wind extremes, at least, the most catastrophic outcomes may not intensify as rapidly as the mean conditions do, although the researchers caution that other hazards, such as precipitation extremes associated with a moister, warmer atmosphere, continue to worsen independently of the wind field.

There remain open questions. The observational record of the upper atmosphere is only a few decades long, and reanalysis products carry uncertainties, particularly in the earlier satellite era and in the data-sparse regions of the North Atlantic. Disentangling the roles of tropical upper-tropospheric warming, Arctic amplification, stratospheric variability and ocean circulation in shaping the jet will require further modelling work. Nevertheless, the study offers a clear and somewhat reassuring refinement of the picture: the Atlantic–European winter jet is not collapsing into weakness and chaos, nor is it becoming uniformly more ferocious. Instead, it is strengthening as a background current while its wildest excursions become relatively less extreme—a reminder that climate change rarely moves every aspect of a system in the same direction, and that the most useful science is often found in the details between the averages and the extremes.

Subject of Research: Winter strengthening and reduced extremity of the Atlantic–European jet stream

Article Title: A strengthening yet less extreme Atlantic–European jet during winter

Article References: Brönnimann, S., Brugnara, Y., & Kallabis, P. (2026). A strengthening yet less extreme Atlantic–European jet during winter. Nature Geoscience. https://doi.org/10.1038/s41561-026-02069-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02069-z

Keywords: jet stream, Atlantic–European jet, winter climate, North Atlantic Oscillation, Arctic amplification, storm track, climate variability, reanalysis, stratosphere, extreme weather, climate change, geoscience