The team’s focus was the Atlantic Multidecadal Variability, or AMV, a slow oscillation in North Atlantic sea surface temperatures that swings between warm and cool phases over decades. When the AMV is in its positive phase, a characteristic ‘horseshoe’ pattern emerges at the ocean surface: the subpolar gyre south of Greenland warms strongly, and a band of warm water spreads across the tropical North Atlantic, lagging slightly behind. Decades of research have linked this pattern to European summer climate, yet the observational record of such multidecadal variability is barely 45 to 70 years long — far too short to cleanly separate the tropical and extratropical contributions to the phenomenon using observations alone.

To overcome that fundamental limitation, the researchers turned to one of the largest decadal prediction experiments ever assembled: an 80-member hindcast ensemble built with the low-resolution Max Planck Institute Earth System Model, MPI-ESM-LR. The members were initialised every November from 1960 to 2019, with the ocean nudged toward the EN4 ocean reanalysis through an ensemble Kalman filter and the atmosphere constrained toward observed conditions. Each member was run for at least ten years beyond its start date, and a subset of sixteen members was extended to twenty. By comparing this initialised ensemble with fifty uninitialised historical runs from the MPI Grand Ensemble, the team could isolate exactly what initialisation — the act of telling the model what the ocean is doing right now — buys in terms of predictive skill.

The first result concerns the ocean itself. In the uninitialised historical simulations, the model reproduced the horseshoe pattern of AMV sea surface temperatures only weakly, with the subpolar gyre signal considerably underdone and displaced toward the south and east. In the initialised hindcasts, by contrast, the subpolar gyre anomalies were dramatically stronger and closely matched the observations from the HadISST dataset and the ERA5 reanalysis. When the researchers defined AMV phases using ERA5 rather than the model’s own index, the strong subpolar signal survived intact across forecast lead years one to seven — meaning those temperatures are highly predictable — while the tropical signal essentially vanished, revealing that AMV-driven tropical SST anomalies are poorly predicted at these lead times.

That contrast matters enormously because the two halves of the AMV drive European summer weather through completely different atmospheric pathways. The extratropical pathway is grounded in classic theory: warm subpolar sea surface temperatures act as a diabatic heat source, and the atmosphere responds with a shallow cyclonic circulation a quarter-wavelength downstream, producing a low pressure anomaly west of Great Britain and Ireland. This feature, closely related to the East Atlantic Pattern of variability, advects warm air polewards and has been linked to hotter summers in central Europe. The tropical pathway is more exotic: warm tropical Atlantic waters generate upper-level divergence, launching Rossby waves — vast undulations in the jet stream — that arc from the Caribbean toward the extratropical North Atlantic and Europe.

When the team examined sea level pressure composites, a striking divergence appeared. Observations from HadSLP and ERA5 showed the expected negative pressure anomaly west of Britain and Ireland during positive AMV summers, but the uninitialised historical ensemble produced almost nothing in the extratropics, even though its tropical response was comparable to reality. The initialised hindcasts, however, did produce the cyclonic anomaly — in the right place, but with an amplitude notably weaker than observed. Crucially, when the hindcasts were evaluated against ERA5-defined AMV phases, this extratropical signal persisted, indicating genuine predictability. Correlation skill maps confirmed it: the highest skill for multi-year mean sea level pressure sat precisely off the west coast of Europe, where the AMV response is strongest and statistically significant.

Yet the weakness of the modelled response opened a door to one of climate science’s most perplexing puzzles: the signal-to-noise paradox. In a well-behaved forecast system, the correlation between the ensemble mean and observations should be no higher than the average correlation between the ensemble mean and its individual members. In the North Atlantic-Europe sector, this is routinely violated — the real atmosphere appears more predictable than the models themselves suggest. By sampling 100,000 single-member timeseries of East Atlantic pressure against the ERA5 AMV index, the researchers found that the observed regression slope was stronger than 99 percent of the hindcast samples, and the ratio of predictable components exceeded two with high significance. Most remarkably, the joint distribution revealed a correlation of minus 0.50 between the weak AMV response and the paradox’s diagnostic errors, directly linking the underestimated ocean-driven signal to the reliability failure.

The story grew richer still when the researchers climbed into the upper troposphere at 200 hectopascals. In reanalyses, the surface cyclonic response extends upward as a deep, near-equivalent-barotropic column over the eastern Atlantic. In the model simulations, however, the dominant upper-level response instead resembles a Rossby wave train emanating from the Caribbean — the tropical pathway. By varying the length of the rolling averaging window from one to fifteen years, the team showed why: tropical Atlantic SSTs carry relatively more interannual variability, while extratropical SSTs dominate on longer timescales, and the correlation between the two basins strengthens with window length. On interannual timescales, both models and reanalyses agree on the Caribbean wave train; on decadal timescales, reanalyses shift toward the extratropical response while the model, whose surface response is too weak, remains dominated by the tropical mechanism.

The implications are profound. The study demonstrates that dynamically driven, multi-year predictability for European summers genuinely exists — rooted in ocean temperatures that models can observe and initialise — and that large ensembles are essential to realise it. But it also shows that the extratropical diabatic heating response, though simulated, is severely underestimated, and that this deficiency cannot simply be fixed by adding more ensemble members or post-processing the output, because the two competing mechanisms interfere destructively in the upper troposphere. The authors point toward higher atmospheric and oceanic resolution, better representation of ocean-atmosphere coupling and eddy feedback, and targeted bias reduction as the most promising routes forward. Until the signal-to-noise paradox is solved, they conclude, large-ensemble forecasts with appropriate correction will be necessary to deliver the summer predictions that a continent increasingly vulnerable to extreme heat urgently needs.

Subject of Research: Decadal predictability of European summer climate through Atlantic Multidecadal Variability mechanisms in large-ensemble climate model hindcasts

Article Title: Predictability of European summer climate: the influence of competing mechanisms related to Atlantic Multidecadal Variability

Article References: Williams, N. C., Müller, W. A., & Pinto, J. G. (2026). Predictability of European summer climate: the influence of competing mechanisms related to Atlantic Multidecadal Variability. Earth System Dynamics, 17(4), 1007-1023. https://doi.org/10.5194/esd-17-1007-2026

Image Credits: AI Generated

DOI: 10.5194/esd-17-1007-2026

Keywords: Atlantic Multidecadal Variability, decadal prediction, European summer climate, sea surface temperature, subpolar gyre, Rossby waves, signal-to-noise paradox, MPI-ESM-LR, climate models, North Atlantic, predictability, diabatic heating

Atmospheric blocking occurs when a persistent anticyclone, or high-pressure ridge, parks itself over a region and deflects the usual eastward march of weather systems. Blocks are among the most consequential features of the extratropical circulation, yet predicting them more than two weeks ahead has long been considered near the edge of what is possible. The challenge is especially acute during transitional seasons, when the large-scale circulation is shifting rapidly between winter and summer regimes and the background flow is unusually volatile. The May–June 2023 sequence offered the researchers a rare natural experiment: three successive blocking episodes over Canada, the Ural Mountains, and Europe, each tied to a distinct surface disaster.

To untangle the dynamics, the team combined ERA5 reanalysis data, the Tibaldi–Molteni blocking index for identifying large-scale blocks, and a relatively new diagnostic technique based on the Hilbert transform. This method allows scientists to compute local wave parameters—phase speed, amplitude, and zonal wavenumber—at every grid point and every moment, rather than relying on hemispheric averages that wash out regional detail. They supplemented this with the Takaya–Nakamura wave activity flux, which traces the horizontal propagation of quasi-stationary Rossby wave energy, and with the stationary wavenumber, a theoretical reference scale at which waves tend to become quasi-stationary in a given background flow.

The diagnostics revealed a clear chain of causation in the wave energy budget. During the first episode, from 1 to 19 May, wave activity flux originating over the North Pacific converged over Canada, sustaining a strong quasi-stationary ridge there. When that Canadian blocking collapsed in mid-May, its energy did not simply dissipate; instead, it dispersed downstream toward the Ural Mountains and Europe, promoting the establishment of the ridges that dominated the second episode from 20 May to 5 June. In the third episode, from 6 to 20 June, wave energy propagating across the North America–North Atlantic sector split into two branches, one heading southeast toward the northwestern Atlantic and the other northeast via Greenland, and both converged over Europe, where a new blocking event formed between 11 and 16 June.

The local wave parameters told an equally striking story. All three blocks were dominated by planetary-scale Rossby waves with zonal wavenumbers of roughly 2 to 4, about one wavenumber lower than the climatological average. Because larger-scale disturbances feel the scale effect—broad waves propagate westward relative to the mean flow more readily than narrow ones—these anomalously low wavenumbers translated into exceptionally slow phase speeds, sometimes even westward motion. The result was a circulation that barely moved for weeks, baking Canada, funneling cold air southward into East Asia alongside the Ural ridge and the East Asian trough, and holding Europe under sustained subsidence and clear skies that marine heatwave feedbacks further amplified.

Perhaps most remarkable were the abrupt regime transitions the diagnostics exposed. At the onset of each block, the wave field flipped within just a few days from eastward-propagating, small-amplitude synoptic-scale waves—wavenumbers above six—to quasi-stationary or westward-propagating, large-amplitude planetary-scale waves. Phase speed dropped sharply, in the Canadian case from more than 3 meters per second to negative values, while amplitude surged past one standard deviation and the wavenumber fell below the stationary value. During decay, the sequence ran in reverse. Comparing the diagnosed wavenumber with the stationary wavenumber provided a useful rule of thumb: blocks stayed put when the two were similar, moved westward when the wavenumber was smaller, and resumed rapid eastward progression once it exceeded that threshold.

The second half of the study turned to forecast skill, evaluating the 50-member real-time ensemble of the ECMWF subseasonal-to-seasonal prediction system. The headline finding is genuinely encouraging: for all three episodes, forecasts initialized 15 to 19 days ahead showed high predictability, with 40 to 65 percent of members correctly predicting 500 hPa geopotential height anomalies exceeding one standard deviation and more than 80 percent predicting above-normal heights. Predictability dipped for forecasts made during the following week before recovering strongly at lead times of 2 to 5 days. The Ural episode proved the most skillful, with the best ensemble members reproducing both the intensity and duration of the ridge.

What separated successful forecasts from failures was not luck but wave dynamics. Comparing the ten strongest and ten weakest members for each episode, the researchers found that good forecasts captured the upstream quasi-stationary troughs—a trough over the North Pacific for the Canadian block, and one over the North Atlantic for the Ural and European blocks—along with the subsequent downstream energy dispersion that built the planetary-scale trough-ridge patterns. Poor forecasts, by contrast, let synoptic-scale waves run eastward along the subtropical or mid-latitude jet without ever amplifying into a blocking pattern, and their predictions drifted back toward climatology. Even the best members underestimated the amplification of wave amplitude and spatial scale, particularly for the Canadian and European blocks, a systematic shortfall that points to a concrete target for model improvement.

The study also uncovered a window of opportunity with real operational implications. Because the three blocks were dynamically chained, getting the first one right paid dividends downstream. In forecasts initialized on 17 April, members that correctly predicted the strong Canadian ridge went on to anticipate a stronger Ural ridge in late May, at a lead time exceeding 33 days, while members that missed the Canadian block also missed the Ural block. In other words, the collapse of the Canadian blocking and the associated shift toward a North Atlantic Oscillation-like regime acted as a predictable precursor, extending the horizon of useful guidance well beyond the conventional two-week barrier.

The authors are careful to note the limits of their analysis. The role of vertical planetary wave propagation, stratospheric processes, and lower-boundary forcing remains unexplored, and three case studies cannot establish how generally the wave-parameter transitions apply across seasons and regions. Future work, they suggest, should extend the Hilbert-transform diagnostics to larger blocking samples and probe why models consistently fail to amplify wave amplitude and scale to observed levels. Still, the central message is clear and actionable: the key to subseasonal prediction of persistent, high-impact blocking lies in correctly capturing upstream Rossby wave precursors and the quasi-stationary energy they dispatch downstream. For forecasters and disaster planners alike, the atmosphere’s worst weeks may be legible weeks in advance—if one knows where to look for the wave train.

Subject of Research: Subseasonal predictability and Rossby wave dynamics of atmospheric blocking events in May–June 2023

Article Title: Subseasonal predictability and Rossby wave dynamics of blocking high during transitional seasons: insights from three successive events in May–June 2023

Article References: Li, Z., Lu, J., & Liu, Y. (2026). Subseasonal predictability and Rossby wave dynamics of blocking high during transitional seasons: insights from three successive events in May–June 2023. Weather and Climate Dynamics, 7(3), 1821-1836. https://doi.org/10.5194/wcd-7-1821-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1821-2026

Keywords: atmospheric blocking, Rossby waves, subseasonal prediction, wave activity flux, Canadian wildfires, European heatwave, East Asian precipitation, ECMWF S2S, ERA5 reanalysis, phase speed, planetary waves, transitional seasons

The study, published in the journal Solar Physics, was led by Partha Chowdhury of the University of Calcutta together with Jagdev Singh, V. Muthu Priyal, and Belur Ravindra of the Indian Institute of Astrophysics in Bengaluru. The researchers analyzed digitized images taken in the light of singly ionized calcium, the Ca ii K spectral line, from the Kodaikanal Solar Observatory. These images capture the chromosphere, a thin layer of the solar atmosphere sitting just above the visible photosphere, where magnetic fields heat the gas and make bright regions known as plages blaze in ultraviolet-adjacent wavelengths. Plages are the chromospheric ghosts of sunspots: where magnetic flux crowds the surface, the calcium emission brightens, even when no dark spot is present.

Because plages are visible even when sunspots are not, they offer a more complete census of the Sun’s magnetic activity than sunspot counts alone. The Kodaikanal archive, spanning Solar Cycles 14 through 22, roughly the first nine decades of the twentieth century, is one of the longest continuous records of chromospheric behavior anywhere in the world. After careful digitization and homogenization, a process that corrects for changes in photographic plates, instruments, and observing conditions over the decades, the team measured the total area covered by plages on every available day and compared it with three standard yardsticks of solar activity: the international sunspot number, the total sunspot area, and the 10.7 centimeter radio flux, a microwave emission that tracks coronal magnetic heating.

The correlations the team found are strikingly strong. Across all nine cycles, the chromospheric plage area tracks each of the photospheric and coronal activity indices with correlation coefficients exceeding 0.85, a level of agreement that leaves little doubt that the Sun’s upper and lower atmospheric layers are driven by the same underlying magnetic engine. This matters for more than just bookkeeping. Because plage records extend further back in time than some modern instruments, they can serve as a reliable proxy for reconstructing solar activity across the twentieth century, including the ultraviolet output that subtly influences Earth’s upper atmosphere and climate system.

But the study went beyond simple correlations. The researchers examined whether the plage areas obey two well-known but poorly understood features of the solar cycle. The first is the Gnevyshev-Ohl rule, an empirical oddity noting that in most pairs of consecutive solar cycles, the odd-numbered cycle is stronger than the even-numbered one that precedes it. The team confirmed that plage areas follow this rule, suggesting that the chromosphere inherits the same cycle-to-cycle memory that governs sunspot production. The second feature is the Gnevyshev gap, a temporary dip in activity that often splits the peak of a solar cycle into two humps. The analysis revealed clear Gnevyshev gaps in the plage data during several cycle maxima, confirming that this double-peaked structure is not merely a sunspot quirk but a genuine, whole-Sun phenomenon imprinted on the chromosphere as well.

To dig deeper into the timing of these variations, the team turned to a powerful mathematical technique called Morlet wavelet analysis, which can detect oscillations whose strength and period change over time, something a traditional Fourier analysis cannot do. They supplemented this with wavelet coherence, a method that measures not just whether two signals share a rhythm, but whether those rhythms stay in step with each other, drifting in and out of phase like two musicians who occasionally lose the beat. The technique is widely used in geophysics and climate science, and it is ideally suited to the Sun, whose magnetic activity is famously irregular.

The wavelet analysis uncovered two families of intermediate-term periodicities hiding inside the dominant eleven-year cycle. The first are Rieger-type periods, oscillations of roughly 130 to 190 days. These were first discovered in 1984, when researchers noticed that gamma-ray flares from the Sun seemed to cluster with a period of about 154 days. Since then, similar periodicities have been found in sunspots, flares, and coronal mass ejections, and they are widely interpreted as the signature of magnetic Rossby waves, vast, planet-scale waves of magnetized plasma rolling around the Sun’s interior shear layer known as the tachocline, where the Sun’s differential rotation winds up its magnetic field. The second family comprises quasi-biennial oscillations, or QBOs, with periods of one to four years, which are thought to reflect a secondary magnetic cycle operating in parallel with the main eleven-year one.

Here the story takes an intriguing turn. The Rieger-type periods and QBOs show considerable variability from one solar cycle to the next, and the wavelet coherence analysis reveals that at these intermediate timescales, the chromospheric plage areas and the photospheric activity indices frequently fall out of phase with one another. The rhythms exist, but they do not always march together. This asynchrony suggests that the shorter-period variations may arise from processes that affect the chromosphere and photosphere differently, or from instabilities in the tachocline whose surface manifestations depend on the details of each individual cycle, including the strength of the interior magnetic field at the time.

At the fundamental scale, however, the picture is one of remarkable unity. Within the 9 to 12 year periodicity belt that defines the solar cycle itself, the team found a stable phase synchrony between chromospheric plages and every photospheric and coronal index they examined. In other words, over the long haul, the Sun’s chromosphere and its visible surface rise and fall together like two ends of the same magnetic tide. This tight decadal coupling is exactly what modern solar dynamo models predict: magnetic fields generated by the rotation of plasma at the tachocline buoyantly rise through the convection zone, emerge as sunspots and active regions in the photosphere, and simultaneously light up the overlying chromosphere as plages. The new results provide hard empirical constraints that any credible dynamo model must now reproduce.

The findings also align with theoretical work on tachocline instabilities and magnetic Rossby waves, which have been invoked to explain everything from the double-peaked shape of solar maxima to the so-called seasons of space weather, bursts of intense flaring that come and go over months. By confirming Rieger-type and quasi-biennial signals in a chromospheric dataset spanning nine cycles, the study strengthens the case that these waves are a persistent feature of the solar interior rather than a fluke of a few well-observed cycles. For space weather forecasters, the implications are tantalizing: if the phase relationships between these periodicities and the activity indices can be pinned down well enough, the hidden rhythms of the tachocline might one day help anticipate the timing of the most active and hazardous phases of the solar cycle. For now, the century-old glass plates of Kodaikanal have once again proven that some of the best windows into the Sun’s deepest secrets were captured long before the space age began.

Subject of Research: Long-term chromospheric plage evolution and its relationship to solar activity indices across Solar Cycles 14 to 22

Article Title: Long-term Evolution of Chromospheric Plage Areas from Kodaikanal Observatory Ca ii K Images and Their Relation to Solar Activity Indices

Article References: Chowdhury, P., Singh, J., Priyal, V. M., & Ravindra, B. (2026). Long-term Evolution of Chromospheric Plage Areas from Kodaikanal Observatory Ca ii K Images and Their Relation to Solar Activity Indices. Solar Physics, 301(10), Article 149. https://doi.org/10.1007/s11207-026-02733-y

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02733-y

Keywords: solar physics, chromosphere, Ca ii K, solar plages, solar cycle, sunspots, Rieger periodicity, quasi-biennial oscillations, solar dynamo, tachocline, Rossby waves, Kodaikanal Observatory

The East Asian Summer Monsoon is one of the most consequential climate systems on the planet, delivering the bulk of annual precipitation to China, Korea, and Japan and sustaining agriculture for well over a billion people. When the monsoon misbehaves, the consequences are measured in flooded cities, submerged farmland, and enormous economic losses. The historically extreme summers of 1998, 2016, and 2020, each marked by devastating Yangtze flooding, have long been attributed largely to the lingering influence of El Niño events in the tropical Pacific. The new analysis does not overturn that picture, but it substantially enriches it by identifying the warm pool core as an indispensable intermediary in the chain of cause and effect.

To define their region of interest, the researchers drew a sharp physical boundary: the warm pool core area encompasses the waters where sea surface temperature consistently exceeds a critical threshold of 29.4 degrees Celsius. This is not an arbitrary contour. Above such temperatures, the atmosphere sits atop an enormous reservoir of latent energy, and the ocean exerts an especially strong grip on the convection, cloudiness, and large-scale circulation above it. By focusing on this core rather than the warm pool as a whole, the team isolated the portion of the western Pacific where ocean-atmosphere coupling is most vigorous and where small temperature anomalies carry the largest atmospheric consequences.

The study combined observational analyses with dynamical diagnostics, drawing on the Met Office Hadley Centre HadISST1 sea surface temperature dataset, the NCEP-NCAR atmospheric reanalysis, ERA5 moisture flux data, satellite-derived outgoing longwave radiation as a proxy for deep convection, and three independent precipitation products including the Chinese 160-station in-situ network. The team also employed a dynamical normalized seasonality monsoon index to quantify monsoon strength and used a horizontal Rossby wave ray-tracing technique to follow the pathways of large-scale atmospheric waves across the Indo-Asia-Pacific domain. Numerical experiments with the Isca atmospheric model framework complemented the observational work, allowing the researchers to isolate the response of the atmosphere to warm pool forcing.

What the analysis revealed is a coherent and rather elegant mechanism. When the warm pool core experiences strong positive sea surface temperature anomalies, local convective activity paradoxically weakens. The suppression of convection over the core area induces an anomalous anticyclone — a clockwise circulation in the lower atmosphere — over the western North Pacific. This western North Pacific anomalous anticyclone is a well-known player in monsoon variability, but the new study shows how it interacts with a second anomalous anticyclone over northern China and a cyclonic circulation east of Japan. Together, these features assemble into a cyclonic shear belt, a band of旋转 vorticity stretching from the middle and lower Yangtze reaches through South Korea to Japan, precisely along the track of the Meiyu-Changma-Baiu rainband.

Along this shear belt, the study documents positive moisture flux anomalies and negative divergence anomalies, technical signatures indicating that water vapor from the South China Sea and the western Pacific converges and rises. Moisture convergence plus ascent is the fundamental recipe for heavy rainfall, and its intensification along the Meiyu-Changma-Baiu front translates directly into enhanced precipitation over the Yangtze valley and neighboring regions. Weak warm pool core events produce the mirror-image situation: suppressed convergence, weakened ascent, and reduced summer rainfall along the same corridor. The symmetry of the response strengthens confidence that the warm pool core is genuinely driving these circulation changes rather than merely coinciding with them.

Beyond the immediate circulation response, the researchers traced how warm pool core temperature anomalies modulate the East Asian Summer Monsoon through several interacting channels. These include the local meridional-vertical circulation, the overturning cell that links tropical convection to subtropical dynamics; the Western Pacific Subtropical High, the semi-permanent high-pressure system whose western edge steers moisture and storm tracks into East Asia; the western North Pacific anomalous anticyclone itself; and Rossby wave trains propagating along the Indo-Asia-Pacific teleconnection pathway. Rossby waves are planetary-scale undulations in the atmospheric flow, and their propagation is acutely sensitive to the background winds and heating distributions. By altering tropical heating, the warm pool core effectively adjusts the waveguide along which disturbances travel from the tropics toward the East Asian mid-latitudes.

Perhaps the most striking finding concerns the great flood years. The summers of 1998, 2016, and 2020 each followed El Niño events, and the canonical explanation emphasizes the delayed oceanic and atmospheric memory of El Niño — particularly Indian Ocean warming and the persistent western North Pacific anticyclone — in loading the dice for Yangtze flooding. The new study shows that warm pool core sea surface temperature anomalies serve as a critical pathway for Rossby wave propagation and may amplify or modulate the effects of El Niño on summer precipitation in the middle and lower Yangtze reaches. In other words, El Niño does not act on East Asian rainfall in isolation; its signal is filtered, redirected, and at times intensified by the state of the warm pool core. This finding positions the warm pool core as an oceanic bridge linking El Niño and the East Asian Summer Monsoon to extreme Yangtze precipitation.

The implications for seasonal forecasting are considerable. Current prediction systems devote enormous attention to the El Niño-Southern Oscillation and to the Indian Ocean, but a warm pool core that enhances or dampens the teleconnection could explain why some El Niño years produce devastating Yangtze floods while others, with seemingly similar precursor conditions, do not. The contrast between 1998 and 2016 — two years with comparable preceding El Niño events but notably different rainfall outcomes — has long puzzled scientists, and midlatitude circulation differences have been invoked to explain the gap. The warm pool core now offers an additional, and potentially quantifiable, piece of that puzzle: monitoring its temperature anomalies during the spring and early summer could sharpen forecasts of Meiyu-season rainfall months in advance.

The study also underscores a broader lesson about the climate system: that the regions of maximum ocean warmth are not passive background players but active amplifiers and routers of climate signals. As greenhouse warming continues, the extent and intensity of the warm pool are expected to evolve, with warm waters expanding and the pool’s structure shifting. If the warm pool core exerts this degree of leverage on monsoon variability under the current climate, changes in its behavior under a warmer one could reshape the odds of extreme summer rainfall for hundreds of millions of people across East Asia. The research, supported by the National Natural Science Foundation of China and other Chinese funding bodies, provides both a mechanistic foundation and a practical diagnostic for meeting that challenge, turning a remote expanse of bathwater-warm ocean into a watchpoint for the floods of summers to come.

Subject of Research: The influence of Western Pacific Warm Pool core sea surface temperature anomalies on East Asian summer monsoon variability and extreme Yangtze rainfall

Article Title: Impact of SST anomalies in the Western Pacific Warm Pool core area on the East Asian Summer Monsoon variability and its mechanism

Article References: Yang, R., Li, J., Wang, H., & Yang, Y. (2026). Impact of SST anomalies in the Western Pacific Warm Pool core area on the East Asian Summer Monsoon variability and its mechanism. Climate Dynamics, 64(10), Article 438. https://doi.org/10.1007/s00382-026-08387-7

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08387-7

Keywords: Western Pacific Warm Pool, East Asian Summer Monsoon, sea surface temperature anomalies, El Niño, Yangtze River rainfall, Rossby waves, western North Pacific anticyclone, Meiyu-Changma-Baiu, Climate Dynamics, seasonal prediction, air-sea interaction, teleconnection

The Indian Ocean Dipole, or IOD, is a naturally occurring climate pattern in which sea surface temperatures swing in a seesaw between the western and eastern tropical Indian Ocean. During a positive IOD event, the western basin near Africa becomes unusually warm while waters off Sumatra turn cooler, shifting rainfall and winds across the entire region. During a negative IOD, the pattern reverses. Scientists have long known that these events leave fingerprints on sea level in the northern Indian Ocean, but the relative importance of equatorial wind forcing versus local wind forcing has been difficult to separate, particularly because each IOD event unfolds with its own personality and intensity.

Nikitha Syam of the Department of Physical Oceanography at Cochin University of Science and Technology and Arnab Mukherjee of the National Centre for Polar and Ocean Research in Goa tackled this problem by combining two complementary lines of evidence. The first came from satellite altimeter observations of sea level anomaly, obtained from the Copernicus Climate Data Store, which track how far the ocean surface departs from its long-term average. The second was a linear continuously stratified ocean model, a numerical tool that simulates how the ocean responds to wind stress while faithfully representing the ocean’s continuous density layering with depth. The model was forced with ERA5 reanalysis winds from the European Centre for Medium-Range Weather Forecasts, allowing the researchers to isolate the wind-driven component of sea level variability.

The technical power of the approach lies in a set of damping-based experiments. By selectively switching off either the remote equatorial winds or the local basin-scale winds in the model, and by measuring how the damped ocean response decays, the researchers could quantify exactly how much each forcing agent contributes to the observed autumn sea level anomalies in each basin. This kind of forcing decomposition is essential because the ocean integrates wind information over time: winds along the equator excite eastward-travelling Kelvin waves that bounce off the eastern boundary and propagate as coastally trapped waves around India and Sri Lanka, while local winds stir up their own responses through Ekman pumping driven by wind stress curl and direct wind setup.

The results confirm a broad picture that oceanographers had suspected, but with important new nuance. Positive IOD events are generally associated with negative sea level anomalies across the Bay of Bengal, and the analysis attributes this to a negative contribution from remote equatorial wind forcing. The mechanism is well understood in dynamical terms: during a positive IOD, anomalous easterly winds along the equator drive upwelling Kelvin waves that travel eastward, lower the thermocline in the east, and send reflected, upwelling-favourable signals poleward along the coastal waveguide into the Bay of Bengal, where they raise the thermocline and depress sea level. Negative IOD events generally produce the mirror image, with positive sea level anomalies in the bay as downwelling signals propagate through the same waveguide.

The Arabian Sea tells a more complicated story. Its response to the IOD is systematically weaker and spatially more variable than that of the Bay of Bengal, and the decomposition experiments show why: local atmospheric forcing carries proportionally greater weight in the Arabian Sea than remote equatorial winds do. The geometry of the region helps explain the difference. The Bay of Bengal sits directly downstream of the equatorial waveguide along a coastal boundary that efficiently channels equatorial Kelvin wave energy northward, whereas the Arabian Sea receives a more attenuated share of that energy after it rounds Sri Lanka and sweeps up the west coast of India. Meanwhile, the Arabian Sea’s own monsoonal wind regime, with its strong seasonal reversals and pronounced wind stress curl patterns, imprints a local signal that can either reinforce or fight against the remote one.

Two individual events, 2015 and 2019, illustrate just how sharply the balance can differ from one IOD year to the next. In 2015, the observed sea level anomaly in the Bay of Bengal was actually positive even though the equatorial wind forcing contribution was negative. The decomposition indicates that local forcing over the Bay of Bengal partly offset the remote signal, overriding the expected dipole response. In 2019, the pattern split between basins: over the Arabian Sea, positive local wind forcing partially cancelled a negative equatorial contribution, leaving the basin-mean modelled response only weakly negative, while over the Bay of Bengal the negative equatorial wind contribution dominated outright and the modelled response remained strongly negative. The same climate event, acting on the same season, produced fundamentally different outcomes in two neighbouring basins because the internal balance of forces tipped in different directions.

The model itself performed credibly against observations, capturing the large-scale interannual sea level variability seen by the altimeters, although with reduced amplitude, a common characteristic of linear models that omit nonlinear processes, thermocline feedbacks and steric effects tied to heat fluxes. Regional time-series statistics were used to evaluate how well the model reproduced the observed variability across the different IOD categories. The agreement gives confidence that the wind-driven dynamics isolated in the experiments, rather than other processes such as freshwater fluxes from river discharge and monsoon rainfall, dominate the autumn interannual signal at the basin scale, even if the full observed amplitude requires additional contributions that a linear model cannot generate.

The practical implications extend well beyond academic dynamical bookkeeping. Sea level anomalies in the Bay of Bengal and the Arabian Sea modulate coastal flooding, saltwater intrusion, fishery productivity and the safety of low-lying delta communities that are home to hundreds of millions of people. If the IOD phase provides a seasonal lead indicator of likely sea level conditions, and if the remote equatorial wind signal can be anticipated from emerging dipole forecasts, then the new decomposition framework offers a way to translate climate prediction into more useful regional sea level outlooks. The study’s message is carefully hedged, however: the IOD phase is an important framework for understanding autumn variability, but the magnitude and spatial structure of the response in any given year depend on the race between remote equatorial waves and local wind forcing. As the researchers put it through their experiments, knowing the dipole phase tells you which way the wind is expected to blow, but the ocean’s answer depends on how hard each wind blows, and where.

The work also adds to a growing body of research showing that the northern Indian Ocean is a region where climate teleconnections and local processes are tightly intertwined. Earlier studies have traced Kelvin wave propagation along the equatorial waveguide, documented the role of the East India Coastal Current in transmitting equatorial signals, and examined how ENSO and the IOD together shape sea level along the Indian subcontinent. By quantifying the damping-based contributions of remote and local forcing across twenty-four years of dipole events, positive and negative alike, the new analysis provides a benchmark against which future event-by-event predictions can be tested. As ocean warming continues to raise baseline sea levels and potentially amplify interannual variability, understanding which lever, equatorial winds or local winds, moves the ocean surface in any given autumn becomes an increasingly urgent question for the densely populated coasts of South Asia.

Subject of Research: Autumn sea level variability in the Bay of Bengal and Arabian Sea driven by remote equatorial and local wind forcing during Indian Ocean Dipole years

Article Title: Role of local and remote equatorial wind forcing in autumn sea level variability in the Bay of Bengal and Arabian Sea during IOD years between 1999 and 2022

Article References: Syam, N., & Mukherjee, A. (2026). Role of local and remote equatorial wind forcing in autumn sea level variability in the Bay of Bengal and Arabian Sea during IOD years between 1999 and 2022. Ocean Dynamics, 76(10), Article 101. https://doi.org/10.1007/s10236-026-01857-w

Image Credits: AI Generated

DOI: 10.1007/s10236-026-01857-w

Keywords: Indian Ocean Dipole, sea level anomaly, Bay of Bengal, Arabian Sea, equatorial Kelvin waves, Rossby waves, wind stress curl, linear continuously stratified model, satellite altimetry, coastal waveguide, interannual variability, monsoon winds