Warm Pool Core Emerges as Oceanic Bridge Driving East Asian Summer Monsoon Extremes

Deep in the western tropical Pacific lies a vast reservoir of the warmest ocean water on Earth, a region where sea surface temperatures persistently climb above 29.4 degrees Celsius. Climate scientists call this the core area of the Western Pacific Warm Pool, and a new study published in the journal Climate Dynamics argues that this seemingly remote patch of ocean plays a far more decisive role in shaping summer weather over East Asia than previously appreciated. The research, led by Rong Yang and Jianping Li of the Ocean University of China together with colleagues, demonstrates that sea surface temperature anomalies in this warm pool core act as a critical conduit through which El Niño influences — and at times amplifies — the catastrophic summer rainfall episodes that periodically drench the middle and lower reaches of the Yangtze River.

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