The El Niño–Southern Oscillation (ENSO) is one of the most powerful climate drivers on Earth, with far-reaching impacts on global weather—particularly across the Asia-Pacific region. Accurate forecasting of ENSO events is essential for climate prediction, yet the 21st century has seen a decline in forecasting skill. One major reason is a shift in ENSO behavior, with more activity centered in the central Pacific (CP) rather than the eastern Pacific (EP), as was more common in previous decades.
To uncover the reasons behind this change, a research team led by Professor Wang Fan from the Institute of Oceanology at the Chinese Academy of Sciences, in collaboration with international partners, investigated the role of subsurface ocean mixing in shaping ENSO events. Their findings, published in Nature Communications, reveal a previously underappreciated feedback mechanism that boosts CP ENSO events while suppressing those in the EP.
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The researchers used a new estimation approach to analyze over 40,000 subsurface profiles collected by the Argo float network, producing the first large-scale dataset on subsurface mixing across the equatorial Pacific. Their analysis highlighted the critical influence of turbulent heat flux and mixing within the ocean’s upper layers—factors that directly impact the growth of both El Niño and La Niña events, particularly of the central Pacific type.
They found that the interaction between surface heat flux entering the ocean’s mixed layer and the turbulent heat flux exiting it creates a noticeable warming or cooling effect—what they describe as a diabatic influence, involving heat exchange with the environment. This effect significantly increases sea surface temperatures in the central Pacific, while simultaneously dampening temperature changes in the eastern Pacific.
This zonally uneven diabatic effect plays a key role in encouraging the formation of CP ENSO events and inhibiting EP ENSO development. Moreover, the process feeds into itself: as CP ENSO events alter wind and ocean conditions, they also intensify subsurface mixing, which in turn amplifies the ENSO event. This self-reinforcing feedback is stronger than previously recognized mechanisms.
"This positive feedback continues until counteracting, negative feedback processes begin to take over," explained Dr. Liu Chuanyu, lead author of the study.
What makes this study especially significant is its focus on diabatic feedback—contrasting with earlier theories that relied largely on adiabatic processes, which do not involve external heat exchange. This new perspective provides deeper insight into why ENSO patterns have changed and offers a valuable foundation for improving climate models and predictions in a changing climate system.