Idealized MITgcm experiments reveal how monsoonal heating steers the timing of El Niño events.

Reporting by Helen Hill for MITgcm

The El Niño–Southern Oscillation (ENSO) is a naturally occurring climate pattern centered in the tropical Pacific Ocean. It involves periodic warming or cooling of sea‑surface temperatures, which in turn influences weather patterns around the globe.

Every few years, the world feels the influence of ENSO’s powerful climate rhythm. It shapes rainfall patterns, shifts storm tracks, and affects everything from agriculture to wildfire risk. Yet one of ENSO’s most curious features has long puzzled scientists: although sunlight at the equator peaks twice a year, ENSO events almost always reach their maximum strength in the Northern Hemisphere winter. Why winter, and why so reliably?

A  study published in Geophysical Research Letters in 2025 by P. J. Tuckman, Jane Smyth, Jingyuan Li, Nicholas Lutsko, and John Marshall may offer the answer. The key, the authors argue, lies not in the Pacific Ocean itself but thousands of kilometers away over the continent of Asia. To uncover this tele-connection, the team relied on a modeling framework rooted in the MITgcm.

At the heart of the puzzle is the seasonal cycle of the tropical Pacific. If sunlight alone controlled ocean temperatures, the West Pacific warm pool and the East Pacific cold tongue would each warm twice a year as the sun crossed the equator. But observations show something very different. The West Pacific is warmest in September–October–November, while the East Pacific is warmest in March–April–May. ENSO events follow suit: as the authors note, “almost all ENSO events peaked during DJF [December-January-February], while none peaked during JJA [June-July-August].” Something in the climate system is breaking the symmetry.

The Tuckman et al study points to the South Asian monsoon as the culprit. During boreal summer, the Asian continent heats up dramatically. This creates a large reservoir of warm, moist air that the monsoon circulation then pushes eastward toward the West Pacific. Tuckman et al describes how this warm air “suppresses surface fluxes in the West Pacific, leading to increased temperature there during the following months.” In simpler terms, when warm air sits over the ocean, the ocean loses less heat. The West Pacific warm pool holds onto its warmth, building up heat that peaks in fall.

This fall warming strengthens the temperature contrast across the Pacific, which in turn intensifies the Walker circulation—the system of trade winds and rising and sinking air that helps govern ENSO. A stronger Walker circulation cools the East Pacific even further through enhanced upwelling. The stage is set: the Pacific enters autumn with a sharper warm‑west / cool‑east pattern, and this is precisely the background state in which El Niño events grow most efficiently. By winter, they reach their peak.

To test this mechanism, the authors turned to a hierarchy of idealized models built on the MITgcm,  designed to allow adjustments to land–sea geometry, ocean basin structure, and atmospheric physics while preserving realistic fluid dynamics, a flexibility that makes MITgcm so powerful for fundamental climate research. The team ran two simulations: one with a simplified Asian continent and one with no land (an aquaplanet). The difference was unmistakable. With Asia included, the model reproduced the observed seasonal cycle of the Pacific and ENSO’s wintertime peak. Without Asia, the seasonal cycle became nearly symmetric, and ENSO events lost their preferred timing. 

By stripping the climate system down to its essentials, the MITgcm‑based framework appeared to reveal a clean, mechanistic link between the monsoon and ENSO’s seasonal rhythm. The study suggested that ENSO’s wintertime peak is not an accident of ocean dynamics but a downstream consequence of the way continents heat and cool. It’s a reminder that Earth’s climate is a tightly interconnected system, where processes in one region can set the tempo for phenomena half a world away.

Story image: The seasonality of the equatorial Pacific (Tuckman et al (2025), Fig. 3)

About the Researcher

PJ Tuckman is an NCAR Climate and Global Change Postdoctoral Fellow working at the Department of Geophysical Sciences at Stanford University. He is interested in several aspects of the climate system, especially tropical dynamics and severe convective events. Current projects include how El Niño events will change with warming and how CMIP models represent the energetics of the Earth system. He has been using MITgcm since 2022.

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