Max Planck Institute for Meteorology: Researchers Identify Three Phases of Climate Change in the Tropical Pacific

Experiments with a climate model reveal that the tropical Pacific responds to increased atmospheric carbon dioxide levels in three distinct phases. The first phase is dominated by random fluctuations. After this, the tropical Pacific warms more slowly than surrounding regions, resulting in a relative cooling that is consistent with what has been observed in recent decades. The third phase, during which the eastern tropical Pacific warms more than the western Pacific, is expected to emerge over longer timescales.

World map of sea surface temperature showing also high temperatures in the Pacific Ocean. — Temperature trend in the ocean between 1979 and 2023, in °C/decade. Credit: ERSSTv5/Moreno-Chamarro

Climate models generally agree on how climate change may unfold in the Pacific region over the long term. As the global temperature rises, a pattern similar to the El Niño phenomenon is expected to gradually develop in the tropical Pacific. This means that the sea surface in the east becomes warmer than in the west, the trade winds weaken, and the Walker circulation—the dominant atmospheric circulation pattern in this region—slows down. However, this expectation has not yet materialized. In fact, the eastern tropical Pacific has cooled in recent decades. A new study by the Max Planck Institute for Meteorology shows that this cooling can be understood as one of three characteristic phases of the region’s response to increasing carbon dioxide levels in the atmosphere.

Model experiments show response to rising CO2 levels

The research team led by Eduardo Moreno-Chamarro used the MPI-ESM climate model to create a large-ensemble with 280 simulations in which the atmospheric carbon dioxide concentration was quadrupled abruptly. This idealized “CO₂ hammer” prompted a pronounced response in the simulations, enabling the researchers to determine the typical reaction to increased CO₂. Additionally, the large number of simulations helps to separate the response to increased CO₂ from natural variability.

The three phases of change in the tropical Pacific—the initial, fast, and late responses—can be distinguished from one another in the simulations. The initial phase is characterized by random fluctuations, with no clear large-scale pattern yet emerging. Within a few years, a cooling tendency develops in the eastern tropical Pacific as part of the system’s adjustment to the CO₂ forcing. After several decades, the El Niño-like pattern becomes dominant, with warming in the eastern Pacific overtaking that in the western Pacific and a weakening of the trade winds. These three phases also occur in a more realistic scenario, in which atmospheric CO₂ content increases gradually by one percent per year instead of instantaneously. However, the transition into the late stage occurs with a delay in this case.

“If we relate the results of these idealized experiments to anthropogenic climate change, it suggests that the recent evolution of the tropical Pacific is consistent with the second phase,” explains Moreno-Chamarro. “In the coming decades, we may then expect a transition to the third phase.”

Causes of the transition

The researchers attribute the cooling during the second phase in part to the “ocean thermostat”—a mechanism involving the upwelling of cooler subsurface water in the eastern tropical Pacific. However, the study shows that this cooling effect remains relatively constant across all phases and does not explain the transition to the later warming pattern. This finding challenges previous expectations that the cooling effect of the thermostat mechanism would fade over time, indicating that other processes must be responsible for the shift in temperature patterns.

Instead, the key driver of the transition is a change in the transport of heat away from the equator. During the second phase, stronger trade winds enhance this heat transport, reinforcing cooling in the tropical Pacific. As the climate continues to warm, the trade winds weaken, reducing this heat export. As a result, the eastern Pacific warms.

These changes in the trade winds are linked to the evolving contrast in warming between the land and the ocean. In the earlier phase, continents—especially in the Northern Hemisphere—warm faster than the ocean surface, strengthening the atmospheric circulation and the trade winds. Over time, as the ocean “catches up” and the land-ocean temperature contrast diminishes, the trade winds weaken.

Further work at the institute is exploring how additional factors, such as land warming, melting Antarctic ice, and small-scale ocean processes, may also influence these temperature patterns.

4 different graphical descriptions of Equatorial Pacific cooling and warming — Equatorial Pacific cooling and warming. Sea surface temperature anomalies (in °C; shading) in absolute values (left) and relative to the plotted area average (right) for the fast response (first 10 years) and late response (last 30 years of the simulation). Arrows show the anomalies (in m/s) for surface winds and are the same on the right and left panels. CC BY 4.0 Moreno-Chamarro et al. 2026

Original publication

Moreno-Chamarro, E., Günther, M., Putrasahan, D., Zhang, J., and Kang, S. M. (2026) Three-stage response of the equatorial Pacific to CO₂ forcing controlled by shifting trade winds. npj Climate and Atmospheric Science, DOI: 10.1038/s41612-026-01391-y

Contact

Dr. Eduardo Moreno-Chamarro
Max Planck Institute for Meteorology
eduardo.chamarro@we dont want spammpimet.mpg.de

Back