The mean state of the tropical Pacific is critical for a broad range of climate phenomena with far-reaching global impacts, from ENSO to atmospheric teleconnections to ITCZ shifts. Yet, there is still uncertainty regarding the response of the tropical Pacific to anthropogenic climate change. Specifically, debate continues on the interpretation of the decadal cooling trends observed in the eastern equatorial Pacific over the past 30 years, especially in the context of the model projections for this region. A vast majority of CMIP6 models predict a weakening of the Walker circulation and the development of an eastern equatorial Pacific (EP) warming pattern by the end of the 21^st century (resembling El Niño-like mean conditions), even though the models disagree on the strength of this pattern. Moreover, results from several models suggest that we should already be in such a state with a slightly warmer eastern Pacific compared to the west, which however strongly contradicts the recently observed decadal trends along the equator. A significant part of the problem is the complex interplay between three factors that determine the response of the tropical Pacific to anthropogenic forcing: (i) an ocean thermostat (Pacific or perhaps Indo-Pacific) related to strong equatorial upwelling in the east, which acts to strengthen the east-west equatorial SST gradient, (ii) the effect of anthropogenic aerosols cooling the tropical Pacific, and (iii) a tendency towards a slower warming in the west due to enhanced evaporative cooling. The changes can be further amplified by the Bjerknes feedback, while internal variability (e.g., the Pacific Decadal Oscillation or PDO) can obscure or amplify the signal. We discuss this interplay using examples from CMIP6 simulations and more idealized CO₂-rise experiments as applied to the observed and projected climate trends in the tropical Pacific. Finally, we discuss how these changes in the tropical Pacific mean state may effect ENSO.

References:

Heede, U.K., and Fedorov, A.V., 2023. A stronger Walker circulation and colder eastern equatorial Pacific in the early 21^st century: separating the forced response of the climate system from natural variability. GRL 50, e2022GL101020

Heede, U.K., Fedorov, A.V., 2023: Towards understanding the robust strengthening of ENSO and more frequent extreme El Niño events in CMIP6 global warming simulations. Climate Dynamics https://doi.org/10.1007/s00382-023-06856-x

Heede, U.K., and Fedorov, A.V., 2021. Eastern equatorial Pacific warming delayed by aerosols and thermostat response to CO₂ increase. Nature Climate Change 11, 696–703.