Colder eastern equatorial Pacific and stronger Walker circulation in the past 30 years: an ocean thermostat versus natural variability in observations and climate models

The mean state of the tropical Pacific is critical for a broad range of climate phenomena from ENSO and atmospheric teleconnections to meridional ITCZ shifts. Yet, debate continues on the causes of the decadal cooling trends observed in the eastern equatorial Pacific and the strengthening of the Walker circulation over the past 30 years, especially in the context of the model projections for this region. In fact, 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 21st century, resembling El Niño-like mean conditions. Moreover, several models suggest that the tropical Pacific should already be in such a state with a slightly warmer east compared to the west, which however strongly contradicts the observed trends along the equator. Whether these trends reflect the forced coupled ocean-atmosphere response to global warming or the negative phase of the Pacific Decadal Oscillation (PDO), or perhaps both, remains unclear. Here we show that the observed SST trends during 1980-2020 are dominated by three signals: a spatially uniform warming trend, a negative PDO pattern, and a Northern Hemisphere/Indo-West Pacific warming pattern. The latter pattern closely resembles the transient ocean thermostat-like response to global warming emerging in a subset of CMIP6 models. It shows cooling in the central-eastern equatorial Pacific but warming in the western Pacific and tropical Indian ocean. Together with the PDO, this forced pattern drives the Walker circulation strengthening. Overall, CMIP6 historical simulations appear to underestimate this thermostat-like pattern, contributing to the models’ inability to replicate the Walker cell strengthening. Finally, the potential contribution of atmospheric aerosols to the cooling trends is also discussed.