Tropical eastern Pacific cooling trend reinforced by human activity

Abstract

It remains unresolved whether the La Ni & ntilde;a-like sea surface temperature (SST) trend pattern during the satellite era, featuring a distinct warming in the northwest/southwest Pacific but cooling in the tropical eastern Pacific, is driven by either external forcing or internal variability. Here, by conducting a comprehensive analysis of observations and a series of climate model simulations for the historical period, we show that a combination of internal variability and human activity may have shaped the observed La Ni & ntilde;a-like SST trend pattern. As in observations, SSTs in each model ensemble member show a distinct multi-decadal swing between El Ni & ntilde;o-like and La Ni & ntilde;a-like trend patterns due to internal variability. The ensemble-mean trends for some models are, however, found to exhibit an enhanced zonal SST gradient along the equatorial Pacific over periods such as 1979-2010, suggesting a role of external forcing. In line with this hypothesis, single-forcing large ensemble model simulations show that human-induced stratospheric ozone depletion and/or aerosol changes have acted to enhance the zonal SST gradient via strengthening of Pacific trade winds, although the effect is model dependent. Our finding suggests that the La Ni & ntilde;a-like SST trend is unlikely to persist under sustained global warming because both the ozone and aerosol impacts will eventually weaken.