Despite a steady rise in greenhouse gas concentrations in the atmosphere over the satellite era, a distinct decrease in sea surface temperature (SST) has been observed in the tropical central-to-eastern Pacific, whereas the Northwest and Southwest Pacific have exhibited a marked SST increase over the same period. Most of current and earlier climate model simulations, however, failed to reproduce this La Niña-like SST trend pattern, raising a question as to whether the observed tropical central-to-eastern Pacific cooling trend is either a forced response to greenhouse gas forcing or internally generated unforced variability. In a new study, published in the journal npj Climate and Atmospheric Science, an international team of scientists from South Korea and the United States of America shows that the observed La Niña-like SST trend pattern has been shaped, in large part, by a combination of internal climate variability and human-induced non-greenhouse gas forcing agents.

 It is well known that climate variability in the tropical Pacific exerts a large influence on weather and climate worldwide via atmospheric teleconnection. This implies that the response of tropical Pacific SST to increasing concentrations of greenhouse gases is a key factor determining the overall characteristics of human-induced future climate change. Hence, it is crucial to ensure that the physical processes governing the future evolution of mean state and variance changes for the tropical Pacific SST are sufficiently well depicted in climate models. One of the ways for testing the model performance is to check whether climate models are able to reproduce observed SST change and variability. Comparisons show that unlike observations that exhibit a La Niña-like SST trend pattern over the satellite era, model-simulated ensemble-mean SST trends rarely capture such a spatial pattern (Fig. 1). Considering that the characteristics of climate sensitivity and atmospheric teleconnections are closely linked to SST changes and variability in the tropical Pacific, the apparent observation-model discrepancy implies that the reliability of projected future climate changes might need to be carefully re-evaluated if greenhouse gas forcing is the main cause of the tropical Pacific cooling trends. Some previous studies, however, argued a potential role of internal climate variability and forcing agents other than greenhouse gas increases, which suggests that the observed La Niña-like SST trend pattern might have been driven, at least in part, by a reinforcing combination of low-frequency internal climate variability and non-greenhouse gas forcing agents.