Abstract

Argo data reveal the development of a thickness anomaly in the lower levels of the ventilated thermocline of the South Pacific starting in 2010. Data through 2016 show this anomaly propagating westward and towards the equator. Theory suggests that this anomaly will reduce the velocity of the equatorial undercurrent (EUC) when it approaches the equator; this is supported by climate model results with enhanced vertical resolution in the thermocline, which also show a strong influence on eastern Pacific sea surface temperatures. We propose that the propagation of such anomalies in the structure of the ventilated thermocline from the southeastern Pacific to the western equatorial Pacific may explain some of the decadal variability observed in the equatorial Pacific that also affects global temperature. This mechanism improves our understanding of variability in the global temperature record, implying that the stepwise warming pattern over the 20th century is primarily due to internal variability in the ocean rather than aerosol forcing. It also provides guidance for climate model development that may enable simulations to better capture internal variability andimprove climate forecasts.