Cross-Equatorial Winds Control El Niño Diversity and Change

Over the past two decades, El Niño events have weakened on average and their sea surface temperature anomalies shifted westward towards the central Pacific. The intertropical convergence zone (ITCZ), typically migrating southward from its northerly position during El Niño events, has not crossed the equator since 1998. The causes of these changes remain under debate. Here, using in-situ, satellite, and atmospheric reanalysis data, we show that these changes can be related to the multi-decadal strengthening of cross-equatorial winds in the eastern Pacific. This gradual strengthening of meridional winds is unlikely to be caused by ENSO changes, and contains signals forced both locally and from outside the tropical Pacific, probably from the tropical North Atlantic. Coupled model simulations in which the observed cross-equatorial wind strengthening is superimposed successfully reproduce the key features of the recent changes in tropical climate. In particular, tropical mean state experiences a “La Niña-like” change, ENSO amplitude weakens by about 20%, the center of SST anomalies shifts westward, and the ITCZ now rarely crosses the equator. Thus, cross-equatorial winds are found to modulate tropical Pacific mean state and variability. I will also discuss the implications for past and future climate changes.