Impact of the subtropical South Pacific on the decadal change in El Niño zonal propagation

The propagation of sea surface temperature (SST) anomalies along the equatorial Pacific Ocean during El Niño shifted around the late 1970s, previously propagating westward from the eastern Pacific and shifting to eastward propagation from the central Pacific. This shift has been attributed to decadal changes in the mean state of the equatorial Pacific. This study examined the influence of the subtropical South Pacific on the decadal variation in El Niño zonal propagation through the utilization of observational data and extensive simulations with coupled climate models. We showed that El Niño induced by the positive South Pacific Subtropical Dipole mode (pSPSD) started in the eastern Pacific and developed westward. The mean trade winds in the southeastern Pacific were stronger before the late 1970s, leading to a stronger pSPSD through enhanced wind–evaporation–SST feedback. Then, the pSPSD could trigger westward propagating El Niño. Contrarily, weaker pSPSD did not induce El Niño events after the late 1970s. El Niño initiated from other processes and propagated eastward. Our study reveals the modulation of the subtropical South Pacific on the decadal shift of El Niño propagation.