Impacts of Tropical Indian and Atlantic Ocean Warming on the Occurrence of the 2017/2018 La Nina

The occurrence of the 2017/2018 La Nina, following a weak-to-neutral La Nina in boreal winter 2016/2017, was surprising. Based on observational records and multiple linear regression analysis for the Pacific zonal wind tendency (dU/dt), this study investigates possible reasons why the La Nina condition suddenly happened in late 2017. Similar to previous four double-peaked La Nina events (1983-1985, 1998-2000, 2007-2009, and 2010-2012), we find that the multiyearly persistent easterly anomaly in the central equatorial Pacific is a key condition to the development of the second La Nina. The occurrence of the 2017/2018 La Nina results from large warm sea surface temperature (SST) anomalies in the tropical Indian and Atlantic Oceans that act to force the persistent easterly anomaly in the Pacific via modifying the Walker Circulations. About 24% of the variance of the Pacific dU/dt can be statistically explained by the tropical Indian Ocean and Atlantic SST anomalies. Plain Language Summary The 2017/2018 La Nina appears to be surprising, given that an El Nino-like condition has already developed in the first half of 2017 but actually in opposite to most models' forecasts that issued a false alarm of an El Nino. Previous studies suggested that both the tropical Indian and Atlantic Oceans have exhibited a rapid warming in recent decades, which have caused an easterly trend in the central Pacific. In this study, we examine the possible contributions of the tropical Indian Ocean and Atlantic SSTs to the occurrence of the second La Nina in 2017/2018. Our results highlight the importance of the SST warming in the tropical Indian and Atlantic Oceans for the occurrence of the second La Nina under rapid SST warming in the two basins during recent decades.