Charge in Long-Lasting El Niño Events by Convection-Induced Wind Anomalies over the Western Pacific in Boreal Spring

Abstract

Abstract In this study, El Niño events are classified as long El Niño (LE) events and short El Niño (SE) events based on their durations, and the characteristics of the early stages of these events are investigated. Results indicate that LE events tend to start earlier compared to SE events, initiating in boreal spring and peaking in winter. Their early occurrence is attributed to the western equatorial Pacific (WEP) sea surface wind anomalies that benefit the eastward propagation of warm water by forcing the downwelling Kelvin waves. It is also found that the wind anomalies are potentially induced by the convection anomalies over the WEP in spring. Experiments with a fully coupled climate model forced by convection heating anomalies over the WEP show that El Niño events become stronger and longer after introducing anomalous convection heating. The convection anomalies induce an extensive anomalous westerly belt over the WEP, which charges El Niño by eastward-propagating Kelvin waves. Moreover, induced by the anomalously northward-shifted ITCZ heating and the suppressed heating over the Maritime Continent, the equatorially asymmetric westerly belt reduces the meridional shear of mean easterly wind in the lower latitudes, which maintains an anomalous equatorward Sverdrup transport and in turn prolongs the persistence of El Niño events. A case study of the 2015/16 super El Niño and a regression study by using a rainfall index in critical regions support the above results.