Summer SST anomalies in the Indian Ocean and the seasonal timing of ENSO decay phase

Abstract

ENSO affects the tropical Indian Ocean (TIO) SST in winter-spring in ENSO decay years through an ENSO-induced ‘atmospheric-bridge’ and subsequent air-sea coupling processes. The interdecadal delay of El Nino decay phase has been related to a warming change in the summer TIO since 1970s. A physical linkage between the summer SST anomalies over the TIO and the timing of ENSO decay phase is however still unclear. This study uses multi-source data to distinguish ‘later-decay’ from ‘normal-decay’ El Nino/La Nina events, and performs diagnostic analysis of the changes in various thermodynamic and dynamic processes due to later-decay ENSO for quantifying the partial contribution by each of these processes to the summer SST changes over the TIO. The results show that, at both the interannual and interdecadal timescales, the significant warmer and colder SST anomalies in the spring TIO in later-decay El Nino and La Nina years respectively can persist into summer. Most of the ENSO-induced atmospheric-bridge-related processes contribute positively to the TIO SST changes in summer due to later-decay of ENSO, as they do in spring during normal-delay ENSO year. The exceptions are the surface wind-evaporation-mechanism and sensible heat-flux anomalies in summer, which always contribute negatively to the summer SST anomalies over most parts of the TIO. The negative contributions from these two processes in summer exist no matter whether there is a weakening or strengthening surface wind due to later-decay of ENSO events. Generally, the presence of five later-decay El Nino events after the 1970s is mainly responsible for the observed interdecadal summer TIO warming in recent decades.