ENSO Temporal Diversity and Associated Coupled Processes over Northwestern Pacific and Indian Ocean

This study analyzes significant features and associated air-sea processes affecting ENSO temporal diversity. Based on the Oceanic Niño Index (ONI) index in summer following El Niño peak phase (denoted as DJF0), 1960-2016 El Niño events are grouped into three categories: ONI turning negative before ENSO-decaying summer JJA1 (fast-decay), turning negative after JJA1 (slow-decay) and persisting into next winter DJF1 (prolonged). The fast-decay, slow-decay, and prolonged groups account for 13, 4, and 4 out of 21, respectively. In the composite analysis of 1980-2016 events, the slow-decay and prolonged groups are further combined as type II (4 events) in contrast to type I (fast-decay; 8 events). Relative to Type II, Type I is featured with a stronger positive Indian Ocean Dipole mode in the fall SON0, earlier formation of western North Pacific anticyclone (WNPAC), and an equatorial zonal heating dipole associated with developing La Niña in JJA1, maintaining the WNPAC. In type II, WNPAC extends to tropical Indian Ocean and forms a positive feedback with underlying sea surface temperature (SST) and monsoon in MAM1 and JJA1, while El Niño SST persists to anchor the tropical circulation and maintain local wind-evaporation-SST feedback in the southeast of the WNPAC. Our results highlight different degree of interactions among tropical Pacific SST, WNPAC, and Indian Ocean SST in ENSO temporal diversity.