Revisiting the ENSO–monsoonal rainfall relationship: new insights based on an objective determination of the Asian summer monsoon duration

Traditionally, the boreal summer monsoon season is regarded as a fixed period of June–September (JJAS). In fact, the monsoon commencement and termination are not fixed to the annual cycle, thus the monsoon duration exhibits remarkable variations. Based on the multivariable empirical orthogonal function analysis, the commencement, termination, and duration of the Asian summer monsoon (ASM) are objectively determined in this study. The ASM duration is shown to be closely linked to the sea surface temperature anomalies in the equatorial Pacific Ocean, which tends to be shorter during an El Niño and longer during a La Niña year. Through the anomalous Walker circulation and the equatorial Rossby wave response, a developing La Niña event is generally associated with advanced commencement and delayed termination, thus a longer ASM duration. The importance of monsoon duration is illustrated further by revisiting the relationship between El Niño-southern oscillation (ENSO) and the monsoonal rainfall, which is the total rainfall within monsoon duration (from commencement to termination) rather than within JJAS. The relationship between the JJAS rainfall over South Asia and ENSO has exhibited remarkable interdecadal changes, which becomes weak since the 1980s and is recovered after the early 2000s. In sharp contrast, the linkage between the monsoonal rainfall over South Asia and ENSO remains significant and robust over the past four decades. Via attaching the rainfall during the transition periods (advanced commencement and delayed termination), the longer ASM duration during La Niña shall increase the total rainfall, which can reinforce the canonical ENSO-monsoonal rainfall relationship. The above results suggest that a reasonable definition of ASM duration may help us better understand the monsoon phenomenon and teleconnections.