Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Niño Events

The inverse relationship between the warm phase of the El Niño Southern Oscillation (ENSO) and the Indian Summer Monsoon Rainfall (ISMR) is well established. Yet, some El Niño events that occur in the early months of the year (boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Niño (springtime) and emerging summer El Niño (summertime) on the regional variability of ISMR. The two ENSO categories were formulated based on the time of occurrence of positive SST anomalies over the Niño-3.4 region in the Pacific. The ISMR’s dynamical and thermodynamical responses to such events were investigated using standard metrics such as the Walker and Hadley circulations, vertically integrated moisture flux convergence (VIMFC), wind shear, and upper atmospheric circulation. The monsoon circulation features are remarkably different in response to the exhausted spring El Niño and emerging summer El Niño phases, which distinctly dictate regional rainfall variability. The dynamic and thermodynamic responses reveal that exhausted spring El Niño events favor excess monsoon rainfall over eastern peninsular India and deficit rainfall over the core monsoon regions of central India. In contrast, emerging summer El Niño events negatively impact the seasonal rainfall over the country, except for a few regions along the west coast and northeast India.