Impact of El Niño onset timing on Indian Monsoon Rainfall patterns

Abstract

The adverse impact of El Niño on Indian summer monsoon rainfall (ISMR) is well established, with documented physical mechanisms linking different flavors of El Niño and ISMR. However, few studies have discussed the pathways that connect different onsets of El Niño and ISMR. The present study identifies the two ENSO categories (spring and summer) depending on the occurrence time of positive sea surface temperature anomalies (SSTA) in the Niño-3.4 region of the Pacific Ocean. The distinct impacts of these El Niño types on ISMR were investigated by analyzing extensive composites of rainfall anomalies, Sea Surface Temperature (SST) patterns, and atmospheric circulation dynamics. It was found that the Indian region experiences a significant average negative deviation of 26 % from normal rainfall across 30 % of the country during spring El Niño (SPE) (February to May). Whereas a summer El Niño (SUE) (June to September) is distinguished by a significant negative deviation over 15 % of the Indian region with an average deviation of 22 % across the country. SPE events showed strong warmth with an average of 1.27 °C, while SUE events had milder warmth, averaging 0.84 °C. Further, a strong positive Indian Ocean dipole (IOD) phenomenon was observed during the SPE event, which was associated with a strong easterly wind. This positive IOD phenomenon was progressively developed from June to September, heading to the establishment of low pressure over Western Indian Ocean (WIO), resulting in altered or weakened Walker circulation. The study elucidates the intricate interaction between El Niño events and the Indian monsoon system, providing a vital understanding of the mechanisms that govern seasonal rainfall variability across the Indian subcontinent.