Asymmetrical Impact of El Niño to La Niña Transition in the Tropical Pacific Basin on Hot Extremes over India

Abstract

Several studies have identified the significant impact of El Niño on summer season (March to May) temperature extremes over India. The current study identifies an asymmetry in the impact of ENSO on the hot extremes over the Indian region. During some years, the El Niño peak occurs in the November-December-January (NDJ) season and is followed by a transition to a cold phase of ENSO during the summer season. In such years, significantly more Indian districts experience extreme temperature anomalies in summer months compared to other ENSO years. This study focuses on bringing out the geo-spatial characterisation of this asymmetry with few dynamical pointers of its causes. It is found that the asymmetry in heatwave impact is related to two types of ENSO transitions: (a) El Niño followed by La Niña (ELLA) and (b) El Niño not followed by La Niña (ELNLA). It is also found that Eastern Pacific (Central Pacific) El Niño events dominate the ELLA (ELNLA) events. Over the Indian Ocean, distinct warming over the Bay of Bengal and the South Indian Ocean is also noted during ELLA years as compared to ELNLA years. In the ELLA years, the monthly maximum and minimum temperature conditions are much higher, with lower day-night temperature differences over a large number of districts than those of the ELNLA years during the winter and in the summer months, leading to increased discomfort. The uneven impact of ENSO on heatwaves can be used to explore region-specific risk mitigation strategies across the Indian subcontinent.