Diversity of Indian Summer Monsoon Circulation and Rainfall During the Super El Niño years

Abstract

In general, the Indian Summer Monsoon (ISM) rainfall over India is deficit (excess) during El Niño (La Niña) years, although this is not true for all the El Niño years. In this work, we noted that the spatial rainfall patterns over the Indian region exhibited large diversity among the super El Niño events such as 1982, 1997, and 2015. Positive Indian Ocean Dipole (IOD) and significant changes in Sea surface temperature (SST) anomaly patterns over the Indo-Western Pacific (IWP) are noteworthy features of these events. Mainly strong southeast- northwest tilted cold SST anomalies were noticeable from the southwest Pacific to the southeast Indian Ocean (SEIO) through the Indonesian islands during 1982 and 1997 than that of 2015. These anomalies were caused by the interaction of cyclonic circulation over the South Pacific Convergence Zone(SPCZ) with anticyclonic circulation over south of Australia. However, it is feeble in 2015, which leads to the occurrence of an asymmetric equatorial circulation over the IWP. We further observed the equatorial Indian Ocean zonal winds are modulated by southern annular mode (SAM) forcing, which plays a substantial role in altering the strength of the IOD. It implies that SAM has an influence on ISM rainfall along with super El Niño induced SST anomalies coupled with circulation over the IWP. Though upper-level convergence over the ISM region is somewhat similar among these super El Niño events, more frequent and organized northward propagating convective activity over Central India was observed in 1982 and 1997 than in 2015. High-frequency intraseasonal variability with rainfall peaks are evident due to the northward propagation of convective activity. The influence of WNP convection on active and break signals of the ISM is evident in Quasi-biweekly (10–20-day) oscillations, even in deficit monsoon years associated with El Niño. Even though the super El Niño may occur in the future, the background conditions over the IWP along with Southern Hemisphere synoptic (such as SAM) events influence the rainfall changes over India