Significant changes in the ENSO-monsoon relationship and associated circulation features on multidecadal timescale

Abstract

We observe significant changes in the ENSO–Indian summer monsoon rainfall (ISMR) relationship in past three multidecadal epochs (early epoch: 1931–1960, middle epoch: 1961–1990, and recent epoch: 1991–2015) based on consistent correlation. The rainfall during early epoch was above normal, however, in other two epochs it was relatively dry. The ENSO–ISMR relationship and its changes are mostly pronounced in the monsoon core zone. During the early epoch, the effect of La Nina was more dominant by accounting higher number of La Nina events with higher values of Nino 3.4 index, which can be attributed to the surplus rainfall over most regions of India. The rainfall during the consecutive two multidecadal epochs is below normal and this may be attributed to the strong El Nino events. The dryness in the northeast region cannot be linked with ENSO events as it has insignificant relationship with the rainfall over this region. The weakening of Low Level Jet during the recent epoch can be a reason to reduce monsoon rainfall during recent periods. It is evident that the reduced moisture transport also accounts for the reduced precipitation. The changes in equatorial Walker and regional Hadley circulations during the three epochs show the same mode of variability and in phase with the changes in rainfall and other circulation related parameters. This study emphasizes the basin mode warming of world oceans and the increased frequency of El Nino events during the recent epochs linked to the climate change and its impact of ISMR.