Increase in summer monsoon rains in northeast India during ENSO periods: a multiproxy analysis

El-Niño Southern Oscillation (ENSO) affects Indian summer monsoon. Most of the worst droughts - the most recent being in 2009 - in India have been triggered by ENSO. But given the heterogeneity in rainfall patterns over India, we revisited ENSO influence on Indian summer monsoon. Our analysis based on multiple isotopic (proxy-based) and satellite data set shows significant variation in the spatiotemporal patterns of rainfall over the Indian subcontinent and adjoining oceans. We observed a weaker summer monsoon over central India and relatively stronger summer monsoon over northeast India during strong El-Niño events. Rainfall derived from isotope-enabled general circulation models reproduces weak and strong rainfall patterns during the El-Niño events over central India and northeast India, respectively. These model derived δ¹⁸O_rain (oxygen isotopic composition of rainfall) variation over central India during ENSO events mimic the weaker rainfall conditions. However, significant changes in the model derived rainfall and associated δ¹⁸O_rain is not observed over northeast India during ENSO events. Based on multiple data analysis, we infer that the long term variations (trends) in the Indian summer monsoon strength were controlled by the long term variation in ENSO during the last 50 years (1965 – 2013).

Since these observations were unprecedented and counterintuitive, we further verified our observations from the proxy records. Two speleothems (cave deposits) records from the central India and northeast India were analyzed for the rainfall variation and ENSO influence signatures. These paleo-proxy records showed a similar inverse relation of rainfall patterns over central India and northeast India during ENSO periods, confirming observed ENSO’s role on rainfall. Also, these proxy records showed a long-term pause in ENSO events or stronger La-Niña like conditions, which were persisted during 1625 – 1715 and favored stronger (weaker) rainfall over central India (northeast India).