Multidecadal see-saw of the impact of ENSO on Indian and West African summer monsoon rainfall

Abstract

The strength of the simultaneous linear relationship between El Nino/Southern Oscillation (ENSO) and Indian summer monsoon (ISM) precipitation show strong variations on a decadal timescale. While some studies attribute this to shift in the state of the climate and consequent teleconnection pattern, some other argue this as natural variability between two random time series. In this study, we show that the relationship between West African Summer Monsoon (WASM) precipitation with ENSO also experiences decadal timescale oscillation. While the ENSO–ISM relationship weakened during the past seven decades, ENSO–WASM relationship strengthened to above the 95% significance level. We explain this multi-decadal see-saw of strong–weak impact of ENSO on ISM and WASM through a common mechanism. ENSO impacts ISM and WASM rainfall by modulating the upper tropospheric temperature of subtropical Africa and South Asia. While the impact of ENSO on this temperature anomaly was strong and concentrated over the northwest of Indian region before the 1980, the anomalies are spatially discontinuous and weak after 1980. Moreover, a westward shift of the center of this anomaly after 1980 help strengthen the ENSO–WASM relationship. We also show a dramatic change in the relationship between Atlantic Nino and ENSO before and after the 1980s. While before 1980 ENSO did not have much impact on Atlantic Nino index-3 (ATL3), after 1980 El Nino (La Nina) is coincidental with negative (positive) ATL3 index. Since a negative (positive) ATL3 reduce (enhance) WASM by increased south-westerly moisture flux, the ENSO–WASM relationship strengthens after 1980. Our study suggests that the decadal variations of ENSO–ISM and ENSO–WASM relationship is physically linked and possibly could not be due to pure noise in the time series.