Equatorially Symmetric Impact of the El Nino-Southern Oscillation on the South Asian Summer Monsoon System

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  • Equatorially symmetric asymmetric impacts of El Nino – Southern Oscillation on the South Asian summer monsoon system

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Abstract

A dominant impact of the El Niño-Southern Oscillation (ENSO) on the South Asian summer monsoon interannual variability is identified using the National Centers for Environmental Prediction /National Center for Atmospheric Research reanalysis aided by an ocean general circulation model. It has an equatorially symmetric structure and is most pronounced at the growth phase of ENSO in late summer during the period from the 1960s to mid-1970s when tropospheric biennial oscillation (TBO)-like ENSO dominates. As ENSO develops, an anomalous Walker circulation system over the tropical Indian and Pacific oceans changes from a single-cell regime in summer to a double-cell regime in fall. Meanwhile, rainfall anomalies of an equatorially symmetric structure are induced over the tropical Indian Ocean, accompanied by north-south twin circulation anomalies in the lower troposphere. The northern circulation is dynamically linked with anomalous monsoon rainfall over India especially in late summer. This research suggests that a combination of the wind-evaporation feedback in the Indian Ocean and ocean dynamics in the tropical Pacific is crucial for the regime transition of the anomalous Walker circulation system associated with the TBO-like ENSO. As the seasonality of the ENSO cycle changes before and after the late 1970s, the difference in ENSO impacts between its growth and decay phases may have influenced the ENSO-monsoon relationship and caused its interdecadal change.<br>

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