Impacts of the El Nino-Southern Oscillation on the strength and duration of the Indian summer monsoon

Using the ERA-Interim data, the impacts of the El Nino-Southern Oscillation (ENSO) on the Indian summer monsoon (ISM) are investigated. El Nino and La Nina are associated with weakening and strengthening of the moist southwesterly winds over the Indian subcontinent, respectively. There is a higher possibility for the occurrence of the positive phase of the Indian Ocean Dipole (IOD) during the eastern Pacific El Nino, causing cooler water and drier than normal conditions in the eastern Indian Ocean, but the opposite conditions in its western side. The Hadley circulation also responds to ENSO, such that anomalous subsidence develops over most parts of the Indian subcontinent during both the eastern and central Pacific El Nino events, causing a shorter duration of the ISM. The impact of the central Pacific El Nino on the Hadley circulation is stronger than that of the eastern Pacific El Nino. A strong correlation between sea surface temperature (SST) anomalies in the tropical Pacific and the mid-tropospheric geopotential height anomalies over the Indian subcontinent is identified during the pre-monsoon month of April, with positive and negative geopotential height anomalies during El Nino and La Nina, respectively. Thus, mid-tropospheric geopotential height anomalies in April can be used to predict the strength of the upcoming ISM.