Impacts of Rapid Warm-To-Cold ENSO Transitions on Summer Monsoon Rainfall over the Northeastern Indian Subcontinent

Abstract

In this study, the impacts of warm-to-cold El Nino-Southern Oscillation (ENSO) phase transitions on the summer rainfall over the northeastern Indian monsoon (NEIM) region were investigated. Extremely rapid warm-to-cold ENSO transitions were identified in 1983, 1988, and 1998. In these rapid ENSO transition years, during August, the NEIM rainfall tended to be higher than that in other years, leading to large river water discharge into Bangladesh in August and September through the Ganges and Brahmaputra. In summer (June, July, and August), the western North Pacific monsoon (WNPM) was significantly suppressed in association with the east-west sea surface temperature (SST) anomaly contrast between an above-normal SST over the Indian Ocean and a negative SST anomaly that developed over the central equatorial Pacific. The suppression of WNPM was highly likely to be a key factor linking rapid ENSO transition to the heavy NEIM rainfall in August. WNPM suppression caused a prominent lower tropospheric anticyclonic anomaly, which extended westward over the Indian Ocean and Indian subcontinent. Associated with this circulation anomaly, a lower tropospheric westerly anomaly developed over the Bengal Plain in August, which activated the NEIM rainfall. Thus, especially in August, WNPM suppression significantly correlated with heavy rainfall over a large area over the northeastern Indian subcontinent. In all three rapid warm-to-cold ENSO transition years, WNPM was extremely suppressed in summer. However, in the August of 1983, owing to atmospheric variations at intraseasonal time scales, WNPM suppression was damped, and both the lower tropospheric circulation anomaly and the NEIM rainfall activation were not clear. On the other hand, in 1988 and 1998, WNPM suppression enhanced the NEIM rainfall in August and increased flood risk in Bangladesh.