Impact of the Indo-Western Pacific Ocean Capacitor mode on South Asian summer monsoon rainfall

摘要

The influence of Indo-Western Pacific Ocean Capacitor (IPOC) mode on South Asian summer monsoon rainfall variability is investigated in the present study. It is found from the observations that the IPOC mode induces a tripole pattern in precipitation anomalies over the South Asian region. Strong positive precipitation anomalies over some parts of western Ghats and Sundarbans and Bangladesh region separated by negative precipitation anomalies over the monsoon trough region are found to be the main characteristic features closely associated with the IPOC mode. It is noted that anomalous western north Pacific (WNP) anticyclone and tropical Indian Ocean (TIO) warming maintain a tripole precipitation pattern over the South Asian region. Anomalous low level convergence in the northern flank of WNP anticyclone and orographic lift cause enhanced rainfall over the Sundarbans and Bangladesh region. Meanwhile, westward propagating atmospheric cold Rossby wave as a response to suppressed convection over the WNP region induced moisture divergence and downward motion play important role in maintaining negative rainfall anomalies over the monsoon trough. At the same time, TIO SST warming supported by easterly wind anomalies associated with WNP anticyclone has contributed to the enhanced rainfall over parts of eastern Arabian Sea and western Ghats. Coupled General Circulation Model sensitivity experiments are performed to investigate individual and combined impact of the TIO warming and WNP circulation on south Asian summer monsoon rainfall variability associated with IPOC mode. The model experiments revealed that WNP anticyclone influences the rainfall over the monsoon trough region and Sundarbans-Bangladesh region. Further the moisture convergence induced by local SST warming is mainly helping to sustain enhanced Western Ghats rainfall. Sub-seasonal variability of ISM rainfall associated with the IPOC modes is also discussed. This study advocates the importance of IPOC mode in exerting strong impact on regional summer rainfall variability over the Indian/South Asian summer monsoon region on the interannual time scale. Thus, it is important to consider the possible roles of TIO warming and WNP circulation in predicting the South Asian summer monsoon rainfall.