MJO Affects the Monsoon Onset Timing over the Indian Region

This study examines the linkage between the Madden–Julian Oscillation (MJO) and the South Asian summer monsoon onset through spatiotemporal wave-filtering and composite analyses of observational datasets during 1979-2016, along with convection-permitting regional climate sensitivity experiments. From observations, we identify two major factors in determining the summer monsoon onset timing: the background conditions associated with seasonal transitions and the atmospheric conditions associated with an active MJO. The background conditions undergo sharp seasonal changes 2–3 pentads before the climatological onset dates over the western Indian Ocean while a typical monsoon onset is often associated with the arrival of the wet phase of the tropical MJO over the Indian Ocean, likely due to the promotion by the MJO-initiated eastward-propagating westerly wind anomaly. Conversely, the circulation associated with the leading dry phase of a strong MJO during the climatological mean onset dates may lead to a delayed monsoon onset. Our sensitivity experiments with and without the MJO for the most part simulate the change the Indian monsoon onset due to MJO influence.