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 data sets during 1979-2016. We identify two major factors in determining the summer monsoon onset timing: the background conditions associated with seasonal transitions and the intraseasonal variations associated with an active MJO. The background conditions undergo sharp seasonal transition over the western Indian Ocean two to three pentads before the onset dates, 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 atmospheric circulation associated with the leading dry phase of a strong MJO during the climatological mean onset dates may lead to a delayed monsoon onset. Plain Language Summary The Madden-Julian oscillation (MJO) is one of the dominant modes of intraseasonal variabilities in the tropical atmosphere, while the Asian summer monsoon represents one of the most significant transitions in the large-scale atmospheric and oceanic circulations in the Indo-Pacific regions and beyond. This study examines the linkage between MJO and the South Asian summer monsoon onset through composite analysis of decades of observational data sets along with a global space-time wave-filtering technique. We identify two major factors in determining the summer monsoon onset timing: the background conditions associated with seasonal transitions and the intraseasonal variations associated with an active MJO. The onset of the Indian summer monsoon is often associated with the arrival of the wet phase of the tropical MJO over the Indian Ocean, while a delayed monsoon onset is likely caused by the dry phase of a strong MJO that suppresses background changes associated with seasonal transition during the climatological mean onset dates.