Is the length of the Indian summer monsoon season shrinking?

Abstract The iconic Indian summer monsoon (ISM), that is often ascribed to the season of June–July–August–September (JJAS), is found to be changing in recent decades. An objective definition of the ISM season is used to diagnose the onset and retreat of the monsoon to the precision of a day in the year. This analysis provides compelling observational evidence that the ISM season has distinctly shortened in parts of northern, northeastern, and peninsular India in recent decades. This is caused by a late onset over northeastern India and an earlier retreat date of the ISM in all these regions in the recent decades, resulting in a reduction of 10 days to a month in the length of the ISM season over these regions. Our analysis reveals that in recent decades, the West Pacific High is further east at the time of the onset of the ISM over northeastern India, while at the time of retreat, the rainfall is much stronger in Southeast Asia with the West Pacific High shifted further west. Both these changes in recent decades reduce the moisture flux that is associated with a comparative reduction of the mean daily rain rate. This reduction of the mean daily rain rate then relates to the delay of the onset and the advance of the retreat of the ISM season in some of these regions over India that exhibit a shortened ISM season in recent decades.