Spatial variability and moisture tracks of Indian monsoon rainfall and extremes

We performed a systematic space-time analysis of monsoon seasonal (Jun-Sep) rainfall and extremes (3-day maximum rainfall) over India for the period 1951-2019. Employing Partition Around Medoid (PAM) clustering technique on the seasonal rainfall and extremes, six spatially coherent regions (clusters) were identified that are contiguous in space and consistent with the topography, which are: Central-West India (CW), Northwest and Northern India (NW), Western Ghats (WG), Deccan Plateau (DP), Central-East India including Indo-Gangetic plain (CE) and Northeast India (NE). Integrated Vertical Transport (IVT) of moisture composites for wet and dry years for each cluster indicated that Bay of Bengal is the major source of moisture for extreme rainfall for all of India, except for WG. Arabian Sea and Bay of Bengal both provide moisture for the seasonal rainfall for western and eastern halves of India, respectively. Trend analysis revealed a decline in seasonal rainfall over CE, NE and WG clusters and increase in extreme rainfall over CW region. These are consistent with increasing IVT trends over Bay of Bengal and decreasing over Arabian Sea and Indian Ocean. Teleconnections to tropical Pacific Sea surface temperatures (SSTs) were reminiscent of El Nino Southern Oscillation (ENSO) patterns, with cooler SSTs in central and eastern Pacific favoring stronger monsoon rainfall and to a lesser extent the extremes. Further, warmer Indian Ocean in recent decades is likely a mediator in the moisture transport by reducing seasonal rainfall and enhancing the extremes. These interesting insights brighten the prospects for skillful forecast of monsoon rainfall and extremes.