On increasing monsoon rainstorms over India

Using a high-resolution daily gridded rainfall data set for the period 1951–2015, new objective criteria were developed to determine rainstorms over the Indian region during the south-west monsoon season (June to September). The rainstorms thus identified have rainfall of 125 mm/day or more at the centre, cover minimum 50,000 km2 in area with rainfall of 25 mm or more and sustain for at least two consecutive days. These rainstorms have potential to cause large-scale floods and associated risk over the region in which they are present. The analysis identified 395 rainstorms during the period, 1951–2015, on an average 6 storms per season. About 40% of the rainstorms are associated with the low-pressure systems which form over the North Bay of Bengal and move north-westwards along the seasonal monsoon trough. The present analysis suggests that over northern parts of India, where majority of rainstorms form, frequency and duration of rainstorms have shown statistically significant increasing trends. During the period of 1951–2015, there was an increase from 4 to 8 rainstorms per year and from 12 to 27 rainstorm days per year. This increase has significant repercussions in terms of added risk of large-scale floods and associated causalities. Further analysis suggests that rainstorm activity over northern parts of India is strongly influenced by the colder sea surface temperature anomalies over the east equatorial Indian Ocean and associated moisture divergent flow and strong moisture convergence over the Indian landmass and the Bay of Bengal.