Variability of extreme rainfall events over India during Southwest monsoon season

Abstract

The spatial and temporal variability of extreme rainfall events over India and their long-term trend have been analysed during the southwest monsoon season from June to September (JJAS) by taking daily rainfall data for 55 years (1951-2005). The contribution of extreme rainfall events (Category iii with rainfall > 124.4 mm day^-1) to the total seasonal rainfall is also analysed for the same period. The analysis shows that the occurrence of extreme rainfall events over India during JJAS shows spatial variations with preferred regions of occurrence over the west coast, central parts of India and northeast India. The average frequency of extreme rainfall events along with the contribution of extreme rainfall events to the seasonal rainfall shows a significant increasing trend (above the 98% confidence level) over the Indian region during JJAS, and also during June and July. It is also found that the increasing trend of contribution from extreme rainfall events during JJAS is balanced by a decreasing trend in Category i (rainfall ≤ 64.4 mm day^-1) rainfall events. Using the filtered data it is found that the increasing trend of extreme rainfall over central India is mainly contributed by synoptic scale systems (periodicity between 3 and 7 days). The seasonal mean moist convective instability (CI) during JJAS averaged over central India indicates a significant increasing trend (99.9% confidence level) and is basically due to the increasing trend of the number of days with greater degree of moist CI during JJAS, which may be one possible cause for the increasing trend in the frequency of extreme rainfall events over the region.