Observed changes in extreme wet and dry spells during the South Asian summer monsoon season

Abstract

The South Asian summer monsoon has an impact on over one billion people. This study applies statistical techniques to precipitation observations (over the period 1951–2011) and finds significant increases in daily precipitation variability, the frequency of dry spells and the intensity of wet spells, whereas dry spell intensity decreases. The South Asian summer monsoon directly affects the lives of more than 1/6th of the world’s population. There is substantial variability within the monsoon season, including fluctuations between periods of heavy rainfall (wet spells) and low rainfall (dry spells)1. These fluctuations can cause extreme wet and dry regional conditions that adversely impact agricultural yields, water resources, infrastructure and human systems2,3. Through a comprehensive statistical analysis of precipitation observations (1951–2011), we show that statistically significant decreases in peak-season precipitation over the core-monsoon region have co-occurred with statistically significant increases in daily-scale precipitation variability. Further, we find statistically significant increases in the frequency of dry spells and intensity of wet spells, and statistically significant decreases in the intensity of dry spells. These changes in extreme wet and dry spell characteristics are supported by increases in convective available potential energy and low-level moisture convergence, along with changes to the large-scale circulation aloft in the atmosphere. The observed changes in wet and dry extremes during the monsoon season are relevant for managing climate-related risks, with particular relevance for water resources, agriculture, disaster preparedness and infrastructure planning.