Decreasing Frequency of Monsoon Depressions in the 21st Century

Recent South Asian summer monsoon extremes associated with the monsoon depressions, the controversy surrounding observed trends in their occurrences, and a moisture driven wet future precipitation response in the CMIP5 global climate model ensemble have raised the importance of understanding effects of anthropogenic climate change on their frequency and longevity in the 21st century. We find that increase in radiative forcing will significantly reduce the occurrences of summer monsoon depressions after mid-21st century without any major impact on their trajectories over South Asian land. Although more extremes are expected to occur during monsoon depressions, the change in total number of high-intensity precipitation events is more driven by the occurrences happening on non-depression days. The downward trend in monsoon depressions, which is driven by a decrease in long-lived depressions, is associated with the gradual weakening of monsoon dynamics during the coarse of the 21stcentury. Our results indicate a significant relationship between the occurrences of monsoon depressions and the strength of mid-tropospheric meridional winds in the CMIP5 models, which show a progressive weakening in response to increase in radiative forcing. Overall, these projected changes suggest likely changes in the intra-seasonal summer precipitation dynamics over South Asia, should the emissions continue to rise throughout the 21st century.