Rise in Rainfall of South Asian Monsoon Low-Pressure Systems

Abstract

Cyclonic low‐pressure systems (LPS) are the dominant synoptic‐scale rain-bearing system of the South Asian summer monsoon. Traditionally categorized by intensity as monsoon lows, monsoon depressions, and more intense cyclonic storms, LPS produce intense rainfall and floods in some of the world’s most densely populated regions. Yet the contribution of the relatively weak lows vs. the stronger depressions to extreme rainfall and its trends remains unknown; this knowledge gap is particularly troubling because historical trends in LPS have been difficult to assess due to changes in the observing network. Future projections have also remained highly uncertain due to the inability of many coarse-resolution climate models to accurately simulate LPS.Here we use satellite and gauge-based precipitation estimates with atmospheric reanalyses to show that precipitation in monsoon depressions has become more intense in recent decades. This intensification has occurred as humidity over parts of India increased more rapidly than nearly anywhere else on Earth. Precipitation in depressions has risen at a relative rate larger than that of specific humidity, suggesting that upward motion in depressions has become more intense; vertical motion trends in a state-of-the-art reanalysis, which incorporates nearly all long-term climate forcings, are consistent with this hypothesis. We also examine changes in South Asian LPS precipitation simulated by an ensemble of high-resolution global models, which we find skillfully represent these storms. Future trends in total LPS precipitation, including in monsoon depressions, lie near an approximate Clausius–Clapeyron rate (7%/K) in the multi-model mean. This change in LPS rain rates contributes to a projected future increase in seasonal mean and extreme precipitation over South Asian land. Adaptation to future changes in human exposure to hydrological extremes thus requires careful monitoring, accurate multi-decadal projections, and skilful short-term forecasts of the interaction of the humidity field with the dynamics of monsoon LPS.