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Past and future trends in South Asian monsoon depressions and their extreme rainfall

Presentation Date
Monday, December 11, 2023 at 4:00pm - Monday, December 11, 2023 at 4:10pm
MC - 3008 - West



Much of the rain that falls in monsoon climates is delivered by transient, propagating vortices that are neither typhoons nor classic extratropical baroclinic waves. This is particularly true in South Asia, where monsoon depressions spin up in continental-scale shear zones, then produce extreme rainfall that is responsible for the majority of that region's hydrometeorological disasters. Adapting to future hydrological change in this densely populated region thus requires quantifying the sensitivity of monsoon depressions to the background climate state. However, observed trends in the precipitation rates of monsoon depressions have not been quantified; future projections in monsoon depression behavior have also remained uncertain because many coarse-resolution climate models have been found to inaccurately simulate these storms.

Here we present analyses of historical trends in the rain rates of monsoon depressions, together with high-resolution global climate model projections of future changes in these storms. Using satellite- and gauge-based precipitation estimates with atmospheric reanalyses, we show that, over the past 40 years, rain rates have increased in the rainiest quadrant of monsoon depressions, southwest of the vortex center. We associate monsoon depressions with more than half of the increased number of extreme precipitation events over central India. However, precipitation decreased in the eastern quadrants of monsoon depressions, yielding no clear trend in precipitation averaged over the entire storm diameter. These storm changes occurred in a background environment where humidity increased rapidly over land while warming was more muted. We also examine changes in monsoon depressions simulated by an ensemble of high-resolution global climate models (HighResMIP) that we show skillfully represent these storms. In the ensemble mean, the number of monsoon depressions decreased over the last 65 years by about 15%; the reduction in depression counts then reverses to produce no trend between 1950 and either 2050 or 2100. The ensemble mean projects a shift in storm genesis from ocean to land and an increase in monsoon depression precipitation of at least 7%/K, which together contribute to a projected future increase in seasonal mean and extreme precipitation over central India.

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