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Past, present, and future of the South Asian monsoon

Ummenhofer, Caroline C.; Geen, Ruth; Denniston , Rhawn F.; Rao, Mukund Palat

As a key component of the global climate system, the South Asian monsoon represents the largest of the Earth’s monsoon systems and a lifeline for the region’s nearly 2 billion people, who receive most of their annual precipitation during boreal summer. Traditionally characterized as continent-scale “sea breezes,” where the land heats faster than the ocean during summer, causing rising air over the continent and moist oceanic air to be drawn in, recent theoretical advances view the monsoon as a regional manifestation of the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zones. The South Asian monsoon varies intraseasonally with active and break spells in response to interannual climate modes (e.g., El Niño-Southern Oscillation) as well as on multidecadal and longer timescales. Hydroclimatically sensitive proxy records (e.g., speleothems, tree rings) provide a centennial to millennial context for South Asian monsoon variability and its extensive societal impacts. While an anthropogenically driven weakening of the South Asian summer monsoon in the 20th century is seen, climate model projections suggest a strengthening during the 21st century. Despite progress in operational forecasts, successful predictions of the South Asian monsoon across a range of spatial and temporal scales still represent considerable challenges.

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The Indian Ocean and its Role in the Global Climate System

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