Past warm intervals inform the future South Asian summer monsoon

In the future, monsoon rainfall over densely populated South Asia is expected to increase, even as monsoon circulation weakens. In contrast, past warm intervals were marked by both increased rainfall and a strengthening of monsoon circulation, posing a challenge to understanding the response of the South Asian summer monsoon (SASM) to warming. Here we show consistent SASM changes in the mid-Pliocene, Last Interglacial, mid-Holocene, and future scenarios, characterized by an overall increase in monsoon rainfall, a weakening of the monsoon trough-like circulation over the Bay of Bengal, and a strengthening of the monsoon circulation over the northern Arabian Sea, as revealed by a compilation of proxy records and climate simulations. Increased monsoon rainfall is thermodynamically dominated by atmospheric moisture following the rich-get-richer paradigm, and dynamically dominated by the monsoon circulation driven by the enhanced land warming in the subtropical western Eurasia and northern Africa. The coherent response of monsoon dynamics across warm climates reconciles past strengthening with future weakening, reinforcing confidence in future projections. Further prediction of SASM circulation and rainfall by physics-based regression models using past information agrees well with climate model projections, with spatial correlation coefficients of approximately 0.8 and 0.7 under the high-emissions scenario. These findings underscore the promising potential of past analogs, bolstered by paleoclimate reconstruction, in improving future SASM projections.