The South Asian Summer Monsoon (SASM) is the world's most significant monsoon system, providing approximately 80% of the region's annual rainfall—influencing agriculture, water security, and the livelihoods of over a billion people across the Indian Peninsula, the western Indochina Peninsula, and the southern Qinghai-Tibet Plateau.
Due to the monsoon's broad effects on the region, making accurate projections of its dynamics under climate warming is crucial. However, current projections—that SASM rainfall will intensify while circulation weakens—differ from ancient climate behavior, in which both rainfall and circulation intensified during warming periods.
Since scientists often use past climate behavior to inform future change, it is unclear how past data should be used in this circumstance, where past climate behavior differs from future projections of weakened SASM circulation.
Now, however, a recent study published in Nature seeks to address this contradiction by exploring how the SASM responds to warming under six climate scenarios, spanning from the past to the future. Led by researchers from the Institute of Atmospheric Physics at the Chinese Academy of Sciences, the study develops a unified framework based on thermodynamic (moisture-driven) and dynamic (wind-driven) processes that govern changes in the SASM, suggesting that insights from past warm climates can inform our understanding of the future SASM.
Using multi-model climate simulations and geological data, the study identifies similar SASM changes across past warm intervals—specifically, the mid-Pliocene (~3.3–3 million years ago), the Last Interglacial (~127,000 years ago), and the mid-Holocene (~6,000 years ago)—as well as three future warming scenarios (2071–2100). These warm periods are marked by different external forcings, including elevated CO2 levels, continental greening and reduced ice sheets, and increased summer insolation. The study's findings indicate an overall increase in monsoon rainfall, though with regional differences; a weakening of the monsoon trough-like circulation over the Bay of Bengal; and a strengthening of monsoon circulation over the northern Arabian Sea.
Furthermore, the study reveals that the thermodynamic and dynamic mechanisms underlying SASM changes remain consistent across both past and future warm intervals. Differences in their magnitudes of dynamics help clarify discrepancies found in previous research. Thermodynamically, the phenomenon follows the "wet gets wetter" pattern, implying that rising global temperatures lead to an increase in atmospheric moisture. Dynamically, changes in monsoon circulation are propelled by enhanced thermal contrasts, with the resulting non-uniform dynamic effects primarily mediated by sensible heat flux.
Building on insights from warm climates, the researchers also develop physics-based regression models for the future SASM using historical data. Given the characteristics of warming, these models effectively predict future changes in monsoon circulation and rainfall as projected by climate models, achieving spatial correlation coefficients of approximately 0.8 and 0.7 under high-emissions scenarios. This suggests that past analogs, supported by paleoclimate reconstructions, hold great promise for improving future projections of the SASM.
