Wetting Asian monsoon and drying American monsoon under global warming: Mechanism of zonal asymmetric responses

   Climate model projections reveal zonally asymmetric changes in monsoon rainfall under global warming. American Monsoon rainfall decreases substantially, primarily due to a pronounced weakening of upward air motion, whereas Asian monsoon rainfall generally increases as a result of enhanced atmospheric moisture and minor changes in vertical motion.

   Using abrupt CO₂-quadrupling experiments, we separate the impacts of direct radiative forcing from those mediated by sea surface temperature (SST) changes. First, because the Eastern Hemisphere is dominated by large landmasses while the Western Hemisphere is dominated by oceans, an increase in atmospheric CO₂ can alter large-scale circulation and suppress upward air motion over tropical America, in particular the North American monsoon region. Second, SST warming exhibits a characteristic pattern with amplified warming over the equatorial Pacific relative to the tropical mean warming, and the increase of latent heating over equatorial Pacific induces a Gill-type atmospheric circulation response, suppressing convection and rainfall over tropical American sector. Third, global warming substantially strengthens summertime latent heating over the Tibetan Plateau, and the enhanced heating counteracts the weakening tendency of the Asian monsoon circulation. Therefore, Asian monsoon rainfall changes are dominated by increasing moisture content, while American monsoon rainfall changes are dominated by weakening monsoon circulation.

   These three mechanisms exhibit distinct spatial controls: the first operates at planetary scale and affects both the Asian and American monsoon regions, while the second and third primarily govern changes in the American and Asian monsoons, respectively. The magnitude of equatorial Pacific warming is strongly linked to the historical zonal SST gradient in the tropical Pacific; however, the systematic model bias toward a too-weak historical SST gradient may lead to an underestimation of future drying over the American monsoon regions. Observation-constrained projections suggest that the magnitude of tropical American drying could be up to 1.6 times larger than indicated by raw model projections.

 

References

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