Temporal Change in the East Asian Summer Monsoon Associated with Tibetan Plateau Forcing: A Complementary Means for Assessing Projections of Regional Climate

In response to global warming, the summertime Tibetan Plateau thermal forcing has weakened in recent decades, during which time the summer rainfall over East China has shown a pattern of “wet in the south and dry in the north.” This study establishes a link between the rainfall pattern and weakening of the Tibetan Plateau forcing, based on the results of numerical aqua-planet experiments with various idealized land distributions, designed to reveal the multi-coupling nature of the monsoon system. The results indicate that although the seasonal change in land–sea thermal contrast is essential for monsoon occurrence, the shape and geographic location of land are critical for monsoon formation. No monsoon occurs for land located in the extratropics. For land located in the subtropics, a weak monsoon appears over its southeastern corner, associated with an intense inter-tropical convergence zone (ITCZ). The occurrence of land in the tropics triggers a coupling between the subtropical and tropical climates, and between the Southern and Northern Hemispheres, thereby intensifying the Asian monsoon. The Tibetan Plateau acts to enhance the coupling between lower- and upper-troposphere circulation, and between subtropical and tropical monsoon circulation, resulting in intensification of the East Asian summer monsoon and weakening of the Indian summer monsoon. The intensity of the East Asian summer monsoon is strongly dependent on the development of southerly winds, as the meridional transport of planetary vorticity and energy/moisture can trigger a positive feedback between positive vorticity generation and latent heat release associated with monsoon rainfall.

We then designed more realistic experiments, based on a sophisticated climate model, to investigate how changes in Tibetan Plateau forcing influence the Asian summer monsoon, thereby influencing its rainfall distribution. The results demonstrate that a weakening of sensible heating over the Tibetan Plateau in spring and early summer results in reduced precipitation in the plateau region and a reduction in the associated latent heat release in summer. In turn, these changes result in weakening of near-surface cyclonic circulation surrounding the plateau and weakening of the subtropical anticyclone over the western Pacific. The southerly that normally dominates East Asia, ranging from the South China Sea to North China, then weakens, resulting in turn in a weaker positive feedback between positive vorticity generation and latent heat release. Consequently, the convergence of water vapor transport is confined to South China, forming a specific anomaly pattern in monsoon rainfall of “wet in south and dry in north.” Because the weakening trend of Tibetan Plateau forcing is associated with global warming, the present results provide an effective means for assessing projections of regional climate over East Asia in the context of global warming.