Origins of East Asian Summer Monsoon Seasonality

The East Asian summer monsoon is unique amongst summer monsoonal systems in its poleward reach as well as in its complex seasonality, exhibiting dynamically distinct rainfall stages and abrupt transitions between them. Previous studies have alluded to the seasonally-varying downstream influence of the westerlies flowing around the Tibetan Plateau as central to the rainfall seasonality. Here, we explicitly demonstrate the influence of the Tibetan Plateau in setting up this unique seasonality, using an atmospheric general circulation model that simulates the seasonal transitions with fidelity. Without a Tibetan Plateau, the East Asian summer monsoon exhibits only one primary stage typical of other monsoons, and does not extend as far northwards as seen today. As the Plateau height is increased, the distinct rainfall stages (Spring/Pre-Meiyu, Meiyu, and Midsummer) emerge, and rainfall becomes more intense. This emergence co-incides with a pronounced modulation of the westerlies around the Plateau and downstream, and which changes character following its northward seasonal migration. The role of various mechanisms that lead to the influence on the Tibetan Plateau on East Asian rainfall seasonality - including stationary waves, sensible heating, and the blocking effect of the Plateau - are explored with the aid of idealized simulations. Our results suggest that mechanical forcing of the Plateau on the westerlies is largely responsible for the existence of the distinct seasonal stages, while the thermal influence of the Tibetan Plateau controls the overall intensity of East Asian summer monsoon rainfall; notably, the northward extent of the Plateau facilitates the northward penetration of East Asian monsoonal rainfall. Our results highlight the unique nature of the East Asian summer monsoon dynamics and the unusual role of the westerlies interacting with the Plateau in setting its distinct seasonal behavior.