Origins of East Asian Summer Monsoon Seasonality

We investigate the dynamical origins underpinning the climatological seasonal evolution of the East Asian summer monsoon rainfall. The East Asian summer monsoon possesses a unique and complex seasonality, exhibiting distinct intraseasonal stages - Spring, Pre-Meiyu, Meiyu, and Midsummer – and with abrupt transitions between them. Previous studies have suggested that the northward migration of the westerlies impinging on the Tibetan Plateau controls this intraseasonal evolution, through the latter’s effect on the downstream circulation. We explore this hypothesis using an atmospheric general circulation model that simulates the intraseasonal stages with fidelity.

The presence of the Tibetan Plateau is fundamental to the East Asian monsoon seasonality. Without a Tibetan Plateau, East Asia exhibits only one primary convective stage typical of other monsoons. As the Plateau is introduced, the characteristic East Asian rainband and distinct rainfall stages emerge, and rainfall becomes more intense. This emergence co-incides with a pronounced modulation of the westerlies around the Plateau and extratropical northerlies penetrating northeastern China. The northerlies meridionally constrain the moist southerly flow originating from the tropics, leading to a band of confluence and humidity front that produces the rainband.

We highlight the seasonal evolution of these downstream northerlies as key to the distinct East Asian monsoon seasonality. The northward migration of the westerlies away from the northern edge of the Plateau leads to a weakening of the extratropical northerlies, which, coupled with stronger monsoonal southerlies, leads to the northward migration of the humidity front and hence the Meiyu. When the peak westerlies migrate north of the Plateau during the Midsummer stage, the extratropical northerlies disappear, leaving only the monsoon low-level circulation that penetrates northeastern China; the rainband disappears, leaving isolated convective rainfall over northeastern China.

We thus suggest that the unique East Asian rainfall seasonality arises from the interaction of two distinct circulations: the subtropical monsoon circulation that strengthens as summer progresses, and the extratropical northerlies that weakens as summer progresses. The former circulation is typical of subtropical monsoons, but the latter is unique to East Asia resulting from the effect of the Tibetan Plateau on the westerlies. Thus, the basic ingredients needed to produce an East Asian-like rainfall seasonality appears to be (i) a subtropical landmass and neighboring ocean to the east, to produce the subtropical monsoon; and (ii) a Plateau-like feature to the west of the eastern coastline of this continent, embedded within the westerlies such that the latter straddles the Plateau in the winter, but migrate to its north during the early summer and eventually away from the Plateau’s influence in the peak of summer. Idealized simulations incorporating these features in an atmospheric general circulation model supports this interpretation.