Impacts of the Tibetan and Iranian Plateau on the Onset, Evolution and Maintenance of Asian Summer monsoon

Abstract:

A brief introduction of the Tibetan Plateau (TP) influence on the Asian climate is demonstrated. The influence is mainly through its mechanical forcing in winter half year and thermal forcing in spring and summer. In winter, the TP reacts to impinging mid-latitude westerly winds by exerting a negative mountain torque on the atmosphere. Thus the zonal deviation of streamlines presents an asymmetric dipole with an anticyclone to the north of the TP and a cyclone to its south. So the isotherms in the high latitudes of Asia tilt from northwest to southeast. The cyclonic gyre in low latitudes triggers the dry season in South Asia and the persistent rainy season in Southeast Asia and South China.

In summer, the strong surface sensible heating of the TP lifts the surrounding moist air upward and excites a huge cyclonic circulation over South and East Asia, working like a sensible heat driven air- pump (SHAP). The strong pumping of the TP-SHAP causes convergence and a cyclonic spiral. The TP is therefore an important location for the genesis of vortices that can propagate eastward and produce torrential rain along the Yangtze River in summer.

Vertical coupling of the upper- and lower-level circulations is vital for the ASM onset and its evolution. The TP forcing anchors the spring onset of the ASM over the Bay of Bengal (BOB) by generating the strong but short-life BOB warm pool and producing the local monsoon onset vortex (MOV) in the lower troposphere, and by modulating the South Asian High (SAH) in the upper troposphere. On intra-seasonal timescales, the TP thermal forcing significantly modulates spring rainfall in southern China and generates the bi-weekly oscillation of the SAH in summer. Despite climate warming, the atmospheric heat source over the TP, particularly in spring exhibits a clear weakening trend from the 1980s to 2000, and contributes to the anomalous ‘dry in the north’ and ‘wet in the south’ rainfall pattern observed over East China.

It is demonstrated that for the maintenance or formation of the ASM, the insulation of the Tibetan- Iranian Plateau (TIP) on the dry and cold intrusion from subtropics into tropics is insignificant. Instead, it is the thermal control of the TIP that plays a dominant role in the generation of the northern part of the South Asian summer monsoon and the Eastern Asian summer monsoon.