DIAGNOSTIC INVESTIGATIONS ON THE MECHANISM OF THE ONSET OF ASIAN SUMMER MONSOON AND ABRUPT SEASONAL TRANSITIONS OVER THE NORTHERN HEMISPHERE PART:Ⅱ THE ROLE OF SURFACE SENSIBLE HEATING OVER TI BETAN PLATEAU AND SURROUNDING REGIONS
Zhang Yongshen Wu Guoxiong (Laboratory of Numerical Modelling Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, P.O. Box 2718,Beijing, 100080)
This is the second part of a series of study on the possible physical mechanism towards the onset of the Asian summer monsoon. In this paper, including a case study of 1989 and climatological analyses of 1980-1989, special attention was paid to investigation of the temporal and spatial evolutions of the air temperature, general circulation and surface sensible heat flux from late spring to early summer over the Asian monsoon regions. The role of surface sensible heating over the Tibetan Plateau and surrounding areas was emphasised. It is shown that, during the transitional season the two abrupt increases of the air temperature over the Tibetan Plateau coincide well with the sequential onset of the Asian summer monsoon over South China Sea and the Indian Subcontinent, respectively. The intense warming due to sensible heating over Tibetan Plateau directly gives rise to a relatively strong disturbance of local anti cyclone at the high levels of the troposphere, and such regime greatly modulate the evolutions of the South Asia High and easterly jet over the northern tropics. It was shown that the strong divergence which appears in the entrance of the easterly jets plays a role in triggering the onset of intense convections. Moreover, it was found that the temporal and spatial evolutions of the tropic easterly jets were greatly affected by the reversals of the meridional temperature gradient on the southern sides of the Tibetan Plateau and the Afghanistan Iran region, where the warming centres developed in a spatial sequence from east to west.