DIAGNOSTIC INVESTIGATIONS OF MECHANISM OF ONSET OF ASIAN SUMMER MONSOON AND ABRUPT SEASONAL TRANSITIONS OVER NORTHERN HEMISPHERE PART Ⅰ

In this work, an sttempt had been made to explore the physical processes towards the onset of Asian summer monsoon and the abrupt seasonal transitions in a coupled ocean/land/at mosphere system. In the first paper, special attention was paid to investigate the basic facts for the establishment of Asian summer monsoon and the atmosphere/ocean/land interactions over the tropics and subtropics. It is shown that the Asian summer monsoon system underwent three distinct stages of the onset in the early May, middle May and middle June, resulting in deep convection and intensive rainfall over the southwestern coast of Indo-China, the South China Sea and the southern Indian subcontinent respectively. When the monsoon convection burst out over South Chian Sea, the maximum centre of the surface latent heat flux (LHF) in relation to the development of the cross equatorial flows moved northward from the southern equatorial Indian Ocean, and two intense maximun centres of LHF appeared over Arabian Sea and the Bay of Bengal when monsoon rainfall bust out over southern Indian subcontinent. At meantime land-sea contrast of LHF became distinct over the eastern coast of Africa. In the subtropical-middle latitude of 25.7-37.5°N from winter to summer months, the LHF and surface sensible heat flux (SHF) intensified gradually over the Tibetan Plateau whereas decreased over west Pacific. It should be noticed that the air-sea interaction play an important role during the onset of monsoon rainf all: it resulted in the intense evaporation and then transported the water vapour to the areas where strong convection bust out.