Influence of the Mechanical and Thermal Forcing of Tibetan Plateau on the Circulation of the Asian Summer Monsoon Area

Abstract

The NCEP/NCAR reanalysis data were used to study the seasonal variation of thermal condition over Tibetan Plateau(TP),and its mechanical and thermal forcing on the atmospheric circulation over the surrounding regions,particularly for the tropical Asian monsoon circulation.Results show that the mechanical effects on the circulation,is the strongest in winter,get reduced in spring.The mechanical forcing result in a winter pattern of lower atmosphere deviation circulation,characterized by the cyclonic/anticyclonic circulations over the south/north of the TP,which is called the TP Dipole Mode.Following northward shift of the westerly and transition of total heating of from negative to positive in April,the cyclonic deflection circulation in the southern becomes stronger and moves northward gradually,and becomes a typical summer cyclonic circulation patterns over the whole in June.To the southern TP,the mechanical and thermal forcing result in Indian-Burma trough all the time over the Bay of Bengal, and thus,lead to a stronger sensible heating on the Indian Peninsula than that over the Indochina Peninsula,and a stronger latent heating on the Indochina Peninsula than that on the Indochina Peninsula.The variation of Indian-Burma trough exhibits a semi-annual periodic feature.The maximum strength of Indian-Burma trough in the early February and August correspond to the strongest of the winter mechanical and summer thermal forcing,respectively.The analysis of the tropical meridian wind shows that the thermal forcing before the monsoon onset is very important,particularly for the persistent rainfall in early spring in South China,and occurrence of the earliest Asian monsoon onset in the eastern of the Bay of the Bengal and the latest in the Indian Peninsula.