STUDY ON MODEL VARIATION OF SUBTROPICAL HIGH AND ITS MECHANISM DURING SEASONAL TRANSITION PART Ⅲ:THERMODYNAMIC DIAGNOSES

The mechanisms for the variation in the configuration of subtropical ant icyclone during seasonal transition are explored from energy budget using the NCEP/NCAR reanalysis dat a.Based on the seasonal variations of temperature and heating fields,it is found that the significant diabatic heating associated with spring precipitation over southern China has impacts on subsequent Asian seasonal transition.The reversal of meridional temperature gradient in the vicinity of the WEB (boundary surface between westerly to north and easterly to south) in the middle and upper troposphere also depends on the lat it ude where temperature ridge locates.The northward shift of the temperature ridge results from the fact that the local temperature increase to the north of the WEB is more than that in its vicinity.The diagnostic results through thermodynamic equation show that thermal mecha-nism in seasonal transition is different from area to area over the Asian monsoon region.The abrupt northward shift of temperature ridge over eastern Bay of Bengal is mainly due to meridional warm advection at its initial transition,but due to descent after the transition.The abrupt northward jump of temperature ridge over South China Sea is caused by warm meridional advection,warm zonal advection and diabatic heating associated with precipitation over southern China,among them the warm meridional advection is more significant.The reversal of meridional temperature gradient over South Asia is mainly a result of subsidence.