Based on the studies in Part Ⅰ (see Mao et al.2003),this paper further examines the relationship between the Asian summer monsoon onset and variation in meridional position of the warm temperature ridge with a zonal orientation in mid-upper troposphere.During the Asian monsoon bursting consequentially over the Bay of Bengal,South China Sea,and South Asia,in addition to the reversal of winds in the lower and upper troposphere and deep convection before and after the onset,the atmospheric meridional temperature gradient (MTG) in the vicinity of the ridge-surface of subtropical high (WEB defined in Part Ⅰ) exhibits a significant reversal.Since the establishment of temperature structure with higher over north than over south of the WEB in the mid-upper troposphere (200-500 hPa) characterizes the collective essential that the Asian summer monsoon bursts over different areas,the MTG in mid-upper troposphere,based on the thermodynamics associated with the seasonal transition,should be a reasonable index to measure the Asian monsoon onset.The definition for onset date is proposed,and the time series of onset date for different sections are determined.As compared with the onset dates determined by other indices such as 850-hPa zonal wind and OLR.correlation analyses indicate that the 850-hPa zonal wind is only regional index,but the MTG index is applicable universally to the Asian monsoon regime.
The mechanisms for the variation in the configuration of subtropical anticyclone during seasonal transition are explored from energy budget using the NCEP/NCAR reanalysis data.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 (westerly-easterly boundary) in the middle and upper troposphere also depends on the latitudinal position where temperature ridge locates.The northward shift of the warm 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 physical mechanism responsible for seasonal transition is different from area to area over the Asian monsoon region.The dominant factors responsible for northward shift of the Bay of Bengal warm ridge are the meridional temperature in initial stages of the onset and the descending motion after the onset. The factors for causing the northward jump of the South China Sea warm ridge involve the zonal temperature advection,meridional temperature advection,and diabatic heating associated with the southern China spring rainfall.The subsidence is the factor leading to the northward migration of the South Asia warm ridge.
