Relationship Between The Seasonal Transition Of East Asian Monsoon Circulation And Asia Pacific Thermal Field And Possible Mechanisms

Many observation, reanalysis datasets and CCSM4 data are used to investigate the relationship between the seasonal transition of East Asian monsoon and Asia Pacific thermal contrast and associated possible causes. The main conclusions are follows.(1) Based on the 250hPa air temperature over two selected key areas, the Asia-Pacific thermal difference (APTD) index is calculated. Results show that the APTD index is highly consistent with Asia-Pacific Oscillation (APO) index, which is defined by Zhao et al. in term of different key areas in different seasons. Moreover,the APTD index can be well used to determine the time point of the seasonal transition in association with the Asia Pacific thermal contrast. It is found that the time of seasonal transition determined by the APTD index can represent the characteristic and process of the seasonal change in East Asian atmospheric circulation.(2) The transition characteristics of the circulation from winter to summer type can be summarized as follows. The continental cold high at lower tropospheric level moves eastward to the East China Sea and decreases rapidly in intensity, while Aleutian low decreases fastly and the low-level northerly winds turn to be southerly winds. At mid tropospheric level, the East Asia major trough is reduced and moves eastward. Furthermore,the subtropical high strengthens and appears in the vicinity of Philippines. The South Asia high shifts from the east of Philippines to the west of Indochina Peninsula, together with the prevailing northerly into southerly at upper tropospheric level. Meanwhile, the westerly and easterly jets both jump to the north.The transition characteristics of the circulation from summer to winter type are opposite to the situation from winter to summer type.(3) The seasonal transition of atmospheric circulation is closely related to the thermal contrast, and the possible mechanism can be concluded as follows. Under the background of the APTD seasonal transition, the southerly wind appears firstly at low level, which triggers the ascending motion via changing vertical shear of meridional winds. The resultant latent heating accelerates the transition of heating pattern from winter to summer. The summer heating pattern can further promote the adjustment of circulation, which favors the formation and strengthening of the low-level southerly and upper-level northerly winds. As a result, the meridional circulation of the East Asian subtropical monsoon is established through a positive feedback between the circulation and thermal fields. The situation from summer to winter type is basically opposite to that from winter to summer type. But the winter heating pattern do not show a cooling center in the middle troposphere, the Hadley meridional circulation is established slow comparatively.(4) Moreover, it is concluded that the transition time of East Asian nonsoon circulation is related to the rainfall and surface temperature in China and there is a significant correlation between the seasonal transition time and previous SST anomalies over the tropical central-eastern Pacific Ocean. This conclusion is also verified in the CCSM4 model, which means it can provide a basis for the short-term climate prediction.