The heating sources over the Tibetan Plateau(TP),the East Asian plain,and the western North Pacific(WNP)form a terraced thermal contrast in the west-east direction.Over East Asia and the WNP,this zonal thermal contrast contributes as high as 45%to the seasonal variance based on the EOF analysis and exerts a significant impact on the seasonal transition of the East Asian climate through the enhancement of the year-round southerly to the southeast of the TP in late March and early April.This effect is investigated in this study using a high-resolution regional atmospheric model by doubling the surface sensible heat flux,respectively,over the TP,the East Asian plain,and the WNP in three sensitivity experiments.Comparisons among the experiments reveal that doubling the surface sensible heat flux over the WNP has little upstream response over East Asia.The increased zonal thermal contrast between the TP and the East Asian plain due to doubled heat flux over the TP would induce anomalous northerly over the region with year-round southerly to the southeast of the TP and weaken its seasonal enhancement.Doubling the surface sensible heat flux over the East Asian plain decreases the zonal thermal contrast and leads to southerly anomaly over the region with year-round southerly to the southeast of the TP and South China,which is favorable for the enhancement of the year-round southerly and its eastward extension.
The NCEP/NCAR reanalysis datasets and Climate Prediction Center(CPC) Merged Analysis of Precipitation(CMAP) rain data are used to investigate the large scale seasonal transition of East Asian subtropical monsoon(EASM) and its possible mechanism.The key region of EASM is defined according to the seasonal transition feature of meridional wind.By combining the 'thermal wind' formula and the 'thermal adaptation' equation,a new 'thermal-wind-precipitation' relation is deduced.The area mean wind directions and thermal advections in different seasons are analyzed and it is shown that in summer(winter) monsoon period,the averaged wind direction in the EASM region varies clockwise(anticlockwise) with altitude,and the EASM region is dominated by warm(cold) advection.The seasonal transition of the wind direction at different levels and the corresponding meridional circulation consistently indicates that the subtropical summer monsoon is established between the end of March and the beginning of April.Finally,a conceptual schematic explanation for the mechanism of seasonal transition of EASM is proposed.
