季节转换期间副热带高压带形态变异及其机制的研究Ⅱ:亚洲季风区季节转换指数

STUDY ON MODAL VARIATION OF SUBTROPICAL HIGH AND ITS MECHANISM DURING SEASONAL TRANSITION PART Ⅱ: SEASONAL TRANSITION INDEX OVER ASIAN MONSOON REGION

在“季节转换期间副热带高压带形态变异及其机制的研究Ⅰ :副热带高压结构气候学特征研究”的基础上 ,进一步讨论亚洲夏季风爆发与当地对流层中上层东西向暖脊的经向位置变化关系。亚洲夏季风相继在孟加拉湾、南海和南亚爆发期间 ,除了对流层高、低空风场及深对流活动在季风爆发前后具有反相的变化以外 ,副热带高压脊面附近大气经向温度梯度亦具有明显的反相特征。对流层中上层 (2 0 0～ 5 0 0hPa)脊面附近建立的北暖南冷的温度结构 ,能够反映亚洲各季风区夏季风爆发共同的本质特征 ,根据季节转换的热力学基础 ,指出对流层中上层经向温度梯度作为度量季风爆发的指标是合理可行的。文中提出了以副热带高压脊面附近对流层中上层大气经向温度梯度作为表征季节转换的指数 ,给出了确定季节转换开始日期的具体定义以及历年季节转换日期序列 ,同时给出由85 0hPa纬向风和OLR表征的季风爆发日期序列。相关分析表明 ,85 0hPa纬向风只是个区域性指标 。

The relationships between Asian summer monsoon onset and meridional displacement of the local temperature ridge in the middle and upper troposphere are further explored based on the previous part I. 850 hPa zonal wind and deep convection display opposite characteristics before and after the seasonal transition over the Bay of Bengal, South China Sea and South Asia, respectively. The most significantly reversed element is the meridional gradient of middle-upper temperature (200-500 hPa) in the vicinity of the boundary surface between westerly to north and easterly to south (WEB in brief). The reversal of meridional temperature gradient near WEB reflects the common characteristics of summer monsoon onset for the different Asian monsoon region. According to the thermodynamic foundation of seasonal transition, a new and uniform index, which is define as the time when the meridional gradient of area-averaged middle-upper temperature in the vicinity of WEB changes direction, is proposed for measuring the seasonal transition and monsoon onset over Asian monsoon region. The time series of seasonal transition date is then given by using this index. Correlation analyses indicate that 850 hPa zonal wind is only regional index, but the meridional temperature gradient is an appropriate and uniform index.