季节转换期间副热带高压带形态变异及其机制的研究Ⅰ:副热带高压结构的气候学特征

STUDY ON MODAL VARIATION OF SUBTROPICAL HIGH AND ITS MECHANISM DURING SEASONAL TRANSITION PART Ⅰ: CLIMATOLOGICAL FEATURES OF SUBTROPICAL HIGH STRUCTURE

利用NCEP/NCAR再分析资料研究了季节转换期间副热带高压结构的气候特征。在亚、非季风区 ,冬季副热带高压带是相对对称的 ,具有脊线连续的带状结构 ,脊面随高度增加向南倾斜 ;夏季副热带高压带中低层是间断的 ,高层是连续的 ,脊面随高度增加向北倾斜。副热带高压脊面倾斜受热成风关系的制约 ,总是倾向暖区。 5月份副热带高压形态变异最显著 ,不同地域副热带高压的结构、性质存在较大差异。夏季型副热带高压于 5月初首先出现在孟加拉湾东部 ,5月第 3候稳定建立在孟加拉湾东部、中南半岛及南海西部地区 ;5月第 4～ 5候在南海建立 ;6月第 1～ 2候在印度中部建立。夏季型副热带高压建立的 3个阶段与亚洲夏季风爆发的 3个阶段存在着较好的对应关系。孟加拉湾夏季风的建立在很大程度上取决于高空副热带高压脊面附近经向温度梯度的反转。

Climatological characteristics of subtropical high structure during seasonal transition are investigated based on NCEP/NCAR reanalysis data. The ridge the surface of subtropical anticyclone is defined by the boundary surface between westerly to the north and easterly to the south (WEB in brief). Under the constraint of thermal wind relation, the WEB is always tilted toward warmer region in the vertical. In Afro-Asian monsoon area, the subtropical anticyclone belt whose ridgelines are consecutive in wintertime takes on relatively symmetrical and zonal structure, the WEB tilts southward with increasing height; In summer, the subtropical anticyclone belt is discontinuous at lower level and continuous at upper level, the WEB tilts northward from the bottom up. The structure and properties of subtropical anticyclone during seasonal transition are different from area to area. May is the period when subtropical high modality most significantly varies, its ridgelines at middle and low levels start to break. Based on the formation and variation of the vertical axis of subtropical anticyclone, a new concept 'seasonal transition axis' is proposed. The summer pattern of subtropical high firstly appears over eastern Bay of Bengal in the beginning of May, then stablizes over eastern Bay of Bengal, Indochina, and western South China Sea on the 3rd pentad of May. It exists over the South China Sea in the 4th-5th pentad of May and establishes over central India in the 1st-2nd pentad of June. The three consequential stages when summer modal subtropical high occurs correspond to that of Asian summer monsoon onset, respectively. The onset of the Asian summer monsoon circulation is closely associated with such 'seasonal transition axis'. To a great extent, the monsoon onset over the Bay of Bengal depends on the reversal of meridional temperature gradient in vicinity of subtropical high ridge in upper troposphere. The meridional temperature gradient at middle and upper levels in troposphere can be used as good index for defining Asian monsoon onset.