2003年夏季异常天气与西太副高和南亚高压演变特征的分析

Weather Abnormal and Evolutions of Western Pacific Subtropical High and South Asian High in Summer of 2003

通过入梅前后东亚环流与多年平均的对比,表明2003年夏季我国东部旱涝分明、长江以南高温少雨、淮河流域持续多雨、强降水过程频繁等特点是与西太平洋副热带高压(下称西太副高)活动特征有关。该年副高偏强、位置异常偏西,垂直结构呈现较强的动力性特征,即低层为辐散下沉运动、高层为辐合,脊线随高度向北倾斜。副高西北侧的梅雨锋区强,且风切变明显。动力和热力诊断结果表明,200 hPa南亚高压脊线以北东传的负涡度平流产生的辐合下沉运动,有利于对流层中下层副高的加强西伸。500 hPa副高西北侧的雨带位置与对流层上层副热带急流的位置和东伸的南亚高压对应。高层西风带和南亚高压的动力作用也会引发副高短期的东西振荡。降水区对流层高层产生的潜热释放,使副高西北侧加热率垂直方向的非均匀分布增加,导致了副高加强西进。热带天气系统的活动,对副高维持和加强也有不可忽视的作用。正是西风带、副热带和热带系统相互作用,动力和热力因素的综合影响,造成了该年副高及其所带来的天气异常。

By comparing with the climatic mean circulations over East Asia, it is demonstrated that, before and during the Meiyu period of 2003, the subtropical anticyclone over Western Pacific （SAWP） is stronger than normal and shifted westwards, presenting more dynamic features in its configuration with convergence in the upper troposphere, divergence and descending motions in the middle and lower tropospheres, and a northward tilting ridge surface with increasing height. The rainbelt associated with the Meiyu front that is located over the northwest of the 500 hPa SAWP is related to a strong upper tropospheric subtropical jet and the eastward extension of the South Asia High（SAH）. The negative vorticity advection along the ridgline of the eastward extended SAH produces the upper layer convergence, in favor of the westward development of the SAWP. The dynamic impacts of the upper layer westerlies and the SAH can also induce the short-term zonal movement of the SAWP. The development of the deep convective precipitation over the northwest of the SAWP intensifies the in situ vertically non-uniform heating, contributing to its westward movement as well. Besides, the development of the deep convection in the tropical monsoon trough also affects the maintenance and intensification of the SAWP. It is concluded that the anomaly of the SAWP in the early summer of 2003 is forced dynamically as well as thermodynamically by the interactions among the weather systems in different latitudes.