1998年7月南亚高压影响西太平洋副热带高压短期变异的过程和机制

IMPACT OF SOUTH ASIA HIGH ON THE SHORT-TERM VARIATION OF THE SUBTROPICAL ANTICYCLONE OVER WESTERN PACIFIC IN JULY 1998

利用逐日NCEP/NCAR再分析资料,对1998年7月二度梅期间南亚高压影响西太平洋副热带高压短期变异的过程和作用机制进行了诊断。结果发现,梅雨间歇期副高单体异常向西、向北发展期间,高层南亚高压曾离开高原上空东移(伸)至120°E以东;南亚高压返回高原上空时,中层副热带高压减弱南落,两者有相向然后相背移动的趋势。进一步研究这两个高压系统密切关联的原因又发现,南亚高压通过两种作用机制影响中层副高的短期变异:南亚高压在东移过程中,高空负涡度平流动力强迫的下沉运动在中层副高区域产生辐散,从动力上影响副高内的负涡度发展;另一方面,强烈下沉运动伴随的绝热加热效应又有利于纬向温度梯度维持,有利于南北风发展,从热力上间接影响西太副高的发展。最后,利用R42L9/LASG大气环流模式进行敏感性数值试验,通过改变高空南亚高压东移产生的负涡度效应,发现中层副热带高压区确有强迫产生的动力辐散和绝热加热出现,并对应副高的异常加强,成功地验证了诊断所揭示的1998年夏季副高短期变异过程的两种机制。

Evidence already shows that the variations of the Subtropical Anticyclone over the Western Pacific（SAWP） appear to be closely related with that of the South Asian High （SAH） in the upper layer. The seasonally northward advancement of the SAWP corresponds to the seasonally meridional movement of the SAH. When the main center of the SAH is over the Tibetan Plateau, it has also been shown that the longitudinal shift of the SAH tends to be followed by the certain variations of the SAWP. Specifically, the eastward shift of the SAH seems to be accompanied with the northwestward development of the SAWP, and vice versa. However, what links the two highs at different levels together dynamically？ It remains unclear yet, especially at the short time scale. In the summer of 1998, the variation of the SAWP, from the belt-brokening and the anomalous developing to the weakening, that resulted in the double-MeiYu in the mid-lower drainage area of Yangtze, has been studied from various views. Our earlier investigations on this case have indicated some close relations between the short-term variation of the SAWP and the upper layer circulations. The main subject of this paper is to try to find some internal and dynamic linkages between the upper layer SAH and the lower layer SAWP through both diagnostics and numerical simulations. The daily NCEP/NCAR reanalysis data are applied to investigate the impacts of the South Asian High （SAH） at 200 hPa on the short-term variation of the Subtropical Anticyclone over the Western Pacific （SAWP） at 500 hPa in the second stage of the 1998 meiyu season （July）. Results show that during the breaking period of meiyu, the eastward shifting of SAH beyond 120°E was accompanied with the northwestward development of SAWP along the east coast of China; while the westward return of SAH to the Tibetan Plateau was accompanied with the weakening of SWAP and the resuming of meiyu. The two anticyclone systems at different levels tended to move towards each other at first and then move away from each other. It is also shown that the eastward shifting of SAH influenced the short-term variation of SAWP at lower level through the following two ways： First, the descent flow dynamically forced by the negative vorticity advection at the upper layer diverges at the middle level subtropical anticyclone area, contributing to the vorticity development of SAWP;Second, the effect of adiabatic warming by the descent also enhances the longitudinal temperature gradient, contributing to the development of South/North winds at the western/eastern side of SWAP and to the variation of SWAP thermodynamically. These mechanisms are verified with a series of numerical experiments based on the atmospheric circu- lation model R42L9/LASG.