1998年夏季西太平洋副热带高压的变异分析

该文利用NCEP再分析资料和其他相关资料分析了1998年夏季西太平洋副热带高压的变异特征和机理。与多年平均相比，1998年夏季副高总体偏强、偏西。副高的南北进退出现显著异常，尤其是7月中旬，副高突然大幅度南撤，南撤后，副高偏南维持至8月初，长江中下游及以南地区出现“二度梅”。副高南撤现象从副高带的东部开始，逐渐西传。分析表明:副高的异常变化是东亚大气环流异常调整的结果，其中副热带西风急流的异常变化对副高南撤起着重要的作用，急流位置和强度的变化及伴随的入口区和出口区次级环流和不稳定均有利于副高在7月中旬南撤和南撤后偏南维持。T106L19全球谱模式的敏感性数值试验表明，高度场对急流减弱的响应与观测分析结果一致。热源和水汽汇分析、OLR分析以及热源与位势高度场SVD分析均表明:在副高南撤前，热带季风区非绝热加热显著减弱；这一结果有利于副高偏南维持；经圈环流的分析结果也与此相吻合。

Researches on Anomalous Characteristics of the Subtropical High over the Western Pacific During 1998 Summertime

Using NCEP reanalysis data, the anomalous characteristics and mechanisms of the subtropical high over the western Pacific during 1998 summer was analyzed. Contrast to the case of climate mean, the subtropical high of 1998 summer was stronger with its ridge extending westward far more. In addition, the abnormal latitudinal fluctuation of the subtropical high occurred, especially on withdrawing abruptly from north to south at the mid of July and then maintaining at lower latitudes until the beginning of August. This caused the second Meiyu precipitation over the Yangtze regions. The latitudinal fluctuation started at east of the subtropical high belt over the mid-Pacific, propagated westward gradually. It shows that the anomalous variation of the 1998 subtropical high is due to the adjustment of the East Asian circulation, in which anomalous variation of the subtropical westly jet is important for the retreat of the subtropical high. The variation on location and intensity of the jet and the induced secondary circulation at entrance/exit of the jet, as well as the instability impacts on the anomaly of the subtropical high on July. The sensitivity numerical experiment by T106L19 global spectral model indicates that the response of geopotential height to weakening of the jet is consistent with the result by observational analysis. Heat source/vapour sink analysis, OLR analysis and SVD analysis with heat source and geopotential height presents that the diabatic heating over the tropical monsoon region is notably weakened before the retreat of the subtropical high, which makes for the subtropical high maintaining at lower latitude. The analysis on the meridional circulation also proves above view.