西太平洋副热带高压双脊线及其对1998年夏季长江流域“二度梅”的影响

利用NCEP/NCAR再分析资料、日本气象厅提供的TBB资料研究了 1 998年 7月西太平洋副热带高压突然偏南的原因。结果表明 ,西太平洋副高脊线突然“南撤”有其一定局限性 ,事实上应是副热带高压脊线在南侧的一次“重建”过程。针对这次重建 ,发现 1 998年 7月上中旬在西太平洋副热带地区存在南北两个高压脊 ,据此本文提出了副热带高压双脊线的概念 ,并着重揭示了这次西太平洋副热带高压双脊线的基本演变特征、环流场和温湿场结构、可能的形成机制及其对 1 998年夏季长江流域“二度梅”的影响。分析表明西太平洋副热带高压双脊线时期具有与单脊线时期明显不同的环流特征和温湿场结构 ,其北侧脊线附近的特征与传统上单脊线副热带高压的特征较一致 ,但南侧脊线附近则更多的具有低纬度系统的特点 ;这次双脊线过程与赤道缓冲带北上并与副热带高压打通合并变性及热带对流云团的演变有密切关系。此外 ,文中还通过中国台站降水资料探讨了副热带高压双脊线的维持对中国东部雨型的影响 ,指出西太平洋副热带高压双脊线的出现改变了原有的水汽输送路径 ,从而在中国东部出现两条雨带 ,呈倒 7字型 ,分别与副热带高压北、南侧脊线相对应。这些结果为西太平洋副热带高压演变规律和机制的研究提供了新的线索

INFLUENCE OF THE DOUBLE RIDGES OF WEST PACIFIC SUBTROPICALHIGH ON THE SECOND MEIYU OVER THE YANGTZE RIVER VALLEY DURING 1998

Based on the NCEP/NCAR reanalysis data and the TBB data from Meteorological Institute Research of Japan, the reason that the ridge of the west Pacific subtropical high (WPSH ) in July 1998 suddenly locates much more southward than its normal position is investigated. The results show that the viewpoint on the sudden retreat southward of the WPSH ridge has its limit to some extent. This is in fact a rebuilding process of the WPSH ridge in the south. It is found that there simultaneously exist two WPSH ridges in the same longitude over the west Pacific subtropical region in the first and middle of ten days of July 1998. Accordingly, a new conception, the double WPSH ridges, is presented. Especially, the basic evolution features, circulation field structures, temperature and relative humidity field signatures,and the possibly formative mechanism of the double WPSH ridges in July 1998 are investigated, and its influence on the second MeiYu over the Yangtze River valley in 1998 is explored as well. It is clear that the circulation features and the temperature and relative humidity patterns during the period of the double WPSH ridges in July 1998 markedly differ from those during the single WPSH ridge in other times. During the double WPSH ridges, the features around the northern ridge are consistent with the ones about the single ridge in tradition, while around the southern ridge, there possess more features of the systems in the low latitudes. Moreover, this process is closely relative with the northward movement, the through-get and consolidation of the equatorial buffer band with the WPSH, and the tropical convective cloud cluster as well. Besides, the influence of the maintenance of the double WPSH ridges on the rainband in the East China is studied by using of China 740 station precipitation data. The original paths conveying the water vapor are changed by the occurrence of the double WPSH ridges, consequently, there appear two rainbands taking on the shape of reverse 7, which correspond to the northern and southern ridges of the WPSH respectively. These results provide a new clue for the study of the variation mechanism of the WPSH.