南亚高压的南北偏移与我国夏季降水的关系

Meridional Variation of South Asian High and Its Relationship with the Summer Precipitation over China

该文定义了一个能较好反映南亚高压南北偏移的指数,并发现该指数与我国夏季降水,尤其是华北和长江流域的降水,无论在年际变化上还是长期趋势上都具有十分显著的相关关系。南亚高压位置偏北时,在我国东部至日本上空存在一个显著的异常反气旋,其中心自上而下向南倾斜,在高层给华北地区带来辐散,在低层使得气流在长江流域辐散,在华北地区辐合,造成华北地区降水偏多,长江流域降水偏少。同时,南亚高压偏北对应着高层两风急流以及中层西太平洋副热带高压偏北,使得我国整个雨带偏北。此外,通过与海温的相关分析发现,南亚高压的长期偏南趋势可能受到印度洋增暖的直接影响。南北偏移指数可作为预测我国夏季区域降水的重要指标,在气候预测业务中有一定的应用价值。

South Asian High （SAH） is the most intense and stable a member of the East Asian Summer Monsoon system, SAH upper level anticyclone in boreal summer. As plays an important role in the regional climate anomaly over China. The meridional variation of SAH is analyzed by using the monthly mean data derived from the Europe- an Center for Medium-Range Weather Forecasts （ECMWF） 40-year reanalysis （ERA-40） from 1958 to 2002. An index of SAH （SAHI） is defined to measure its meridional variation in summer and to analyze its relationship with the summer precipitation over China. The results show a significant correlation between the meridional position of SAH and the summer rainfall over China for both the interannual timescale vari- ability and the long-term linear tendency. The correlation coefficients between SAHI and the summer rain- fall in North China and in the Yangtze River Valleys are 0. 577 and --0. 604, respectively, both of which exceeds 0.01 level. When SAH locates further northward （southward）, North China and South China are wetter （drier） than normal, while the Yangtze River Valleys is drier （wetter） than normal. And the southward linear trend of SAH corresponds with the decreasing trend of rainfall in North China and the in- creasing trend of rainfall in the Yangtze River Valleys. Linear regression analysis of the circulation reveals that when SAH locates further northward, an a- nomalous anticyclone controls eastern China with its center tilted southward from 200 hPa to 850 hPa. In the upper atmosphere, the anomalous anticyclone forms a divergence zone over North China. In the lower atmosphere, it results in flows diverging over the Yangtze River valley, and converging over North China. Besides, the northward movement of SAH would cause the upper-level westerly jet and the Western Pacific Subtropical High move northward, with the rainbelt locating in North China. The meridional anomalous variation of SAH is closely related to the sea surface temperature anomalies （SSTA） of the Tropical Indian Ocean （TIO）, the Central and Eastern Equatorial Pacific, and the northern Pacific. And the TIO SSTA might modulate its meridional position directly. Positive TIO SSTA might lead to a southward expansion of SAH. Due to strong correlation with the summer rainfall over China and being modulated by the TIO SSTA, the meridional variation of SAHI could be considered as an important indicator used to predict the regional climate anomaly.