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Meridional Variation of South Asian High and Its Relationship with the Summer Precipitation over China
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摘要: 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西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。Abstract: South Asian High (SAH) is the most intense and stable upper level anticyclone in boreal summer. As a member of the East Asian Summer Monsoon system, SAH 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 European 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 variability and the long-term linear tendency. The correlation coefficients between SAHI and the summer rainfall 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 increasing trend of rainfall in the Yangtze River Valleys.Linear regression analysis of the circulation reveals that when SAH locates further northward, an anomalous 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.
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图 1 南亚高压南北偏移指数的定义
(a)1958—2002年夏季平均200 hPa位势高度场 (单位：gpm)，(b) 南亚高压南北偏移指数标准化序列，(c) 南亚高压南北偏移高指数年 (实线) 和低指数年 (虚线)200 hPa位势高度场合成 (单位：gpm)
Fig. 1 Definition of the South Asian High Index (SAHI)
(a) climatology of summer geopotential height (unit: gpm) at 200 hPa from 1958 to 2002, (b) standardized time series of SAHI, (c) composites of geopotential height (unit: gpm) at 200 hPa in case of high SAHI (solid line) and low SAHI (dashed line)
图 2 南亚高压南北偏移与我国夏季降水的关系
(a) 南北指数回归的我国夏季降水异常分布 (回归系数大小由色阶表示；相关系数达到0.05显著性水平的区域用粗线标出)，(b) 华北地区降水标准化序列 (及趋势) 与南北偏移指数序列的对比，(c) 长江流域降水标准化序列 (及趋势) 与南北偏移指数序列的对比
Fig. 2 Relationship between SAHI and precipitation over China
(a) pattern of regression of anomalous precipitation over China against SAHI (correlations exceeding 0.05 level are highlighted by thick lines), (b) standardized time series of precipitation (with the linear tendency) in North China compard with SAHI series, (c) standardized time series of precipitation (with the linear tendency) in Yangtze River Valleys compared with SAHI series
图 3 南亚高压南北偏移指数回归的环流场 (矢量) 以及高度场 (等值线，单位：gpm) 异常(阴影区表示纬向风或经向风异常达到0.05显著性水平)
(a)200 hPa，(b)500 hPa，(c)850 hPa (仅环流场异常)
Fig. 3 Patterns of regression of anomalous horizontal wind (vector) and geopotential height (contour, unit: gpm) against SAHI at 200 hPa (a), 500 hPa (b) and 850 hPa (anomalous horizontal wind only)(c)(significant values of anomalous horizontal wind exceeding 0.05 level are shaded)
图 4 南亚高压南北偏移与海温异常的关系
(a)1958—2002年夏季海温异常线性变化趋势，(b) 南亚高压南北偏移指数回归的海温异常异常场 (相关系数达到0.05显著性水平的区域用粗线标出)
Fig. 4 Relationship between SAHI and SSTA
(a) linear tendency of SSTA, (b) pattern of regression of SSTA against SAHI (correlations exceeding 0.05 level are highlighted by thick lines)
图 5 标准化印度洋海温异常与南亚高压南北偏移指数的时间序列
Fig. 5 Standardized time series of Indian Ocean SSTA and SAHI with their linear tendency
表 1 南亚高压各项指数与我国各区域夏季降水的相关关系
Table 1 Correlations of SAHI to precipitation in North China and the Yangtze River Valleys
指数 华北地区 长江流域 南北偏移指数 0.577** -0.604** 面积指数 0.061 0.382** 中心强度I1 0.291 0.170 强度I2 0.175 0.323* 强度I3 0.349* 0.169 注：*表示相关系数达到0.05显著性水平，**表示相关系数达到0.01显
著性水平。另外，南北偏移指数外其余4项指数的定义[21]：① 面积指
数：区域内位势高度大于或等于12500 gpm总格点数；② 中心强度I1：
高压中心位势高度值；③ 强度I2：区域内位势高度大于12500 gpm时所
有格点上的位势高度值与12500 gpm之差的总和；④ 强度I3：I2与面积
指数之比。
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