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东亚冬季风经向异常与后期嘉兴梅雨关系
The Relationship Between East Asian Winter Monsoon Meridional Anomaly and Meiyu Precipitation of Jiaxing
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摘要: 利用1948—2005年NCEP/NCAR再分析资料, 定义了一个可以反映中高纬度间冬季风强弱差异的冬季风经向变化指数 (WMMCI), 该指数与东亚冬季风指数有较好的对应关系。强 (弱) 冬季风年后期嘉兴梅雨偏多 (少) 为主, 年际、年代际关系对应为正相关, 并对后期降水有持续影响, WMMCI指数持续增强 (减弱) 2年并处于强 (弱) 冬季风年时, 后期嘉兴梅雨持续偏多 (少), 嘉兴梅雨降水升高或降低的变化趋势滞后于该指数变化。对500 hPa高度场距平合成分析表明:强 (弱) 冬季风年北正 (负) 南负 (正) 的距平分布使中高纬度经向 (纬向) 环流发展, 有利 (不利) 于冷空气南下影响, 并对后期大气环流产生持续影响, 从而有利 (不利) 于降水。冬季风经向异常对嘉兴后期降水持续影响的一个可能解释是, 持续增强 (减弱) 的强 (弱) 冬季风使赤道太平洋地区盛行风向发生改变, 使赤道太平洋海温变化出现异常, 触发El Niño (La Niña) 事件, 从而对后期大气环流造成持续影响。Abstract: Using NCEP/NCAR reanalysis monthly average SLP data, a winter monsoon meridional change index (WMMCI) is defined to show the intensity difference of winter monsoon between mid and high-latitudes. The index is well in accordance with the East Asian winter monsoon index (EAWMI). Generally, when WMMCI is strong, so is EAWMI. And WMMCI is weaker when EAWMI is weaker, too.The correlation between WMMCI and Meiyu precipitation of Jiaxing is studied. The results show that the abnormal Meiyu precipitation in the coming summer is caused by the abnormality of East Asian winter monsoon and the influence lasts two or three years. There is a positive correlation between the WMMCI and later Meiyu precipitation of Jiaxing on both interannual and interdecadal scales. Jiaxing Meiyu precipitation is generally heavy (light) when preceding winter monsoon is strong (weak). The variation trend of the Jiaxing Meiyu precipitation is behind the index. The Meiyu precipitation of Jiaxing is continuously heavy (light) when WMMCI continuously strengthens (weakens) for 2 years and the winter monsoon is strong (weak). The effect on later precipitation caused by weak winter monsoon is slower than that caused by strong winter monsoon.The composite anomalies of 500 hPa height show that the development of meridional (zonal) circulation over mid-and high-latitudes is favored by the pattern of north positive (negative) and south negative (positive). The southward movement of the cold air is benefited (not benefited) by the meridoinal (zonal) circulation. Also, the pattern has a persistent effect on the circulation, by which the Meiyu precipitation is favored (not favored). A possible explanation of the continuous effect of the winter monsoon meridional abnormality on the Meiyu precipitation of Jiaxing is that the abnormality of westerly (easterly) in the equatorial Pacific Ocean is caused by the continuously strengthening (weakening) strong (weak) winter monsoon, by which the occurrence of El Niño (La Niña) through abnormal variation in sea surface temperature is triggered.
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图 1 冬季风经向变化指数 (○) 和东亚冬季风指数 (△) 的变化曲线 (■为嘉兴梅雨降水距平百分率)
Fig. 1 The curve of the winter monsoon meridional change index (○) and the East Asian winter monsoon index (△) (■ is for anomaly percentage of Meiyu precipitation in Jiaxing)
图 2 强 (a, b)、弱 (c, d) 冬季风年同期冬季 (a, c)、后期6月 (b, d) 500 hPa高度场距平合成分析 (单位:gpm)
Fig. 2 Composite winter (a, c) and later June (b, d)500 hPa height anomaly fields (unit:gpm) (a, b) strong winter monsoon years, (c, d) weak winter monsoon years
表 1 强、弱冬季风年与后期嘉兴梅雨降水关系
Table 1 The relation between strong or weak winter monsoon years and the Meiyu precipitation of Jiaxing in the next summer
表 2 持续增强 (减弱) 的强 (弱) 冬季风对后期嘉兴梅雨的影响
Table 2 The influence of the continually strengthening (weakening) strong (weak) winter monsoon for the later Meiyu precipitation of Jiaxing
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