Why was the strengthening of rainfall in summer over the Yangtze River valley in 2016 less pronounced than that in 1998 under similar preceding El Niño events?—Role of midlatitude circulation in August
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It is widely recognized that rainfall over the Yangtze River valley (YRV) strengthens considerably during the decaying summer of El Niño, as demonstrated by the catastrophic flooding suffered in the summer of 1998. Nevertheless, the rainfall over the YRV in the summer of 2016 was much weaker than that in 1998, despite the intensity of the 2016 El Niño having been as strong as that in 1998. A thorough comparison of the YRV summer rainfall anomaly between 2016 and 1998 suggests that the difference was caused by the sub-seasonal variation in the YRV rainfall anomaly between these two years, principally in August. The precipitation anomaly was negative in August 2016—different to the positive anomaly of 1998.
Further analysis suggests that the weaker YRV rainfall in August 2016 could be attributable to the distinct circulation anomalies over the midlatitudes. The intensified “Silk Road Pattern” and upper-tropospheric geopotential height over the Urals region, both at their strongest since 1980, resulted in an anticyclonic circulation anomaly over midlatitude East Asia with anomalous easterly flow over the middle-to-lower reaches of the YRV in the lower troposphere. This easterly flow reduced the climatological wind, weakened the water vapor transport, and induced the weaker YRV rainfall in August 2016, as compared to that in 1998. Given the unique sub-seasonal variation of the YRV rainfall in summer 2016, more attention should be paid to midlatitude circulation—besides the signal in the tropics—to further our understanding of the predictability and variation of YRV summer rainfall.
Key wordsYangtze River valley summer rainfall super El Niño sub-seasonal variation Silk Road Pattern
一般认为, 在厄尔尼诺衰退年, 长江流域夏季降水会明显增强, 比如1998年灾难性的大洪水. 然而, 同样是超级厄尔尼诺衰退年, 2016年夏季降水则较1998年明显偏弱. 本文研究发现它们的差异主要来自长江流域降水的次季节(主要是8月份)变化. 2016年8月份长江流域降水有明显负异常, 与1998年相反. 本文进一步的研究发现中纬度显著的环流异常导致了2016年8月偏弱的长江流域降水. 在2016年8月, “丝绸之路遥相关”和乌拉尔山地区的位势高度异常显著, 为近40年来最强, 它们会导致中纬度东亚地区出现显著的反气旋式环流异常, 引起长江中下游地区异常的东风异常, 进而会减弱副高外围气候态风场和水汽输送, 导致长江流域降水偏少. 基于2016年独特的次季节变化特征, 本工作表明研究长江流域夏季降水可预测性和变化特征时, 不能仅仅考虑热带的信号, 更需要关注中纬度环流的特征.
关键词长江流域 夏季降水 超级厄尔尼诺 次季节变化 丝绸之路遥相关
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We thank the two anonymous reviewers for their valuable comments. This study was supported by the National Natural Science Foundation of China (Grant Nos. 41320104007, U1502233, 41675078 and 41461164005).
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