Subseasonal Change in the Seesaw Pattern of Precipitation between the Yangtze River Basin and the Tropical Western North Pacific during Summer
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There is a well-known seesaw pattern of precipitation between the tropical western North Pacific (WNP) and the Yangtze River basin (YRB) during summer. This study identified that this out-of-phase relationship experiences a subseasonal change; that is, the relationship is strong during early summer but much weaker during mid-summer. We investigated the large-scale circulation anomalies responsible for the YRB rainfall anomalies on the subseasonal timescale. It was found that the YRB rainfall is mainly affected by the tropical circulation anomalies during early summer, i.e., the anticyclonic or cyclonic anomaly over the subtropical WNP associated with the precipitation anomalies over the tropical WNP. During mid-summer, the YRB rainfall is mainly affected by the extratropical circulation anomalies in both the lower and upper troposphere. In the lower troposphere, the northeasterly anomaly north of the YRB favors heavier rainfall over the YRB by intensifying the meridional gradient of the equivalent potential temperature over the YRB. In the upper troposphere, the meridional displacement of the Asian westerly jet and the zonally oriented teleconnection pattern along the jet also affect the YRB rainfall. The subseasonal change in the WNP–YRB precipitation relationship illustrated by this study has important implications for the subseasonalto- seasonal forecasting of the YRB rainfall.
Key wordsYangtze River basin tropical western North Pacific precipitation subseasonal change
长江流域夏季降水与热带西北太平洋夏季降水之间存在显著的反相关关系. 本文的研究结果表明, 两者的反相关关系经历了显著的次季节变化: 反相关关系在前夏较强, 而在中夏减弱. 我们进而分析了前夏和中夏影响长江流域降水的大尺度环流异常. 结果表明, 前夏长江流域降水主要受热带环流异常的影响, 即热带西北太平洋降水异常所激发的反气旋或气旋式异常. 中夏, 长江流域降水主要受热带外环流异常的影响. 在对流层低层, 长江流域北部的东北风异常通过增加相当位温的经向梯度使长江流域降水增多. 在对流层高层, 亚洲急流的经向偏移以及沿急流传播的纬向波列也对长江流域降水有重要影响. 本研究对长江流域降水的次季节-季节预测具有重要的指示意义.
关键词长江流域 热带西北太平洋 降水 次季节变化
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We thank Dr. Yunhuan HAN and Ms. Yawen DUAN for offering the screened daily station rainfall data. We also thank the two anonymous reviewers for their comments, which were helpful in improving the presentation. This work was supported by the National Natural Science Foundation of China (Grant No. 41320104007).
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