华北夏季降水异常与华南前汛期降水异常的关系

Relationship between Summer Precipitation Anomaly in North China and Precipitation Anomaly in the Pre-Flood Season in South China

为改进华北夏季降水异常的预测能力、寻找前期异常信号的监测预测指标,本文利用1961-2020年华北、华南夏季降水资料,美国环境预报中心和大气科学研究中心(National Centers for Environmental Prediction/National Center for Atmospheric Research,NCEP/NCAR)的再分析环流资料、向外长波辐射资料(Outgoing Longwave Radiation,OLR),采用相关、合成和环流异常回归重构等方法,分析了华南前汛期(5-6月)降水异常与华北夏季(7-8月)降水异常的联系。结果表明:(1)华北夏季(7-8月)降水异常通常与华南前汛期(5-6月)降水异常呈反位相关系,即如果华南前汛期降水偏多,对应华北夏季降水就会偏少,反之亦然。华南前汛期降水异常可以作为华北夏季降水异常监测预测的一个前期指标。东亚夏季风造成对流层低层水汽输送异常和北半球夏季热带低频信号传播造成对流层中低层环流异常是两地降水呈反位相变化的联系机制。(2)在水汽输送方面,如果5-6月东亚副热带夏季风偏强,即东亚西南南风(SSW)显著偏强,华南水汽来源会偏少,造成华南前汛期(5-6月)降水异常偏少。到7-8月,印度夏季风显著偏强、东亚副热带夏季风进一步增强,华北水汽来源显著偏多,造成华北夏季(7-8月)降水异常偏多。反之,华南前汛期(5-6月)降水异常偏多,华北夏季(7-8月)降水异常偏少。(3)在动力环流方面,如果5-6月,热带印度洋低频信号活跃、西北太平洋低频信号不活跃,伴随印度洋低频信号向东北方向传播,在850 hPa层华南沿海附近形成反气旋性环流异常,对流减弱,造成华南前汛期(5-6月)降水偏少。到7-8月,热带印度洋低频信号仍然十分活跃,原来位于华南沿海的反气旋性环流异常向东北方向传播到长江下游至东部海上,反气旋北侧的华北地区为气旋性环流,对流偏强;另一方面,伴随低频扰动在对流层中高层向高纬传播,会形成沿高空西风急流传播的扰动波列,扰动位置相对固定,在贝加尔湖附近为负距平、朝鲜半岛附近为正距平,造成贝加尔湖高空槽加深、副热带高压在朝鲜半岛附近加强,形成“东高西低”的阻挡形势,有利于华北产生上升运动。即华北夏季对流上升运动增强,造成华北夏季(7-8月)降水偏多。反之,华南前汛期(5-6月)降水偏多,华北夏季(7-8月)降水偏少。(4)华南前汛期降水异常对华北夏季降水异常有很好的指示意义。但在有些年份关系并不好,这可能是因为这种关系还受到其他因子如海温变化、热带环流、青藏高原以及中高纬环流变化的调节与影响。

To improve the prediction ability for summer precipitation anomalies in North China and determine the indicators of early abnormal signals,this paper uses the summer precipitation data in North and South China from 1961 to 2020,and the reanalysis circulation data from the National Centers for Environmental Prediction/National Center for Atmospheric Research(NCEP/NCAR).Via correlation,synthesis and circulation anomaly regression reconstruction,the relationship between the precipitation anomaly in the pre-flood season(May-June)in South China and the precipitation anomaly in summer(July-August)in North China is analyzed.The results show that:(1)the summer precipitation anomaly in North China(July-August)is usually inversely related to the precipitation anomaly in the pre-flood season in South China(May-June),that is,the higher the precipitation amount in the pre-flood season in South China,then lower the precipitation amount in North China in summer and vice versa.The precipitation anomaly in the pre-flood season in South China can be used as an early indicator for the prediction of summer precipitation in North China.The water vapor transport anomaly in the lower troposphere,caused by the East Asian summer monsoon,and the circulation anomaly in the middle and lower troposphere,caused by the propagation of tropical low-frequency signals in the northern hemisphere in summer,represent the connection mechanism of the inverse phase change of precipitation between the two areas.(2)In terms of water vapor transport,if the East Asian subtropical summer monsoon is stronger from May to June,that is,the East Asian southwest wind(SSW)is significantly stronger,there will be less water vapor in South China,resulting in less precipitation in the pre-flood season(May-June)in South China.From July to August,the Indian summer monsoon is significantly stronger,the East Asian subtropical summer monsoon is further strengthened,and there are significantly more water vapor in North China,resulting in more precipitation in summer(July-August)in North China.On the contrary,there are more precipitation anomalies in the pre-flood season(May-June)in South China and less precipitation anomalies in summer(July-August)in North China.(3)In terms of dynamic circulation,if the tropical Indian Ocean low-frequency signal is active and the Northwest Pacific low-frequency signal is inactive from May to June,with the Indian Ocean low-frequency signal propagating to the northeast,an anticyclonic circulation anomaly will be formed near the South China coast at 850-hPa layer and convection will be weakened,resulting in less precipitation in the pre-flood season(May-June)in South China.From July to August,the low-frequency signal of the tropical Indian Ocean is still highly active.The anticyclonic circulation anomaly originally located along the coast of South China propagates to the northeast to the lower reaches of the Yangtze River to the eastern sea.North China on the northern side to the anticyclonic is subjected to a cyclonic circulation with strong convection.On the other hand,with the low-frequency disturbance propagating from the middle and upper troposphere to the high latitude,a disturbance wave train propagating along the upper westerly jet will be formed,and the disturbance position is relatively fixed.There is a negative anomaly near Baikal Lake and a positive anomaly near the Korean Peninsula,resulting in the deepening of the high trough of Baikal Lake and the strengthening of the subtropical high near the Korean Peninsula,forming a blocking situation of"high in the East and low in the West",which is conducive to the upward movement in North China.That is,the convective upward movement in North China in summer is strengthened,resulting in more precipitation in summer(July-August)in North China.On the contrary,there is more precipitation in the pre-flood season(May-June)in South China and less precipitation in summer(July-August)in North China.(4)The precipitation anomaly in the pre-flood season in South China is a good indicator of the summer precipitation anomaly in North China.However,in some years,this relationship is not straightforward,most likely because it is also regulated and affected by other factors,such as the change in the sea surface temperature,tropical circulation,the Qinghai-Xizang Plateau,and the change in the circulation in middle and high latitudes.