夏季中国华北与印度降水之间的关联及其成因分析

本文基于1951~2012年的再分析资料以及站点观测资料,针对中国华北夏季降水和印度夏季风降水的协同变化(正相关)关系,利用集合经验模态分解法(EEMD)对两个降水序列进行时间尺度分解,并在年际尺度上分别考察了对两者正相关关系形成的有利和不利环流形势.结果表明,印度夏季风降水和华北夏季降水序列的较好正相关关系主要来自周期为2~3年的年际尺度分量,两者在该时间尺度上的相关系数为0.34,达到99%的信度水平.在年际尺度上,与印度夏季风降水异常有关的对流层中高层环半球遥相关型(CGT)波列能够衔接伊朗高原和环渤海地区上空的同位相环流异常(反气旋式或气旋式),从而有利于华北夏季降水和印度夏季风降水之间的协同变化.然而,这一协同变化关系并不总是成立.当伊朗高原上空异常中心的位置偏西时,CGT波列无法形成.这时,即使印度夏季风降水出现显著异常,华北地区却易受东亚—太平洋型或西太平洋型遥相关型的影响而其降水形势可能与印度夏季风降水形势相反.这些结论有助于进一步理解印度夏季风降水与华北夏季降水的正相关关系,从而对华北夏季降水的预测具有参考意义.

Relationship between Summer Rainfall over North China and India and Its Genesis Analysis

Based on reanalysis and observational data, the relationship between the summer rainfall over North China and India was analyzed. Using the ensemble empirical mode decomposition (EEMD) method, the two rainfall time series were decomposed into different time scales. For the formation of the positive correlation between them on the interannual scale, this study analyzed the favorable and unfavorable circulation patterns. The results showed that the relatively good correlation between the two rainfall time series was primarily contributed by the interannual scale component of 2-3 a, for which the correlation coefficient was 0.34, exceeding the 99% confidence level. On the interannual scale, anomalous Indian summer monsoon (ISM) rainfall could induce the circumglobal teleconnection (CGT) pattern in the middle and upper troposphere, which would act as a bridge to make the anomalous circulations over the Iranian Plateau and the region around the Bohai Sea (cyclonic or anticyclonic anomaly circulation) to vary in phase, thus causing the summer rainfall anomalies over North China. However, such a relationship does not always hold. The CGT is sometimes absent when the anomalous circulation over the Iranian Plateau associated with the ISM rainfall is mainly situated in the westward position. In this situation, even if there is a significant anomalous ISM rainfall, the summer rainfall over North China is likely to be influenced by the East Asia-Pacific (or West Pacific) teleconnection pattern and is thus out of phase with the ISM rainfall anomaly. These results are helpful in deepening our understanding of the relationship between the summer rainfall over North China and India, thus providing some clues for the prediction of North China summer rainfall.