Decadal Change in the Influence of the Western North Pacific Subtropical High on Summer Rainfall over the Yangtze River Basin in the Late 1970s

Abstract

It is well known that on the interannual timescale, the westward extension of the western North Pacific subtropical high (WNPSH) results in enhanced rainfall over the Yangtze River basin (YRB) in summer, and vice versa. This study identifies that this correspondence experiences a decadal change in the late 1970s. That is, the WNPSH significantly affects YRB precipitation (YRBP) after the late 1970s (P2) but not before the late 1970s (P1). It is found that enhanced interannual variability of the WNPSH favors its effect on YRB rainfall in P2. On the other hand, after removing the strong WNPSH cases in P2 and making the WNPSH variability equivalent to that in P1, the WNPSH can still significantly affect YRB rainfall, suggesting that the WNPSH variability is not the only factor that affects the WNPSH–YRBP relationship. Further results indicate that the change in basic state of thermal conditions in the tropical WNP provides a favorable background for the enhanced WNPSH–YRBP relationship. In P2, the lower-tropospheric atmosphere in the tropical WNP gets warmer and wetter, and thus the meridional gradient of climatological equivalent potential temperature over the YRB is enhanced. As a result, the WNPSH-related circulation anomalies can more effectively induce YRB rainfall anomalies through affecting the meridional gradient of equivalent potential temperature over the YRB.

摘 要

西太平洋副热带高压 (副高) 的年际变动对长江流域夏季降水有重要影响. 一般来说, 副高偏西 (东)时, 长江流域降水增多 (减少). 本文发现, 副高和长江流域降水的这种对应关系在 1970s 末发生了一次年代际转变, 即副高在 1970s 末之后可以显著地影响长江流域降水, 但在此之前其影响很弱. 其中一个原因是副高的变率在 1970s 末之后增强, 使得其对长江流域降水的影响增强. 此外, 我们去除后一阶段的强副高个例, 使前后两个阶段的变率基本一致, 发现副高的变动依然可以显著地影响长江流域降水, 表明副高变率的增强并不是导致其影响加强的唯一原因. 进一步的研究结果表明, 热带西北太平洋热力背景场的改变为副高和长江流域降水关系的增强提供了有利条件. 1970s 末之后, 热带西北太平洋的低层大气变得更暖、 更湿, 增强了长江流域相当位温的经向梯度. 因此, 副高的变动可以更有效地引起长江流域的相当位温梯度异常, 进而影响长江流域降水.