Weakened Impact of the Indian Early Summer Monsoon on North China Rainfall around the Late 1970s: Role of Basic-State Change

摘要

Previous studies have found a link between north China and Indian rainfall during summer, with significantly increased rainfall in north China related to a stronger Indian summer monsoon. This link is weakened after the late 1970s, generally attributed to the reduced magnitude of interannual variability in the Indian summer rainfall. This study reveals a similar change in this rainfall link in early summer after the late 1970s. Related to a heavier Indian early summer rainfall, rainfall in north China enhances significantly before the late 1970s but not thereafter. The change in rainfall teleconnection is caused by the weakened impact on north China rainfall of a midlatitude wave train along the Asian jet in the upper troposphere. After the late 1970s, the portion of the wave train over East Asia displaces eastward, leading to an eastward shift in the associated ascending motion and, subsequently, enhanced rainfall from north China to the Yellow Sea. Moreover, the change in the midlatitude wave train is attributed to the change in the basic state over East Asia (i.e., a northward shift of the East Asian upper-tropospheric westerly jet after the late 1970s). The latter reduces stationary Rossby wavenumber and increases wavelength of the midlatitude wave train, leading to an eastward shift of the wave train over East Asia. Therefore, in this study a mechanism is proposed for the change in early summer, different from the previous mechanism for the entire summer period.