Three modes of the east Asia summer monsoon influencing Eastern China

Abstract

East Asia Summer Monsoon (EASM) is the primary circulation affecting boreal summer precipitation in eastern China and is mainly dominated by the recession and extension of the Western Pacific Subtropical High (WPSH). Based on monthly datasets, previous studies have mainly focused on the intensity and variability of the WPSH, in order to obtain potential predictions of the EASM. Here, based on daily wind anomalies, in addition to the most common mode of WPSH, two additional modes are found in the summer circulation over China. The second mode (M2) seems irrelevant to the influence of WPSH over China. The positive phase of M2 favors precipitation in the areas south of the Yangtze River and the western Northeast China, but is unfavorable to precipitation in North China. M2 poses an India Ocean-East China (or west-east) dipole pattern. The positive phase of M2 seems associated with a strong, large, spatially continuous but southerly shifted subtropical high that connects the WPSH and the subtropical high over the India Peninsular/Indian Ocean, leading to a strong eastward wind flow from the northern India Ocean to the northwest Pacific. Accompanying with this large subtropical high system is a cyclonic circulation over the area of Southeastern China and the East China Sea. The positive phase of the third mode (M3) reflects a northward shifted WPSH, which favors precipitation in northern China and a small part of South China, but unfavorable to precipitation in the Yangtze River basin. M3 is associated with a strong south-north dipole structure of sea level pressure. The south side of this structure is over the Maritime Continent, and its north side is over the Asia continent, but the variation of sea level pressure over the Maritime Continent is small compared with that over the Asian continent and the western North Pacific, so this dipole structure can also be regarded as a uniform structure over the Asia continent. Comparatively, M3 has a more pronounced decreasing trend in the past decades, while M2 mainly poses decadal changes.