The first jump of the East Asia summer monsoon trough

The features of the first jump of the seasonal march associated with the interannual variabilities of the East Asia summer monsoon (EASM) trough are studied. The northward march of the monsoon trough has been identified and two kinds of EASM are categorized. The evolution and the physical characteristics for the first jump are investigated. Low-level meteorological parameters over the region of 110°E-120°E and 0°N-50°N for the northward march of the low-level monsoon trough from 1979 to 2000 are analyzed. The first northward jump of the low-level trough occurs from the South China Sea (SCS) to the Yangtze River region after the onset of the South China Sea Summer Monsoon (SCSSM). But an interannual variability occurs in the speed of the jump. The trough jumps faster than the climatology in some of the years but it jumps relatively slower in other years. Based on the features of the precipitation pattern, the northward speed of the first jump can be defined as the speed of the northward move of the maximum precipitation from the time of the SCSSM onset to the Yangtze River region. Two kinds of EASM are then categorized by considering the variation in the speed of the first northward jump in the precipitation pattern. The occurrence of the first jump of the EASM trough is associated with two conditions: the position of the subtropical high and a jet in the SCS. Based on the 850-hPa winds, the evolution of the first jump of the EASM begins with the propagation of a low-level jet from the Bay of Bengal (BoB). The first jump occurs as the jet enters the SCS with the westward extension of the subtropical high. The low-level jet and the trough in the SCS are “pushed” by the southerly associated with the subtropical high to the SC and Yangtze River. The so-called ‘Mei Yu’ forms and the first jump of the seasonal march is completed.

Author: Lee, Wai Pong

Source: City University of Hong Kong

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