A Case Study of Double Ridges of Subtropical High over the Western North Pacific: The Role in the 1998 Second Mei-yu over the Yangtze River Valley

摘要

Double ridges in the Western North Pacific Subtropical High (WPSH) in 1998 are studied as a particular case using NCEP/NCAR reanalysis data and the Black Body Temperature (TBB) data. In contrast to typical single ridge in WPSH, double ridges are found during July in 1998 and associated with the second Meiyu activity in the Yangtze River valley. The second ridge of WPSH is built up in the south of the first one and contributes to the stagnancy of WPSH in the south in middle of July. It is different from the previous view that the stagnancy of WPSH in the south resulted from the sudden southward withdrawal of the original ridge. The characteristics of double ridges including the structure, evolution and possible mechanisms are investigated with the case of 1998. The results show that there are a lot of differences in the large-scale circulation, temperature and humidity between the single ridge and double ridges of the WPSH. The north branch of the double ridges has coherent characteristics with the traditional single ridge, while the south branch is characterized by the tropical systems. There are evidences to support that the formation of the double ridges is associated with the northward movement of equatorial buffer zone and the persistence is related to the evolution of tropical convections. The rainfall pattern in eastern China associated with the double ridges of WPSH is examined using the instrumental records of precipitation in China. Two rain belts shaped as an italic "L"-like pattern in eastern China are found to be related to the double ridges. This study provides new evidence to understand the variability of WPSH and associated East Asian summer monsoon activity on synoptic scales.