Characteristics and mechanisms study of abnormal meridional movement of the Western Pacific Subtropical High in July 2020

The climate in China is extremely abnormal in July 2020, which is largely attributed to the anomalous activities of the Western Pacific Subtropical High (WPSH). In the present study, statistical analysis and diagnostic calculation of individual terms of the complete form of vorticity equation are conducted to investigate features of meridional movement of WPSH in July 2020, and the mechanism for the anomalous meridional movement of the WPSH is also explored. Results indicate that in July 2020, the western segment of the WPSH ridge line persistently remained to the south of its climatological position, especially in the lower troposphere. Meanwhile, the eastern segment of the ridge line continuously stayed to the north of its normal position. Such a configuration leads to a large meridional span of the WPSH ridge line and is directly responsible for the super long Meiyu season in eastern China in the summer of 2020. The persistent southward shift of the WPSH ridge line in its western segment is attributed to the maintenance of strong positive vorticity anomalies to the north of the WPSH ridge line. Whereas the abnormally frequent southward invasion of cold air and the activities of troughs in the mid- and high-latitude regions are the main reasons for the maintenance of the vorticity anomalies to the north of the WPSH ridge line. In addition, the anomalously southward WPSH combined with the Indian ocean SST anomalies inhibited the development of tropical convection in the South China Sea—Philippines, which subsequently weakened the vorticity anomalies to the south of the WPSH ridge line and made the P-J teleconnection pattern exhibiting in abnormally negative phase, and resulting in anomalous cyclonic circulation and negative geopotential height developed in the middle and lower troposphere over southern Japan, which further caused the WPSH centroid locating southward and restricted the northward movement of the WPSH.