A Diagnostic Analysis of Activities of the Western Pacific Subtropical High During the Second Meiyu Period in 1998

The relationship between atmospheric meridional circulation and activities of the Western Pacific Subtropical High during the second Meiyu period in 1998 over the Yangtze River is analyzed. The results show that the advance and retreat of the Western Pacific Subtropical High are closely connected with the variation of the meridional circulation. The northward jump of the Western Pacific Subtropical High is not directly caused by the northward movement of the descending branch of the counter-circulation, which is connected with the Western Pacific Subtropical High. The development of the deep ascending motion at the lower latitudes obstructs the maintaining of the Western Pacific Subtropical High in the lower latitudes. Whereas, the sudden southward movement of the counter-circulation at the middle latitudes is in favor of the buildup of the Western Pacific Subtropical High in the farther north latitudes. Then, the Western Pacific Subtropical High jumps northward abruptly. This is form of the re-adjustment of the meridional circulation that ends Meiyu in the Yangtze-Huai River Region. The Western Pacific Subtropical High moves northward or southward along with the connected counter-circulation while remaining steady. Before the Western Pacific Subtropical High retreats from north, a new counter-circulation originates near the tropopause. When the new counter-circulation moves south rapidly, the Western Pacific Subtropical High moves south along with it.