Subseasonal relationship between the zonal oscillation of the Western Pacific Subtropical High and the South Asia High

Abstract

This study investigates relationship between the zonal oscillation of the western Pacific subtropical high (WPSH) and the South Asia High (SAH). As the WPSH extends westward, in upper-troposphere the SAH shows conspicuous eastward extension in 10–25-day period, while weak zonal movement in 25–50-day period. In 10–25-day period, the zonal oscillation of the SAH lags the movement of the WPSH by about one day. It is closely related with a westward moving anomalous high from the south of the Japan to the continent. Further analysis shows that the WPSH-related precipitation plays the significant role in the variation of this anomalous high. Accompanied with the westward extension of the WPSH, in lower-troposphere an anomalous anticyclonic circulation occupies the western Pacific. It enhances the moisture transportation to the east side of the SAH, causing above-normal precipitation here. With the enhancement of the precipitation, the anomalous diabatic heating induces divergence anomaly in upper-troposphere, and further stimulates anomalous high by the horizontal divergence effect, leading to the eastward extension of the SAH. In 25–50-day period, due to the weaker convective anomaly, the WPSH is unable to extend farther westward. As the result, less moisture is transported and the anomalous moisture convergence is restricted to the north of Yangtze River, and so is the precipitation. At the east side of the SAH it is dominated by the anomalous cooling, unfavorable for the stimulation of the anomalous high, and thus, the zonal oscillation of the SAH.