Evolution of the western North Pacific subtropical high and impact of Asian precipitation from spring to summer

Abstract

Based on the JRA55 monthly reanalysis datasets and simplified numerical experiments, this study identifies several key regions in which precipitation has an evident influence on the seasonal march of the western North Pacific Subtropical High (WNPSH) from spring to summer. In May, there is an evident positive feedback between the developed trough low and the increased precipitation mainly around the Bay of Bengal with the aid of the thermal supply from the underlying ocean. This positive feedback facilitates the breakdown of the SH and the formation of the WNPSH as an independent circulation system. In June, Meiyu precipitation occurs and in turn stimulates an anticyclonic anomaly in the northwestern Pacific, which contributes as much as approximately 77 % to the first northward advance of the WNPSH therein. The second northward advance of the WNPSH in July is closely associated with the increased precipitation around Indian subcontinent, as the latter can explain as much as approximately 50 % of the observed vorticity anomaly over East Asia. After August, the precipitation increment pattern is almost reversed with respect to that during the previous months. Accordingly, the WNPSH retreats southward and gradually merges with the Iran High, restoring to a zonally uniform distribution pattern.