Comparison of Intraseasonal Variation of the Meridional Displacement of the Western North Pacific Subtropical High in Early and Late Summer

Abstract

The meridional displacement of the western North Pacific subtropical high (WNPSH) on an intraseasonal time scale is investigated, with emphasis on differences between early (May–June) and late (July–August) summer. The intraseasonal variation (ISV) of the meridional displacement of the WNPSH is dominated by the 10–30-day period, and the variation amplitude is larger in late summer. The ISV of the WNPSH is attributed mainly to the evolution of an anomalous cyclone/anticyclone north of the WNPSH in early summer, whereas it is due to a south-to-north dipole of an anomalous anticyclone and cyclone over East Asia in late summer. Moreover, the WNPSH tends to shift westward when it moves northward, and vice versa, especially in early summer. Both tropical convection and mid- to high-latitude teleconnection across Eurasia are responsible for the ISV of the meridional displacement of the WNPSH in early and late summer. The role of mid- to high-latitude teleconnection is more important in early summer, whereas tropical convection over the South China Sea is more crucial in late summer, through triggering a Pacific–Japan (PJ) pattern. In early summer, as the WNPSH shifts northward, rainfall increases over the Yangtze River valley and decreases over Southeast China, and vice versa. In late summer, deficient rainfall over North China persists when the WNPSH is at its southernmost location and during its northward shift, and vice versa. The characteristics, underlying processes, and impacts of the 10–30-day meridional displacement of the WNPSH are significantly different in early and late summer.