Meandering Subtropical Jet and Precipitation over Summertime East Asia and the Northwestern Pacific

Abstract

t has been revealed that in summertime, precipitation is enhanced to the south of the upper-level tropopausal potential vorticity contours, which are accompanied by instantaneous jets, over the eastern coastal region of China to the northwestern Pacific. It is frequently exhibited as precipitation bands ranging in size from over a thousand to several thousands of kilometers long. In this study, an analysis was conducted to quantify the relationship depending on the phase of upper-level disturbances. With composite analysis, it is shown that the enhancement along the contours occurs at all phases; it occurs not only to the east but also to the west of the upper-level troughs, although it is weaker. The midtropospheric distributions of upwelling and the Q-vector convergence are collocated with the precipitation enhancement, suggesting the importance of dynamical induction by geostrophic flow at all phases. The effects of upper-level disturbances and low-level jets (LLJs) with a southerly component are investigated by using an idealized nondimensional quasigeostrophic model supporting latent heating. While upper-level waves induce upwelling and downwelling to the east and west, respectively, of the upper-level troughs, LLJs tend to offset the downwelling, enabling precipitation to the west too. Both in the observational composite and the idealized model with LLJ, confluence and diffluence contribute to the Q-vector convergence to induce upwelling along the subtropical jet irrespective of upper-level disturbance phases. This induction is explained as a general feature of a veered jet where geopotential isolines rotate clockwise with height without requiring wind variation along the jet.