Orographic Effects of the Tibetan Plateau on the East Asian Summer Monsoon

This project investigates the dynamical processes through which the Tibetan Plateau (TP) affects the formation of the East Asian summer monsoon (EASM) within the framework of the moist static energy (MSE) budget using both observations and atmospheric General Circulation Model (GCM) simulations. Here, we focus on the most prominent feature of the EASM, i.e., the so-called Meiyu-Baiu, which is characterized by a well-defined, southwest to northeast elongated quasi-stationary rainfall band, spanning from eastern China to Japan and into the northwestern Pacific between mid-June to mid-July.

Observational analyses of the MSE budget of the MB front indicate that horizontal advection of moist enthalpy, and primarily of dry enthalpy, sustains the front in a region of otherwise negative net energy input into the atmospheric column. A decomposition of the horizontal dry enthalpy advection into mean, transient and stationary eddy fluxes shows that the longitudinal thermal gradient due to zonal asymmetries and the meridional stationary eddy velocity are the most influential factors determining the pattern of horizontal moist enthalpy advection. Numerical simulations in which the TP is either retained or removed show that the TP influences the stationary enthalpy flux primarily by changing the meridional stationary eddy velocity, with reinforced southerly wind over the MB region and northerly wind to its north. Changes in the longitudinal thermal gradient are mainly confined to the Meiyu region and it appears that warm air advection from the TP has a lesser impact on the rainfall over the MB region.