Diagnosis of Effect of Southwesterlies on Tibetan Vortex Moving East

Tibetan Plateau Vortices (TPV), when moving eastward out of the plateau, usually cause catastrophic weather in East China.This study diagnoses the effect of southwesterly wind along the south periphery of Tibetan Plateau (often called the south branch of the subtropical westerly jet) on three such unique TPVs moving eastward out of the plateau (with their lifespan >48 hrs downstream) by means of the 325K isentropic surface, 500 hPa vapor transfer and cyclonic vorticity advection with NCEP reanalysis data.Evidence suggests that i) the TPVs are located on the NE-SW sloping isentropic surface, with southwesterly flows to the east that are reinforced when the TPVs migrate off.The three TPVs are associated with the Tibetan shear, the transverse shear line to the north of Hetao, and the shear at the trough line end.The southwesterly affects more on the former two than the latter one;ii) The TPVs travel faster when the southwesterly is amplified, shifting eastward and vice verse.When close to the southwesterly, the TPV is displaced quickly with it and vice verse;iii) the TPV-associated soththwesterly is responsible for transporting vast amounts of vapor into the vortex, invigorating rising motion and producing stronger cyclonic vorticity along with its advection inside, a situation favorable for TPV moving out of the tableland and persisting for long (>48 hr) downstream.These fruits, thus, expands the scope of knowledge of east-traveling TPVs, providing a scientific basis for predicting TPV-related rainstorms and flooding events in the plateau per se and downstream.