Diagnostics of westward propagating East Asian monsoon low-pressure systems that reach the Indian monsoon region

Abstract

The East Asian monsoon low-pressure systems (MLPSs) are important rain-producing weather disturbances in East Asia during the summer (June, July and August). After formation, they primarily move westward and impact southern China and the Indochina Peninsula. We analyzed the climate distribution characteristics of generation locations and frequency of different intensity events in westward East Asian MLPSs during the summers of 1979–2012. The East Asian and the Indian MLPSs are relatively independent weather systems and the Indochina Peninsula area separates them. The westward East Asian monsoon low-pressure system (MLPS) rarely moves to the Indian monsoon region (IMR). When an East Asian MLPS crosses the boundary between the East Asian monsoon region (EAMR) and the IMR, the potential vorticity (PV) axis near the low-pressure center remains vertical with height, while the PV axis near the center of the East Asian MLPSs that do not cross the boundary shifts to the northeast with height. The East Asian MLPSs that reach the IMR are stronger before crossing the boundary than the MLPSs that do not cross the boundary. The strength of the MLPS that approach the boundary gradually weakens during the westward movement. The middle and high-level PV center shifts to the northeast of the low-pressure center, which causes the MLPS to turn and fail to cross the boundary between EAMR and IMR. Variation of the PV budget at 500 hPa over time showed that a strong positive PV tendency forms on the north side of low-pressure center mainly due to horizontal adiabatic PV advection. The PV center gradually shifts to the north and the MLPS turns. The PV budget at 500 hPa for the East Asian MLPS that reach the IMR shows that the horizontal and vertical adiabatic advection PV transport has a strong positive tendency on the northwest side of the low-pressure center, causing the MLPS to continue moving northwestward and eventually cross the boundary to reach the IMR.