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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. [ABSTRACT FROM AUTHOR]

The relationships among heavy precipitation in the eastern region of China, the atmospheric heat source over the Tibetan Plateau and its surrounding areas, and atmospheric circulation in East Asia were investigated using the multi-variate empirical orthogonal function (MV-EOF) method and synthetic analysis on daily meteorological data from May through August 2010, which were compared with data from 2013. The MV-EOF decomposition results revealed that the atmospheric heating over the eastern region of the Tibetan Plateau and the Bay of Bengal exhibited opposite trends when heavy precipitation events occurred in South China, West China, and the middle and lower reaches of the Yangtze River. These results indicated that the land–sea thermal contrast between the eastern region of the Tibetan Plateau and the Bay of Bengal was likely one of the key factors leading to the occurrence of heavy precipitation events in the eastern region of China. The results of the synthetic analysis revealed a possible physical mechanism: When the atmospheric heating was weak over the Tibetan Plateau and strong over the Bay of Bengal, there was a strong ascending motion over the Bay of Bengal and its surrounding areas, which was conducive to maintaining the South Asian high and the western Pacific subtropical high (WPSH) in southerly positions. This also resulted in weak water vapor transport in the southwest, thus forming continuous heavy precipitation in South China. After the increase in atmospheric heating over the Tibetan Plateau, the convergence and ascending motion of the lower atmosphere were strengthened, and the South Asian high moved northward to the plateau, with a strengthened eastward extension. The WPSH then lifted northward, and the airflow around it conveyed more water vapor to West China and the middle and lower reaches of the Yangtze River, resulting in heavy precipitation in these regions. [ABSTRACT FROM AUTHOR]