Characteristics of subtropical monsoon and rainfall over eastern China and western North Pacific and associated reasons.

Using the NCAR/NCEP reanalysis data and the NOAA's Climate Prediction Center's merged analysis of precipitation (CMAP) during 1981-2000, we investigated the seasonal evolution of the southwesterly wind and associated precipitation over the eastern Chinathe subtropical western North Pacific area and its relationship with the tropical monsoon and rainfall, and analyzed reasons responsible for the onset and development of the wind. It was found that the persistent southwesterly wind appears over southern China and the subtropical western Pacific earliest in the early spring, and then expands southwards to the tropics and advances northwards to the midlatitudes. From winter to summer, the seasonal variation of surface heating over western China and the subtropical western Pacific may result in an earlier reversal of the westward tropospheric temperature gradient over the subtropics relative to the tropics, which may contribute to the earliest beginning of the subtropical southwesterly wind. Additionally, the strengthening and eastward expanding of the trough near the eastern Tibetan Plateau as well as the strengthening and westward moving of the western Pacific subtropical high also exerts positive influence on the beginning and developing of the subtropical southwesterly wind. In the early summer, the northward expansion of the southwesterly wind over southern China is associated with a northward shift of the subtropical high, and the southward stretch of the southwesterly wind is associated with a southward stretch of the trough in the east side of the plateau. With the begining and northward expansion of the subtropical southwesterly wind (namely-southwest-monsoon), convergences of the low-level air and water vapor and associated upward motion in front of the strongest southwesterly wind core also strengthens and moves northwards, leading to an increase of rainfall intensity and a northward shift of the rain belt. Accordingly, the subtropical rainy season occurs earliest over southern China in spring, moves northward to the ChangjiangHuaihe River valley in early summer, and arrives at North China in mid summer. Compared to the subtropical rainy season, the tropical rainy season begins later and stays mainly over the tropics, not pronouncedly moving into the subtropics. Clearly, the meiyu rainfall over the ChangjiangHuaihe valley in early summer results from a northward shift of the spring rainy belt over southern China, instead of a northward shift of the tropical monsoon rainfall belt. Before the onset of the tropical monsoon, water vapor over the subtropical monsoon region comes mainly from the coasts of the northern Indochina Peninsula and southern China. After the onset, one branch of water vapor flow comes from the Bay of Bengal, entering into eastern China and the subtropical western Pacific via southwestern China and the South China Sea, and another branch comes from the tropical western North Pacific, moving northwestwards along the west edge of the western Pacific subtropical high and entering into the subtropics.