The seasonal transition of the characteristics of the lower-level circulation systems around the Baiu front in China from spring to summer and their relation to the Northern Summer Monsoon are examined by using the observational data of 1979.The Baiu front in May (stage (A)) is the system within the baroclinic zone in middle latitudes. The surface high pressure belt with cooler air and large divergence is located just to the north of it and the subtropical high at the surface level to the south of the Baiu front is not so clearly found.The subtropical high with a strong lower-level southerly wind toward the Baiuu front around China was enhanced abruptly in the middle of June, nearly simultaneously with the onset of the Northern Summer Monsoon (the beginning of stage (C)). In stage (C) the low OLR area (intense heat source area) expands widely in South Asia (EQ-25°N/60°E-105°E). The pressure in the lower layer decreases there, which results in the geostrophic enhancement of the southerly wind in the subtropical high area around 120°E by producing the eastward pressure gradient. At the same time the belt-shaped low OLR area extending from India to Western Pacific region (-10°N/60°E-140°E) is formed, which intensifies the local Hadley cell with strong lower-level divergence around 20°N as its northern branch.It is concluded that the change in the lower-level wind structure of the subtropical high to the south of the Baiu front in the middle of June is due to the superposed effect of the two characteristics of the heat source distribution associated with the Northern Summer Monsoon. The additional analysis by using the pentad mean data from 1979 to 1983 supports this conclusion.
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