The interannual variability of spring rainy belt over southern China and the associated atmospheric circulation anomalies

Using the daily accumulated precipitation data from Chinese 693 rain gauge stations and the daily National Centers for Environmental Prediction National Center for Atmospheric Research (NCAR/NCEP) reanalysis data during 1960-2008, the interannual variations of spring rainy-season precipitation in intensity and position over southern China and the associated atmospheric circulation anomalies were investigated by means of the statistical analysis methods. The results show that on the interdecadal time scale, the spring rainy-season precipitation to the south of the lower valley of the Yangtze River was less in the middle and late 1960s and was more in the middle 1970s to the early 1980s. It began decreasing from the early 1990s again. On the interannual time scale, when the subtropical high over the western North Pacific and the low-level low-pressure system to the east of the Tibetan Plateau strengthen and their anomalous centers appear to the south of 20°N and 30°N in spring respectively, anomalous southwesterly winds mainly prevail to the south of the Yangtze River valley. Remarkable convergence of air mass appears over the region where southwesterly winds gradually weaken, accompanying the local strengthened low-level convergence of air mass, the tropospheric ascending motion, and the low-level convergence of water vapor. These anomalies lead to the increase of precipitation over there. At this time, the water vapor coming from the western North Pacific arrives at the South China Sea, not causes the local convergence, and turns northwards into southern China. While both the subtropical high and the low pressure system strengthen and their anomalous centers appear to the north of 30°N, anomalous southwesterly winds prevail over most of eastern China, with the anomalies of low-level convergences of air mass and water vapor flux and of the tropospheric ascending motion shifting northward to the valleys of the Yangtze Huai Rivers. Accordingly, more precipitation appears over the valleys. Meanwhile, the anomalies of divergences of air mass and water vapor flux and the tropospheric descending motion appear to the south of the Yangtze River valley and over the coasts of southern China, with a decreased precipitation over these regions. In this case, water vapor comes from the subtropical western North Pacific. Because the above observed results are very consistent with those from the model sensitivity experiments of changing thermal contrasts between the East Asian land and its adjacent oceans, the observed anomalies in precipitation and atmospheric circulation may result from the forcing of the thermal contrasts.