THE RELATIONSHIP BETWEEN THE SUMMER PRECIPITATION IN THE YANGTZE RIVER VALLEY AND THE BOREAL SPRING SOUTHERN HEMISPHERE ANNULAR MODE: Ⅰ BASIC FACTS

The relationship between the boreal spring (April-May) southern hemisphere annular mode (SAM) and the following summer (June-August) rainfall in China was examined statistically. The results show there are a significantly positive correlation between the boreal spring SAM index (SAMI) and the following summer precipi tation in the middle and lower reaches of the Yangtze River valley. In the spring, a pair of anomalous anticyclones exists in the Mongolia Plateau and Tianshan Mountains respectively while there is spring strong SAM. Meanwhile the anomalous northerlies prevail from the middle latitudes of East Asia to South China throughout northeast of China. These anomalous circulations may continue to the following summer and weaken East Asian summer monsoon (EASM). The west ridge of western Pacific subtropical high strengthens and extends westward in summer following the spring strong SAM. These circulation anomalies are related to the above normal precipitation in the middle and lower reaches of the Yangtze River valley. Moreover, the spring strong SAM is followed by increased ascending vertical velocity, specific humidity and relative humidity, which provide necessary water vapor conditions for increasing the summer precipitation in the middle and lower reaches of the Yangtze River valley, and vice versa. The boreal spring SAM variation provides a potential valuable signal for predicting the summertime precipitation in the middle and lower reaches of the Yangtze River valley.