EAST ASIAN SUBTROPICAL SUMMER MONSOON INDEX ANDITS RELATIONSHIPS TO RAINFALL

Using monthly means of the reanalyzed datasets from the NCAR/NCEP and rainfalldata from 160 surface stations in China for the period 1961-1999 and referring to atmospheric circulations in East Asia and the western Pacific, a simple index of summer monsoon over the East Asian subtropics is defined. Relationships between the index and the variability of the atmospheric circulation and rainfall in China are examined and comparisons among different indices are made. The results show that the monsoon index in the present study displays the variability on the decadal scale. The higher-index values mainly occurred in the 1960s and 1970s and the low-index values mainly occurred in the 1980s and 1990s. The index reflect the variations of both the thermal low pressure centering in Mongolia and the subtropical high in the western Pacific very well and also indicate an intensity of summer monsoon in the East Asian subtropics and an anomaly of rainfall along the valleys of the Yangtze River. Compared to the subtropical high, the low pressure exerts a greater effect on the anomalous rainfall. Corresponding to a lower index, the low pressure and the subtropical high are weaker. Meanwhile, the northerly wind anomalies prevail in the lower troposphere over the mainland of China, with the southwesterly wind anomalies in the upper troposphere. The northerly wind anomalies indicate the more frequent activities of cold air at mid latitudes of East Asia. This can cause the convergence and ascending motion to strengthen along the Mei-yu front, accompanying with the greater rainfall along the valleys.