An East Asian Subtropical Summer Monsoon Index and Its Relationship to Summer Rainfall in China

Using the monthly mean NCEP/NCAR reanalysis data and the monthly rainfall observations at 160 rain gauge stations of China during 19611999, and based on major characteristics of the atmospheric circulation over East Asia and the western Paci c, a simple index for the East Asian subtropical summer monsoon (EASSM) is de ned. The relationship between this index and summer rainfall in China and associated circulation features are examined. A comparison is made between this index and other monsoon indices.The results indicate that the index de ned herein is re ective of variations of both the thermal low pressure centered in Siberia and the subtropical ridge over the western Paci c. It epitomizes the intensity of the EASSM and the variability of summer rainfall along the Yangtze River. Analysis shows that the Siberian low has a greater e ect on the rainfall than the subtropical ridge, suggesting that the summer rainfall variability over the eastern parts of China is to a large extent a ected by anomalies of the atmospheric circulation and cold air development in the midlatitudes. Taking into account of the e ects of both the Siberian low and the subtropical ridge can better capture the summer rainfall anomalies of China.The index exhibits interannual and decadal variabilities, with high-index values occurring mainly in the 1960s and 1970s and low-index values in the 1980s and 1990s. When the EASSM index is low, the Siberian low and the subtropical ridge are weaker, and northerly wind anomalies appear at low levels over the midlatitudes and subtropics of East Asia, whereas southwesterly wind anomalies dominate in the upper troposphere over the tropics and subtropics of Asia and the western Paci c. The northerly wind anomalies bring about frequent cold air disturbances from the midlatitudes of East Asia, strengthening the convergence and ascending motions along the Meiyu front, and result in an increase of summer rainfall over the Yangtze River.