Using the NCEP/NCAR reanalysis data (Version 1.0) and the observation data of China from January 1951 to February 2007,a new index of East Asian winter monsoon circulation (I_(EAWM)) was defined based on the comparison of previous different winter monsoon indices and circulation factors influencing the winter climate over China.Its relationships with winter temperature over China and large-scale circulation were analyzed.Results show that IEAWM Can successfully describe the variation of China's mainland winter temperature and the East Asian winter monsoon (EAWM) system.This index reflects the integrated effect of the circulations over high and low latitudes and the thermal difference between the continent and the ocean.While in the previous studies,most monsoon indices only describe the single monsoon member. The I_(EAWM) is a good indicator of the intensity of the EAWM.Positive values of I_(EAWM) correspond to the strong EAWM,the stronger Siberian high and East Asian trough than normal,and the strengthening of the meridional shear of 500-hPa zonal wind between high and low latitudes over East Asia,and therefore,the southward cold advection becomes stronger and leads to the decrease in surface temperature over China; and vice versa.The I_(EAWM) inter decadal change is obviously positive before the mid-1980s,but negative since the mid-1980s,in good agreement with the fact of the winter warming in China after 1985.
Using a monthly precipitation dataset of 160 stations over China and a daily and monthly National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis dataset from 1961 to 2006, we here define an East Asian land-sea atmospheric heat source difference index ILSQD and investigate its relationship to summer rainfall in China and East Asian general circulation. The results show that ILSQD more closely reflects the anomalous variations in summer monsoon phenomena; in the high-index (HI) cases, the strong low-level southerlies over East China and the strong high-level westerlies over middle latitudes indicate an active summer monsoon, and vice versa in the low-index (LI) cases. This index also reflects summer rainfall anomalies over East China; in the HI (LI) cases rainfall increases (decreases) over North China and at the same time decreases (increases) over the mid-lower Yangtze River valley and the southern Yangtze River. Hence, ILSQD can be utilized as a summer monsoon index. There is also remarkable correlation between ILSQD in March and the following summer rainfall over the mid-lower Yangtze River valley. Finally, the Community Atmospheric Model Version 3.1 (CAM3.1) of NCAR is used to run numerical experiments, which verify that the anomalous summer precipitation in simulations is similar to that of diagnosis analysis based on the anomalous summer atmospheric heating forcing. Similarly, the atmospheric heating rate in March can force summer rainfall anomalies in the simulations just as observed in the data.
