Based on daily observations and NCEP/NCAR reanalysis data from 1960 to 2006, the characteristics of temporal and spatial variations in land-air temperature difference are analyzed by using EOF (Empirical Orthogonal Function) analysis. The results show that there are three principal spatial patterns of spring land-air temperature difference in China. The most dominant pattern shows a spatial distribution of "-" "+" "-" "+" from south to north in the spring land-air temperature difference. The second pattern shows spatial distribution of "+" "-" "+". The third pattern shows a spatially uniform distribution. There are also three principal spatial patterns of the summer land-air temperature difference in China. The most dominant pattern shows opposite spatial distributions of land-air temperature difference in eastern and western China. The second pattern shows a spatial distribution of "-" "+" "-". The third pattern shows a spatial distribution of "+" "-" "+".
The analysis identifies a significant positive correlation between the land-air temperature difference and the summer rainfall in the middle and lower reaches of the Yangtze River, and a negative correlation over North China that more significant in the summer. The analysis results suggest that with increased thermal contrast between eastern and western China, northwesterly wind anomalies become more significant in eastern China. This will result in weaker than normal summer monsoon and is unfavorable for northward moisture transport. Moreover, under this circulation pattern, ascending motions and moisture convergence are located at southeastern China, which contribute to an increase in summer rainfall over the middle and lower reaches of the Yangtze River.