Variations of East Asian Monsoon and Its Relationships with Land-sea Temperature Difference in Recent 40 Years

Using the NCEP/NCAR reanalysis grid data for the period of 1957—2000, East Asian Monsoon indexes defined by Qiao Yunting are calculated with interrelationship and annual changes of the indexes are examined. Relationships between the indexes and the variability of the weather elements and characteristics of the climate change in the east of China are analyzed by using surface data from 194 surface stations. The results show that the changes of the southwest monsoon, the southeast monsoon and the north monsoon are different. The intensity of southwest monsoon has been weakening until 1990s as well as the area's change. The southeast monsoon becomes weaker in recent 40 years. On the contrary, its range increases. The north monsoon changes little both in the intensity and area in winter. However the summer monsoon affecting the China continent weakens in liner changes in recent 40 years. In summer, the precipitation of Yangtze-Huaihe River valley increases apparently and decreases over the other east parts of China in summer. The precipitation of every part in monsoon region and its corresponding land-sea temperature difference show negative correlation, and the precipitation of Northeast China and North China have positive correlation with the land-sea temperature difference of Yangtze-Huaihe River valley. It means that when the land-sea temperature difference of Yangtze-Huaihe River valley drops, the precipitation of Yangtze-Huaihe River valley increases and that of Northeast China and North China decreases. The relationship between land-sea temperature difference and precipitation in two different phases are examined, and it is found that the influence of land-sea temperature difference on precipitation in summer enhances in the second phase (1979—2000) than in the first phase (1957—1978). The temperatures of the east part of China continent and the corresponding sea increase in recent 40 years, and the increasing velocity of the sea is larger than that of the continent, which resulting in the land-sea temperature difference decreasing year by year. That may be the cause why the summer monsoon changes weakly. During the period from 1957 to 1978, the temperatures of 850 hPa in summer drop over the middle and west part of China except north part of Xinjiang and rise over the east of China. During the second period from 1979 to 2000, the temperature of 850 hPa in summer rises all over China except middle and north part of Xingjiang and Qinghai, the north part of Gansu and west part of Inner Mongolia. The area where temperature of 850 hPa in summer increases most greatly moves from southwest of Northern China and east part of Yangtze-Huaihe River valley to west part of Yangtze-Huaihe River valley and the southeast sea to China continent.