关键词: 中国北方地区, 湿润指数, 气候干湿带界线变化

Abstract:

In this paper mean locations of borderline for wet and dry climate zone in northern China over the past 40 years from 1961 to 2000 and their decadal changes were studied. Over 40 years in northern China, boundary line of semiarid and subhumid zones where W (humid index) is 0.5 moved gradually toward the east.In the eastern part east of 100E, especially in the 1990s it moved toward the east and the south compared with that in the 1960s,which resulted in the enlargement of semiarid area and the shrinkege of subhumid area,and the climate tended to become dried;and in the western part west of 100E,the extremely dry area was dwindled and the climate tended to become wetted evidently.By combining the temperature with W we can call the eastern part east of 100E where the North China Plain and the Loess Plateau are located a sustaining dry and warm type; the northwestern part west of 100E can be called transitional zone from dry and-warm type into humid and warm one, of them the Hexi corridor and East Xinjiang Basin started to change in the early 1970s while the mountain region, oasis, and desert in northern Xinjiang started to change around the mid 1980s.The deterring factor for changes of borderline in the wet and dry climate zone is the variation rate of precipitation vs. potential evaporation. Over the past 40 years both precipitation and potential evaporation tended to decrease in the eastern part east of 100E,but the decreasing rate of precipitation is higher than that of potential evaporation. We think that the cause for wetting in the western part of 100E was that the potential evaporation has been decreasing with the increase of precipitation, moreover the absolute value for decreasing rate of potential evaporation is higher than that of increasing rate of precipitation.

Key words: northern China, humid index, change for the borderline of wet and dry climate zone