中国北方地区40年来湿润指数和气候干湿带界线的变化

本文研究了中国北方地区 196 1～ 2 0 0 0年 4 0年间气候干湿带界线分布和 10年际变化。 4 0年来中国北方地区 ,在东经 10 0°以东地区 ,半干旱区和半湿润区的分界线不断波动向东推进 ,2 0世纪 90年代比 6 0年代向东和向南扩展 ,半干旱区面积扩大 ,半湿润区面积缩小 ,气候趋向干旱化 ;东经 10 0°以西地区 ,极端干旱区面积在缩小 ,湿润指数有增大趋势。如果把温度和湿润指数相结合 ,东经 10 0°以东的黄淮海区和黄土高原区为持续的干暖型 ;东经 10 0°以西的西北地区 ,则由干暖型向湿暖型转变 :河西走廊和东疆盆地转型的时间发生在 2 0世纪 70年代初 ,北疆山地绿洲荒漠地区转型的时间发生 2 0世纪 80年代中期前后。气候干湿带界线的变化取决于降水和潜在蒸发的变化速率。 4 0年来 ,在东经 10 0°以东地区 ,降水和潜在蒸发都呈下降趋势 ,但降水减少速率大于潜在蒸发下降速率 ;在东经 10 0°以西地区变湿的原因 ,研究认为除了降水有所增加外 ,潜在蒸发也在下降 ,而且潜在蒸发下降速率的绝对值大于降水增加速率。

Changes of humid index and borderline of wet and dry climate zone in northern China over the past 40 years

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.