近代中国北方干湿变化趋势的多时段特征

Multi-Scale Temporal Characteristics of the Dryness/Wetness over Northern China During the Last Century

利用CRU(Climate Research Unit)月降水和月平均气温资料, 通过构造一个既包含降水变化又考虑温度变化对潜在蒸发影响的干湿指标, 对我国北方近100年(1901～1998年)、 50年(1951～2002年)、 20年(1981～2002年)和近10年(1991～2002年)干湿变化趋势进行了系统的检测和分析, 突出了在全球增暖背景下温度变化对干湿变化的重要影响, 揭示了中国北方四个不同时段干湿变化趋势的基本特征.结果表明: 由于受温度升高的影响, 近100年我国西部地区降水尽管增加但并不存在变湿趋势, 而东部地区降水显著增加的地区明显呈现出变湿趋势, 显著变湿的范围较降水增加的范围大, 且强度明显增强, 这与这个地区温度的降低有关.在近50年, 100°E以东的北方地区是明显的干旱化趋势; 西北西部显著变湿的范围较降水显著增加的范围为小, 而东部干旱化区域的范围较降水显著减少的区域大.这充分说明了增暖能够减弱降水增加对地表水分收支的贡献, 也就是加剧降水减少的干旱化程度.在近20年, 新疆北部尽管降水量有所增加, 但并未改变该地区干旱化的时空格局, 也未发现显著的变湿趋势存在, 这个时段北方大部分地区仍然以干旱化趋势为主.特别值得注意的是, 在有些地区干湿指标的变化趋势与降水的变化趋势完全相反.在20年和近100年时段上, 我国西部大部分地区仍处在一个干旱化的进程中, 而华北地区在20年和50年时段上均表现为一个干旱化的趋势.

A dry-wet index, including the impact of both surface air temperature and precipitation on the potential evaporation, is established by using monthly mean temperature and monthly precipitation data from Climate Research Unit （CRU）. The CRU air temperature and precipitation data in China are compared with the observations from China 160 stations, and the gridded data agree well the observations in general. Based on the index, the dryness/wetness trends over northern China during the last century at different time scales （1901 - 1998, 1951 - 2002, 1981 - 2002, and 1991 - 2002） are detected and analyzed systematically, and the warming impact on the surface dryness/wetness state is studied. The characteristics of the dryness/wetness trends in four periods in northern China are presented. The results show that there are different variation trends of dryness/wetness during four periods 1901 - 1998, 1951 - 2002, 1981 - 2002, and 1991 - 2002, respectively. Over the last 100 years, there exists no wetting trend in western China due to the warming impact in spite of the precipitation increase. The reason is that warming results in the increase of potential evaporation, and the increasing precipitation cannot compensate the loss of water at surface due to the increasing potential evaporation. As a result, although the precipitation may become more in some regions, the warming effect will offset the wetting impact of precipitation increase and the wetting trend cannot occur in these regions. However the obvious wetting trend is located in the eastern regions where the precipitation increases, and the wetting range is wider than the range of the precipitation increase, even with more enhanced amplitude for the wetting trend. This phenomenon can be attributed to the local temperature decrease, which leads to the reduced potential evaporation and enhances the wetting trend. Over the last 50 years, there is the obvious aridification in northern China to east of 100°E, and the marked wetting range in the western part of Northwest China is smaller than the range of precipitation increasing. And the drying range in the eastern part of Northwest China is larger than the range of precipitation decreasing. This shows that warming can weaken the effect of precipitation increase on the surface water budget, in other words, it enhances the drought resulting from the reduced precipitation. Over recent 20 years, although the precipitation increases in the northern part of Xinjiang Uygur Autonomous Region, it has not changed the spatial pattern of aridity, and the arid trend still exists in most regions of the northern part under regional warming. It should be noticed that the precipitation and the index in some areas of northern China show the opposite trends for wetting or drying variation, which reflects that the impact of air temperature on surface dryness or wetness should be considered when the drying or wetting trend is analyzed. At the temporal scales over the last 100 years and 20 years, the aridification in most regions of western China is still in progress. And the arid trends occur in North China at the scales of latest 20 and 50 years.