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

A drywet 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 (19011998, 19512002, 19812002, and 19912002) 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 19011998, 19512002, 19812002, and 19912002, 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.