Aridity-wetness Characteristics over Northwest China in Recent 44 Years

Northwest (NW) China covers Xinjiang, Qinghai, Ningxia, Shaanxi and the western part of Inner Mongolia; it is located to the north and northeast of the Qinghai-Tibetan Plateau, far away from oceans. This extensive region is one of the aridest areas in the world and also a principal arid and semi-arid expanse in China, including totally 85%of such land in China. This region is under the impacts of westerly, plateau and monsoon climates so that rainfall undergoes great variability and there is a high frequency of drought, serving as a zone sensitive to climate change and ecologically vulnerable. The inference is presented by Shi Yafeng that the NW climate is changing from a dry, warm one to a moist, warm stage one, which is awaited for further studies with more climate elements. Whether climate pattern changes or not depends completely on the change in hydrological cycle whose critical ingredient is evaporation, by which, in combination with rainfall and runoff, a regional hydrological equilibrium is determined. Rainfall is focused on by previous studies on NW China climate change and the pattern conversion and it is considered by few researchers whether the climate is changing from a warm, dry one to a warm, moist pattern by means of rainfall and evaporation in combination, which are two no negligible aspects of water equilibrium on a regional basis. It is apparent that drought is led to by deficient precipitation and evaporation would increase such that it is feasible to take into account the budget of rainfall and precipitation in the study on transformation of arid-moist climate for the NW China, which is of far-reaching strategic and practical significance to NW China socio-economic development. Based on 1960—2003 rainfall and small-sized evaporation pan measurements from 131 stations in NW China, an aridity-wetness homogenized index of rainfall and evaporation is proposed as the crucial components of hydrological equilibrium, with EOF, REOF, tendency analysis, M-K sudden change check and Morlet wavelet analysis, the aridity-wetness evolution characteristic is analyzed over northwest China in recent 44 years. Results show that a regional consistent anomaly serves as the paramount mode of the 1960—2003 yearly dry and wet features, as well as the opposite feature in the NW (NE) to the SE (SW) which is also the important mode for the annual dry and wet abnormality. The space patterns for the arid and moist anomaly include the westerly, tableland and monsoon climates. The annual arid and wet features display greatly humidification trends in the whole NW, westerly and tableland climate zones, with the westerly climate humidification stronger than the trend of the tableland sub-zone, their abrupt change is from arid to moist in the mid-1970s. While the area on the fringe of monsoon climate in the southeast of NW China shows a trend of turning aridity, their abrupt change is from moist to arid in the early 1990s. In spite of the period surge which is not the same as in NW and three key climate zones, inter-decadal oscillation of over 8-year is a main period in all regions, 3—5-year inter-annual surge is also shown in all regions, but the intensity of amplitude is weak.