Study On The Climate Change From Arid To Wet In The Northwestern Of China

Applying the data of monthly precipitation, temperature, ground wind speed and water surface evaporation in northwest of China (Shanxi, Gansu, Ningxia, Qinghai, Xinjiang and west Inner Mongolia) in 1960 ~ 2003 and NECP/NCAR monthly mean reanalyzing data, the basic tendency which the temperature, precipitation, evaporation as well as the ground wind speed have changed in northwestern China while the global warming was analyzed, and the wet index was constructed considering synthetically the most essential factors about moisture revenue and expenditure, the precipitation and evaporation, then the model characteristics which the climate has transited from arid to wet in northwestern China and the relation between these characteristics and the continuity adjustment of atmosphere circulation in different latitude were studied . The main conclusions are:(1) In recent 44 years, the temperature has risen, precipitation has increased in a majority of areas in Northwestern China, but the water surface transpiration rate has reduced. The break occurred in 1976. It is related closely with the reduced surface wind speed, it is consistent with also the overall tendency which the local sand storm has reduced.(2) The abnormal annual arid-wet characteristics in northwestern China could be divided into three essential areas, such as the westerly zone climate region, the plateau climate region and the monsoon climate region, for short, the westerly wind area, the plateau area and the monsoon area. There is obvious oscillation cycle between the entire northwest area and the arid-wet index in three districts. The abnormal consistency is the most important modality which the arid-wet characteristics in northwestern China distributed spatially in recent 44 years, but the reversed change between the northwestern part and the southeastern part and the reversed change between the northeastern part and the southwestern part also are the important modalities in Northwestern China.(3) The arid-wet characteristics in westerly wind area, plateau area and entire northwestern represent obvious tendency which is changing from arid to wet, the most remarkable tendency is in the westerly wind area, the plateau area is next, they brook separately and in 1976 and 1974. But the arid-wet characteristics in southeastern area of northwestern China changed from wet to drought because it is located the edge of the region which monsoon affects, but this kind of tendency is weak. In view of the spring,summer, fall and winter, there is the phenomenon which the climate has changed suddenly from drought to wet in entire northwestern area and every regions, but the tendency has the obvious topicality and the seasonal difference.(4) The inconsistency between the climate arid-wet break in northwestern region and the arid-wet changing tendency in various regions is the result caused by the continuity adjustment of atmosphere circulation in different latitude under the complex surface condition, and relates with the drought in north and water logging in south. There is remarkable counter-correlation between AO in summer and the East Asian monsoon in summer, and changed separately from weak to strong in 1976 and 1976, this corresponded with the break of arid-wet index in northwestern area. Comparing with AO in summer, as a whole, the correlation between the East Asian summer monsoon , precipitation in northern area and arid-wet is reverse distribution, namely when Arctic Oscillation positive(negative) phase is stronger, the East Asian summer monsoon is generally weaker(stronger), the precipitation in southeast part of northwestern area reduces(increases), the climate change to drought(wet), the precipitation in westerly wind area and the plateau area increases(reduces), the climate change from drought(wet) to wet(drought).(5) With a view to the arid-wet index constructed by precipitation and evaporation synthetically, it may be objective to reflect the climate arid-wet evolution characteristics in northwestern area. The arid-wet index in summer represents the relation between the arid-wet evolution characteristics and AO in summer and the continuity adjustment of East Asian summer monsoon more remarkable than that of only the rainfall, also the inter-annual correlation obviously is higher than that of the annual. Applying the arid-wet index to study the climate transiting model from arid to wet in northwestern area is necessary and effective.