| Northwest China is located in the hinterland of Eurasia,far from the sea,with a total area of more than three million square kilometers,nearly one third of China's territory,and a population of more than 100 million.This region is one of the major arid and semi-arid regions in the middle latitudes of the northern hemisphere,with dry climate,water shortage and sparse vegetation.It is not only a typical fragile ecological environment area in the world,but also a sensitive area responding to global climate change.This region plays an extremely important role in the global climate environment system,and the climate types in Northwest China are diverse.Therefore,the climate change in Northwest China has long been a key scientific issue of great concern to the scientific community at home and abroad and the Chinese government.Since the end of last century,the climate change in Northwest China has presented new characteristics,especially the wetting trend in western of this region.After that,the wetting trend gradually strengthened and expanded eastward.Due to the wetting have direct relationship with precipitation,air vapor is one of the preconditions of precipitation,at the same time,the research shows that potential evapotranspiration(PET)has a direct impact on climatic wet and dry,therefore,it is necessary to study the change of air vapor and PET in the region under the new characteristics of climate change.Because of global warming increased extreme climate events significantly,the variation characteristics of extreme temperature and extreme precipitation in this region also need to be further clarified under the warming-wetting background.This paper mainly uses the latest centenary grid data set of Climatic Research Centre(CRU)of the University of East Angola(UK),reanalysis data,the forecast data of future climate scenarios(SSP2-RCP4.5),and the data of soil water content,meteorological observation data in Northwest China and radiosonde data.Through methods of correlation analysis,trend analysis,air water vapor integration,variable substitution and variance analysis,and so on.The spatio-temporal evolution characteristics,intensity variation characteristics of the warming-wetting in Northwest China were analyzed firstly.Secondly,the long-term trend and spatial distribution characteristics of the vertical accumulation of water-vapor content over the Northwest China were analyzed,as well as the annual variation of water-vapor content in the Qilian Mountains,a typical mountainous area in the Northwest China.Finally,the spatial characteristics,multi-year variation trend of potential evapotranspiration(PET)and its response to warming and wetting were analyzed,and also the influence of related meteorological factors on PET.The contribution of temperature,relative humidity,sunshine and wind speed to PET was quantified.The main conclusions are summarized as following:(1)The periodicity and spatial distribution characteristics of warming-wetting in Northwest China are revealed.Research show that the temperature in the western and eastern parts of Northwest China has experienced two warming periods from the1940 s to the 1960 s,and the current warming period is stronger.In terms of precipitation,it has the characteristics of seesaw changing during many periods in western and eastern parts of Northwest China,but since the beginning of this century,the regional overall have entered the wetting period.The temperature of the whole Northwest China presents a consistent increase spatially,and the warming degree gradually increases.However,the boundary of precipitation has been continuously expanding eastward since the last three climatic periods.At present,the range of precipitation has expanded to the whole Northwest China,which has been particularly prominent since the beginning of this century.However,the enhancement and eastward expansion of the current warming and wetting trend in this region have not changed the basic climate patterns.(2)The water-vapor content proportion increases in the west wind influenced region and decreases in the monsoon influenced region.Most area of the Northwest China is affected by westerlies,and only a small part of the southeast is affected by monsoons.The vertical accumulation of water-vapor content in the west wind influenced region and the monsoon influenced region both showed an increasing trend over the years.However,the proportion of the water-vapor content from the westerlies showed an increasing trend,while that in the monsoon influenced region showed a decreasing trend.The results show that the westerly circulation and the East Asian summer monsoon circulation are consistent with the precipitation variation in the western and eastern in this region respectively.Since the beginning of this century,the west wind circulation and summer wind circulation have changed from the seesaw pattern during the 20 th century to a new consistent strengthening pattern,which is consistent with the characteristics of the increasing and expanding trend of climate wetting in Northwest China.(3)The contribution of meteorological factors affecting PET was quantified in this region.By analyzing the response of PET to warming and wetting in Northwest China,results showed that the PET in this region had obvious periodic changes,and the most significant changes occurred in the 1970 s and 1980 s.Factor effect analysis showed that PET had a positive response to temperature,the highest and lowest temperatures in the region continued to rise,resulting in the increase of PET.Relative humidity reduced the PET in the region.Sunshine duration has continued to decrease rapidly since the 1980 s,resulting in a corresponding decrease in PET.Similarly,the PET has been a significant decrease corresponding to wind speed decrease.In general,PET in the eastern parts of Northwest China has mainly been affected by the temperature,whereas it has mainly been affected by the wind speed in the western part of the region.Relative humidity and sunshine duration have relatively little effect on the PET(below 20% in most places).Combined with PET and precipitation,the theoretical threshold of the first spring rainfall in the eastern part of Northwest China was determined.Furthermore,the typical circulation pattern and sea surface temperature pattern were synthesized when the rainfall occurs earlier or later.The results can be used in the monitoring and forecasting of climate prediction.(4)The warming-wetting in Northwest China is accompanied by the change of extreme temperature and extreme precipitation in this region.Specifically,both extreme low and extreme high temperature gradually became higher,and the former is more obvious,while the intensity and variability of extreme precipitation increases.At the same time,the days and precipitation proportion of light rain in this region showed a decreasing trend,while the days and precipitation proportion of heavy or torrential rain and extra torrential rain showed an increasing trend. |
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