Study on Spatial and Temporal Variation of the Boundary and Area of the Semi-arid Region in Northern China over the past 60 Years
The semiarid region of Northern China is a transitional zone between humid and arid regions,which distribution is roughly the same as that of the agriculture-animal husbandry ecotone and the transition zone affected by the oscillation of the northern margin of the summer monsoon.Due to the shortage of water resources and the large interannual variability of precipitation in this region,it is a typical eco-environment fragile zone and ecological transition zone,and it is very sensitive to climate change.Therefore,it is of great scientific significance to study the spatial and temporal variation characteristics of summer precipitation and its causes in the semi-arid region of northern China under the background of global warming,which is helpful to better understand the climate change law in the semiarid region.Based on the meteorological station data,circulation characteristic index data,NCEP/NCAR reanalysis data and other basic geographic information data from 1960 to 2016,this paper analyzed the variation law of the semi-arid area in northern China and the spatial-temporal variation of precipitation in midsummer(July and August),and the relationships between the typical spatial-temporal precipitation pattern and atmospheric circulation/water vapor transport has also been discussed.The main conclusions were as follows:(1)From 1960 to 2016,the multi-year weighted average positions of the arid and s emi-arid boundary lines in northern China were(98°59′46′′E,37°10′5′′N)and(108°26′31″E,38°57′25″N).During the past 57 years,the arid boundary has a tendency to move westward in the longitude,and a tendency to move northward in the latitude;The semi-arid boundary has a tendency to move westward in the longitude,and a tendency to move southward in the latitude.(2)The changes of arid and semi-arid boundaries in different regions were complex,and the anomalous years were also different.The variation range of drought boundary was relatively small in Hetao plain area.In the inland river area of Hexi Corridor there was a trend of becoming wet in the 1990 s and the trend of drying has been obvious from 2000 to 2009.The semi-arid boundary tends to dry in the western part of the Yellow River Basin,and the most obvious characteristic in the central and eastern parts of the Yellow River Basin,the Haihe River Basin was that the 1990 s is the most obvious period of drought in theses areas.The semi-arid boundary changes fiercely in Liaohe River basin,and the phenomenon of wetting was obvious in the 1980 s.and it tends to dry obviously from 1990 s.However,it is worth noting that the arid and semi-arid regions in the last ten years have alleviated their phenomena of drought,there were signs of gradually becoming wet.(3)The arid and semi-arid boundaries generally have different cycles of change in the meridional and latitudinal movements,the primary periods of the arid boundary in the meridional and zonal shift were 8a and 12 a respectively in general,while that of the semiarid boundary in the meridional and zonal shift were 4a and 12 a.(4)The area change of semi-arid area in northern China from 1960 to 2016 was very intense,and it was a sensitive area for dry-wet change.In the past 57 years,the area change of semiarid area in China has a slight decreasing trend,and its area change shows a stage characteristic: the area of semi-arid areas showed an increasing trend from 1960 to 1969,a decreasing trend from1970 to1989,an obvious increasing trend from 1990 to 1999,and a decreasing trend after 2000.Among them,the period from 2000 to 2009 was the period of the largest semi-arid area and the most severe drought in the past 57 years.(5)There were two dominant spatial modes of July and August precipitation in semiarid region of Northern China from 1960 to 2016,which can be summarized as the "regional inphase type" and the "subregional out-of-phase type".The "regional in-phase type" showed that the precipitation in July and August of semi-arid area is uniformly large or less in the same phase.In this mode,the precipitation in July and August has shown a decreasing trend in recent 57 years,but the decreasing trend in August is more obvious.The "subregional outof-phase type" indicates that the distribution of precipitation in the east and the west of the semi-arid area of Northern China has shown an opposite trend,and the interannual variability of precipitation in July and August of the model is relatively small,but the interdecadal variability is obvious.(6)Under the "regional in-phase type" mode,there were differences in the atmospheric circulation configuration of precipitation changes in July and August in the semi-arid regions of northern China.The Eurasian teleconnection and the mid-latitudes westerlies were closely correlated with the precipitation variations in July,while the WPSH was the main factor affecting the interannual variations of precipitation in the semiarid region August.Under the "subregional out-of-phase type" mode,when a south-high-north-low barometric height field was formed along the coast of East Asia,the anticyclonic circulation controls the semiarid northern China,it was easy to form a distribution pattern with more precipitation in the western part of the northern semi-arid region and relatively lower precipitation in the eastern part.