| As one of the most destructive natural disasters in the world,drought has a serious impact on social economy,natural environment and human activities.Shaanxi Province is located in the hinterland of China,which has always been said to be “has drought nine years out of ten”.Under the background of changing global climate,it is very important to study the characteristics and causes of meteorological drought for water resources management,drought resistance and disaster reduction in Shaanxi Province.Based on spatial-temporal characteristics of climate elements,the spatial-temporal characteristics of meteorological drought were studied by using standardized precipitation evapotranspiration index(SPEI)and various statistical methods in Shaanxi Province from 1960 to 2016.Then the effects of climatic factors,atmospheric circulation and sunspot activities on drought were studied by correlation analysis and cross-wavelet analysis.The main conclusions are as follows:(1)In the past 57 years,the average annual precipitation was 649 mm in Shaanxi Province.The annual precipitation showed a decreasing trend in Shaanxi and three sub-regions.Except for the increasing trend of precipitation in summer,the other three seasons showed a decreasing trend.The distribution of average annual precipitation was very uneven in space,gradually increased from north to south.The average annual temperature was 11.4 C in Shaanxi Province.The annual average temperature showed a significant increasing trend in Shaanxi and three sub-regions.The average temperature showed an increasing trend in the seasons,especially in spring,autumn and winter.The average annual temperature gradually increased from north to south.The average annual PET was 972 mm in Shaanxi Province.The annual PET showed a decreasing trend in Shaanxi,central and southern parts,while showed an increasing trend in northern parts.The PET showed an increasing trend in spring,autumn and winter,while showed an obviously decreasing trend in summer.The average annual PET gradually increased from southwest to northeast.(2)In the past 57 years,it showed a trend of acidifications on the annual scale in Shaanxi and the three sub-districts.The frequency of drought became higher and higher,and the scope of influence became larger and larger,and the degree became more and more serious.The annual drought index showed a decreasing trend,especially in Shaanxi,central and southern parts.The first three modes were: “regional consistent type”,“northwest and southeast difference type”,and “north-south and middle difference type”.The average frequency of drought was 67.1% in Shaanxi,which was dominated by light drought and moderate drought.The frequency of drought had the characteristics of “high in central and low in north-south”.The provincial drought was the most frequent in Shaanxi and three sub-regions.The ratio of number of stations with drought and drought intensity showed an increasing trend,among which ratio of number of stations with the light drought had a decreasing trend,while with the medium drought,heavy drought and extra drought had an increasing trend.In the 1990 s,the frequency of drought was the highest,the scope of influence was the widest and the degree was the most serious in Shaanxi,central and southern parts.While in northern parts,the frequency of drought was the highest and the range of drought was the largest in the 2000 s and the degree of drought was the most severe in the 1990 s.(3)In the past 57 years,there has been a trend of drought in spring,autumn and winter,and wetting in summer.The frequency,extent and severity of drought in spring,autumn and winter showed an increasing trend,while showed a decreasing trend in summer.The SPEI index showed a decreasing trend in the other three seasons except in summer,and the decreasing trend was obvious in spring and autumn.The first three modes in spring and winter were: “regional consistent type”,“north-south difference type”,“north-south and middle difference type”,in summer and autumn were: “regional consistent type” “northwest-southeastern difference type”,“north-south and middle difference type”.It was most prone to winter drought in Shaanxi.The frequencies of drought in the four seasons were 54.0%,55.8%,51.9% and 61.2% respectively,which were dominated by light drought and moderate drought.The seasonal drought frequency tended to decrease with the increase of drought grade.The frequency of drought had the characteristics of “low in north and high in south” in spring and winter,had the characteristics of “high in west and low in east,high in north and low in south” in summer,and had the characteristics of “high in north low in south” in autumn.The provincial drought was the most frequent in spring,autumn and winter,while the regional drought was the most frequent in summer.The ratio of number of stations with drought and drought intensity showed an increasing trend,especially in spring,and showed a decreasing trend in summer.In the 2000 s,the frequency of drought was the highest,the scope of influence was the widest and the degree was the most serious in spring and summer,while it appeared in the 1990 s in autumn.In winter,the frequency of drought was the highest and the range of drought was the largest in the 1980 s and the degree of drought was the most severe in the 1990 s.(4)The SPEI index could better reflect the change of drought area,and had good adaptability in the meteorological drought assessment in Shaanxi Province.(5)Precipitation was closely related to meteorological drought.In recent 57 years,the trend of drought has been mainly affected by the decrease of annual precipitation and the increase of annual average temperature in Shaanxi,southern and central parts.And it has been affected by the decrease of annual precipitation,the increase of annual potential evapotranspiration and average annual temperature in northern parts.The trend of drought has been affected by the decrease of precipitation,the increase of average temperature and potential evapotranspiration in spring,autumn and winter.The trend of wetting was a response to the increase of precipitation and the decrease of potential evapotranspiration in summer.(6)The enhancement of El Ni?o,the abnormally cold sea surface temperature in the eastern North Pacific,the southward invasion of the Arctic cold air and the sunspot activity would cause different degrees of drought.Among them,sunspot activity had the strongest impact and the ENSO event of large-scale circulation had a strong impact on the drought,followed by the Arctic Oscillation and finally the Pacific Decadal Oscillation.(7)Sunspot activity was the main driving force of drought.On the one hand,it had an important impact on precipitation,temperature and potential evapotranspiration,which directly affected the drought.On the other hand,it had an indirect impact on the drought by affecting the anomalous factors of atmospheric circulation. |
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