Characteristics Of Meteorological Drought Across The Loess Plateau And Their Linkages With Large–scale Climatic Factors

The Loess Plateau is located in a typical ecologically fragile area and climate sensitive area in China,with arid climate,little and concentrated precipitation,and serious soil erosion.Global warming combined with the impact of human production and life,the Loess Plateau region is deeply affected by drought disasters,causing certain losses to local agriculture and economy.Studying the characteristics of meteorological drought in the Loess Plateau is conducive to a correct understanding of local climate change,and can provide a valuable basis for the rational planning and utilization of water resources in the region and the prevention of soil erosion,and is of great significance to reducing drought losses.In this paper,using precipitation data on the Loess Plateau and large-scale factor data,Mann-Kendall mutation detection method and Sen slope method are used to analyze the characteristics of precipitation changes at different time scales on the Loess Plateau from1961 to 2017.Based on the Standardized Precipitation Index(SPI),the annual and seasonal drought characteristics of the Loess Plateau and the temporal and spatial variation characteristics of different grades of drought are analyzed.Continuous wavelet transform(CWT)and Cross Wavelet Transform(CWT)are used to study the influence of Southern Oscillation Index(SOI),Pacific interdecadal oscillation(PDO)and multivariate ENSO index(MEI)on the climate of the Loess Plateau,and the influence of teleconnection factors on the climate of the Loess Plateau is analyzed.The run theory was used to extract the drought characteristic variables of drought events from the SPI indices of different time scales in the Loess Plateau from 1961 to 2017,and the spatial distribution of joint probability and joint return period under the two variables of drought duration and drought intensity is analyzed.The analysis reveals the characteristics of meteorological drought in the Loess Plateau in an all-round way,and the research draws the following conclusions:(1)The annual precipitation change rate of the Loess Plateau from 1961 to 2017 is-0.076mm/a,the spring precipitation change rate is 0.096mm/a,the summer precipitation change is-0.183mm/a,and the autumn precipitation change rate is-0.023 mm/a,the change rate of winter precipitation is 0.068mm/a,the increasing trend of winter precipitation reaches a significant level,and the average summer precipitation accounts for the largest ratio of annual precipitation;there is no significant abrupt change in annual and seasonal precipitation in the Loess Plateau.The average annual rainfall in southern Shanxi,southern Shaanxi,and eastern Gansu is relatively high.The northern part of Ningxia and the central part of Inner Mongolia have relatively little precipitation.The eastern part has more precipitation than the western part,and the southern part has more precipitation than the north.The spatial trends of precipitation in spring and autumn all show that the precipitation in southern Shanxi and Shaanxi and eastern Gansu has decreased,and the precipitation in eastern Qinghai,central Gansu,northern Ningxia and central Inner Mongolia has increased.The area with reduced precipitation in summer accounts for a large area,and the decline in northwestern Shanxi is significant,and increased precipitation in most areas in winter.(2)The annual-scale SPI index of the Loess Plateau shows a downward trend.Analyzing the changes of seasonal SPI,spring SPI increases at an average annual rate of0.001,summer SPI increases at an average annual rate of 0.005,autumn SPI decreases at an average annual rate of 0.0019,and winter SPI increases at an average annual rate of 0.0047.Annual and seasonal SPI change trends show different phase characteristics,but different seasons are dry periods from 1995 to 2000,and humid periods after 2010.The frequency range of the same drought level on the Loess Plateau has little difference under different time scales.Areas above severe drought are mainly concentrated in the western and northern regions of the area.(3)The SPI index at different time scales is positively correlated with SOI,and negatively correlated with PDO and MEI,and as the time scale increases,the degree of correlation increases.Based on the continuous wavelet transform,it is concluded that the SPI drought index and the large-scale climate factors SOI,PDO,and MEI of the Loess Plateau at different time scales have similar periodic changes in a certain period of time.Cross wavelet transform shows that there are 3a,6a,12 a resonance periods between SPI and SOI,PDO,and MEI at different time scales,but the phase relationship is quite different.The long-term SPI has a stronger correlation with the Large scale climatic factors than the short-time SPI.(4)The distribution of drought duration or drought intensity on the same time scale in the Loess Plateau shows a high degree of similarity.The spatial distribution of drought duration or drought intensity is different in different time scales.As a whole,the northeastern part of Qinghai and the vicinity of the border of Shanxi,Shaanxi and Mongolia have experienced a longer drought duration,while the drought duration of Shanxi and southern Shaanxi was shorter.The most severe droughts occurred in central Shanxi and the surrounding areas of Wuzhong in Ningxia.Under the same drought recurrence period,with the increase of time scale,the value of drought duration and drought intensity in most areas also increased.On the same time scale,the two joint transcendence probability spatial distributions constructed by the two drought variables of drought duration and drought intensity corresponding to the return periods of 5 years,10 years,20 years,and 30 years show a high degree of consistency.