Spatiotemporal Variability And Driving Mechanisms Of Drought Based On Standardized Precipitation Evapotranspiration Index

In the past 100 years,the rate of temperature increase in China was consistent with that in the world.However,for the past 50 years,the rate of temperature increase was nearly double that in the world.This shows that the increase in drought caused by global warming is more clearly reflected in China.Drought is one of the most devastated natural disasters due to its complicated driving mechanisms,uncertain frequency and widespread impacts characteristics.The occurrence of large-area drought will bring great threats to agricultural food production,economic and social development,and even human survival.Therefore,studying the temporal and spatial evolution of drought and its driving factors has an important role in drought assessment and monitoring,prevention,and decision-making for future drought.In combination with meteorological variables and teleconnection indices,this research analyzed the spatiotempoal characteristics and driving factors of the standardized precipitation evapotranspiration index?SPEI?in China,different sub-regions and different weather stations.We obtained the change trend,evolution period and spatial pattern of SPEI using the three statistical methods which including modified Mann-Kendall?MMK?,ensemble empirical mode decomposition?EEMD?and empirical orthogonal function?EOF?.In order to make SPEI as a drought index more targeted in China,we re-divided mainland China,according to?P-ET₀?,into four new sub-regions.We also analyzed the correction between 1-month SPEI and teleconnection indices,which including Pacific Decadal Oscillation?PDO?,El Ni?o–Southern Oscillation?ENSO?,Sea Surface Temperature?SST?and Arctic Oscillation?AO?,for each station.In addition,we used the multifractal theory based on the partition fractal function to explore the multifractal characteristics of the SPEI series.The main conclusions are as follows:?1?When mainland China were divided into seven sub-regions.Different sub-regions showed different characteristics of SPEI.Specifically,the arid and semi-arid areas have been humidified since 1990,while the humid and semi-humid areas showed arid trend for recent years.The minimum value of 1-month SPEI distributed randomly.During the entire studying period,there were 227 stations?nearly one third of the total 763 stations?whose extremely serious 1-month SPEI occurred in the 1960s.The trend test results of the 12-month SPEI showed that,in each sub-region,number of sites with increasing trends larger than that with decreasing trends,and this phenomenon is particularly obvious in the northwest China,the northeast Chinaand the central China.During the entire studying period,the northwest,east and southeast of China showed a humidification trend,while the central of China showed an aridity trend.?2?The first four intrinsic mode functions?IMFs?decomposed by ensemble empirical mode decomposition?EEMD?method from the 1-month SPEI indicated that the main average drought cycle in China and each sub-region is less than 1.2 years.The EEMD decomposition results of each station showed that the main average drought cycle of each station is in the range of 0.22-2.95 years,and only a small areas in the northwest of China whose average cycle are longer than 2.95 years.The drought cycle in northern and northwestern China is longer than that in other regions.The first four empirical orthogonal functions?EOFs?identified four drought modes of China and its results showed that the north China plain and northeastern China are two drought sensitive areas.?3?When mainland China were divided into four sub-regions.The analysis of the spatiotemporal evolution of SPEI and its driving mechanisms showed that the fluctuation of SPEI in the four new sub-regions is more obvious than that in seven sub-regions.The SPEI turning points in sub-regions 1 and 2 were both in 1996.The drought conditions in sub-region 4 can almost reflect the average drought conditions of China.The two drought sensitive periods of sub-region 1 is before 1988 and after 2006.The drought sensitive periods of sub-regions 2 and 4 are corresponding to 2001-2010 and 1961-1974,respectively.The year 1998 is the turing year that the SPEI in sub-region 1 turned wetting tendence into drying tendence.The year 1979 is the turing year that the SPEI in sub-region 3 turned drying tendence into wetting tendence.Sub-regions 2 and 4 showed wetting tendence throughout the entire studying period.?4?The teleconnection indices that affected precipitation,evapotranspiration?ET₀?,and SPEI most are corresponding to Ni?o 3.4,Ni?o 3.4 and Sea Surface Temperature?SST?,and Arctic Oscillation?AO?,respectively.The impact of Ni?o 3.4 on precipitation and ET₀ is reversed for each sub-region,which also causes the effect of Ni?o 3.4 on SPEI of each sub-region to be offset.ENSO and SST are major driving factors of spatiotemporal characteristics of drought of the arid and semi-arid areas?sub-regions 1 and 2?.PDO is major driving factor of spatiotemporal characteristics of drought of northern China,north China plain and Qinghai-Tibet Plateau.AO is major driving factor of spatiotemporal characteristics of drought of middle and western China.?5?The analysis of the multifractal characteristics of SPEI showed that SPEI for different sub-regions and different stations have multifractal characteristics,but the strengths of multifractal characteristics are different.With the increase of order q,the better the linear fitting effect of the log?_i?q,e?-logecurve,this indicated that the SPEI has significant multifractal characteristics within the specified scales.As the time scale increases,the multi-fractal characteristics of SPEI become more obvious.The SPEI's multifractal characteristics of Inner Mongolia,Qinghai-Tibet Plateau and north China are more obvious than that in other sub-regions,that is,the fluctuation variability of SPEI in these three sub-regions is stronger.The variation range of the generalized fractal dimension of each sub-region from large to small is as follows:north China plain,Qinghai-Tibet Plateau,southern China,Inner Mongolia,central China,northeastern China,northwestern China.This order indicated that the drought in north China plain has the strongest variability and most complicated causes,while the level of variability of drought in northwestern China is weakest.The multi-fractal spectrum of SPEI at all stations all showed right-biased phenomenon,indicating that the variability of SPEI is dominated by small periods among those internal periods included in the SPEI variation.The degree of right-biased phenomenon of 6-month SPEI is highest than SPEI in other four scales.This study can provide researchers with a reference by revealing the multi-fractal characteristics of SPEI in China,so that they can further study the potential impact mechanism of drought.