Analysis Of The Relationship Between Meteorological Drought And Atmospheric Circulation Index In Northwest China

With global warming,the frequency and intensity of droughts increase constantly.Meteorological drought is closely related to atmospheric circulation and the potential effects of teleconnection cannot be ignored.It has great theoretical significance to explore the response of meteorological drought to atmospheric circulation for early warning of drought and reduction of drought effect.Limited by the current knowledge of the underlying mechanisms,this response relationship is usually described using statistical correlation which fails to reveal the potential impact on drought when large-scale climate signals oscillate violently.This paper takes Northwest China(NWC)as the research object.The SPEI was used for describing meteorological drought.Based on EOF and REOF methods,the study area was divided,the trend of drought in subregion was analyzed by EEMD.Correlation analysis,cross-wavelet transform and random forest method were used to study the response of sub-regional meteorological drought to multiple circulation indexes and the main circulation indexes that drive meteorological drought.Based on kernel density estimation and Copula function,the joint distribution of the characteristic variables of drought events under the extreme circulation phase were fitted.Finally,the probability and return period of drought events under the extreme phase of different sub-regions were analyzed.The main conclusions are as follows:(1)The variation of drought index in NWC had obvious regional regularity.The research area was divided into six sub-regions: the northwest central region,the eastern northern region,the eastern southern region,the northern region,the plateau region and the southern region.The northwest central region,the eastern northern region and the eastern southern region showed a trend of dryness.The other three subregions showed different degrees of wetting trend,with the most obvious trend in the plateau region in recent years.The meteorological drought in the northwest central region tends to dryness monotonously,and the periodicity is not obvious.In the other five subregions,the interannual variation was the main one,supplemented by the interdecadal variation,and the interannual period of drought was more obvious than that of interdecadal period.(2)SPEI-1 was mainly correlated with atmospheric circulation index,while SPEI-12 was mainly correlated with sea surface temperature(SST)circulation index.Circulation changes are concurrent,and the most correlated are not necessarily the major drought drivers.The driving factors of SPEI-1 were mainly NAO,AMO,IOWPA and SN.The driving factors of SPEI-12 are mainly SN,ONI,PDO,TNA-SST and IOD.The driving factors of interdecadal scale drought are mainly SN and AMO.When the scale of drought was extended from interannual to interdecadal,compared with the monthly scale,the influence of atmospheric circulation index disappeared,the influence of SST on drought was further reduced,and the activity of the sun occupied the main component.(3)The different years and periods of each cross-wavelet transform indicated that the circulation index dominated the teleconnection with drought in different time and frequency domains.The different annual and monthly drought periods in each sub-region indicated that the cumulative change of drought index is more significantly affected by the circulation index.(4)Under the extreme circulation phase,drought characteristics had different manifestations.The northwest central region,the eastern southern region,the plateau region,the southern region under the high circulation phase were significantly stronger than those under the low circulation phase.The characteristics of drought events in the eastern northern region and the northern region under the low circulation phase were obviously weaker than those under the high circulation phase.(5)Under the extreme circulation phase,the 2d / 3d drought joint characteristics were different in the same area.The situation of joint drought characteristic value with high probability above 0.9 was complex,with peak value-short duration,low peak-long duration in 2d,and peak value-short/long duration-weak intensity,peak value-short/long duration-strong intensity in 3d,etc.Namely with the same joint distribution,under different circulation phases,the joint of drought characteristics value corresponding to the occurrence point of drought with high probability is different,and there are strong-weak joint and strong-strong joint.(6)The 2-d / 3-d drought return period in each sub-region had different characteristics under the extreme circulation phase.In 2d,under different circulation phases,the joint/co-occurrence recurrence interval of the equal characteristics value in the same sub-region was different.In 3d,by analyzing the high probability drought event points and the return period,it could be seen that the corresponding drought characteristic values of the same return period were different under the extreme circulation phase.In the mass,there was a circulation phase that was more prone to drought events.The above research results can lay the foundation for drought prevention and drought warning and forecast in Northwest China.