Research On Responses Of Aridity And Drought To The East Asia Summer Monsoon In Monsoon Transition Zone Of China

The monsoon transition zone of China is a special area having complex climate change,most of this area is semiarid climate,and its ecological environment is fragile.Activities of the East Asian Summer Monsoon?EASM?which include its intensity,its position of the northermost margin and its duration,and the three aspects all affect climate change of the monsoon transition zone.Firstly,this paper studies the connections and differences among three aspects above of the EASM.Secondly,taking the monsoon transition zone as research object,its aridity and drought are studied.What are the impacts of three aspects above of the EASM on climate of the monsoon transition zone is also studied.Finally,basing on above result,differences amonge the impacts of three aspects above of the EASM and reasons for these differences are analized.We get the following main conclusions:?1?Changes of strength,the northermost margin and duration of the EASM are not consistent.The quantitative descriptions of the summer monsoon activities are varied,such as from the intensity of the EASM,the northernmost margin,and its duration of the EASM.What are the differences among different angles as which are above mentioned is not.deep researched A variety of quantitative indexes of the EASM are defined,especially intensity of the EASM.As it is mentioned that activities of the EASM also include it's the northermost margin and duration,but there are rarely indexes about these two aspects.So according to previous studies,this paper chose three intensity indexes and one northermost margin index,meanwhile a duration index is difined.Then we compare these three indexes in different time scales,and some differences do exist among these three types of indexes.These three types of the EASM all have similar changes in long-term scale,which all show that activities of the EASM weaked during 1961-2013.These three types of indexes also show similar decadal fluctuations in decadal scale.But ininterannual scale,differences among three types of index are very significant.So it is not very reasonable to think that the stronger the intensity of the EASM is,the norther of the northermost margin is,at the same time,the longer duration in research on activities of the EASM about interannual scale in the future.?2?The impacts of intensity,the northermost margin and duration of the EASM on aridity?and drought?also have many differences in the monsoon transition zone of China.By studying responses of aridity?and drought?to the EASM in different time scales,it is found that change of aridity?and drougth?can response well to all three aspects of the EASM in long-term trend scale and decadal scale.It is the reason why the monsoon transition zone of China is drying that activity of the EASM is becoming weaker and weaker during 1961-2013.But on interannual time scale,the response of aridity?and drougth?of the monsoon transition zone to the summer monsoon intensity is not good.While aridity?and drought?of the monsoon transition zone can response well to the northermost margin and its duration,which show that the more norther position of the northermost margin is,or the longer duration of the EASM is,the wetter climate is,and the less drought occur in this transition zone.In terms of indexes,correlation coeffcients between the MSM?index of the northermost margin?and IAI index?aridity index?is 0.15-0.46,and correlation coeffcients between the MSM?index of the northermost margin?and SPEI index?drought index?is 0.15⁰.35.The MDN index?index of duration?defined firstly in this papper has the most significant correlation with I_AI and SPEI indexes.Correlation coeffcients between the MDN?index of the duration?and I_AI index?aridity index?is 0.46-0.65,and correlation coeffcients between the MDN?index of the duration?and SPEI index?drought index?is 0.4⁰.55.It means that climate of the monsoon transition zone is closely depend on duration of the EASM,and excepting intensity of the EASM we should take the northermost margin and duration of the EASM into consideration.?3?There are also some differences in impacts of the EASM on aridity?and drought?in the monsoon transition zone.The most significant impacts occur in the center and west of the transition zone,and comparatively small impact occurs in east of the transition zone.This is because that duration of the EASM has different effect on precipitation and water vapor flux in different areas of the transition zone.The duration of the EASM has a significant impact in the center and west of the transition zone?r=0.63?,but small impcat in east of the transition zone?r=0.53?.In term of systems bringing a large amount of water vapor transmission,they are different in center and west from in east of the zone.The center and west area are mainly affected by anomalous anticyclonic conveying system?southwest water vapor transport?,and the east area is mainly affected by abnormal cyclonic conveying system?northeast or southeast water vapor transport?.Because the MDN is based on southwest in this article defines,the degree of aridity?and drought?have a good response to the MDN in center and west area of the transition zone,and weaker response in east area of the transition zone.?4?It is found from above results that there are different impacts of the duration of the EASM on aridity and drought in the summer monsoon transition zone.The impact is more significant on aridity than on drought in the transition zone.Based on the results above and combining previous researches,we find that this is because aridity and drought themself exist differences.So differences of aridity and drought and the reasons for the differences are studied in the monsoon transition zone which has complex climate by comparing to monsoon region and non-monsoon region.The results showed that the differences are mainly concentrated in less than 6 months of short time scales,especially in the monsoon transition zone,which is the most significant difference between aridity and drought.Therefore,it is not reasonable to use an aridity index as a monitoring index of drought to monitor occurrence of drought for short periods of time in the transition zone.The direct cause of the differences in physical significance lies in the degree of dry climate and drought itself exist difference,the root cause lies in the different adjustment of meteorological elements on both aridity and drought.In terms of physical significance,climate is dry degree on behalf of the dry and wet climate background,and its change occur in the longer time scale,little change in a short time scale.While drought is a climate event,and its change occur more frequently in a short time.Iterms of adjustment of meteorological elements on aridity degree and drought,the elements which affect the aridity in the monsoon transition zone from main to secondary are precipitation,relative humidity,air temperature,wind speed,and it is similar to the non-monsoon region.And influence of drought meteorological elements from main to secondary are relative humidity,precipitation,wind speed,air temperature,and it is similar to the monsoon region.So it is clear that dominant elements affecting aridity and drought in the monsoon transition zone are in a different order.As it is mentioned that differences are the most significant in monsoon transition zine,this is closely connected with the regional climate background.First of all,frequent drought events will lead to more aridity climate,and this characteristic is similar to monsoon region.At the same time,if a year with less precipitation anomalies over the zone,value of aridity degree in this year will be even close to 1,the the relationship of drought occurence and aridity will be more similar to the non-monsoon region.This led to a more uncertain and the complex relationship between aridity and drought in the monsoon transition zone.