Study On The Change Of Circulation Pattern And The Attribution Of Summer Drought In The Huai River Basin

Based on the meteorological station data,NCEP/NCAR reanalysis data and CMIP6 model data of the Huai River Basin(HRB)from 1961 to 2015,the circulation pattern evolution of summer meteorological drought in the HRB was studied and attributed in this paper.Firstly,the spatio-temporal variation and circulation characteristics of meteorological drought in the HRB in summer were analyzed based on SPEI.Furthermore,SOM(self-organizing mapping neural network)classification method was used to study the frequency,transition probability and duration of circulation patterns affecting drought in the HRB.Finally,combining with the CMIP6 model,the basin summer drought was attributed on the basis of circulation patterns to quantify the impact of human activities.The following conclusions are drawn:(1)The spatio-temporal changes and circulation characteristics of the summer drought.The SPEI in summer showed an upward trend in time.It showed an upward trend in the southwest,and a downward trend in the northeast.According to EOF analysis,the typical drought years are 1961,1966,1978,1994 and 2013.The composite analysis shows that the high latitude in drought year is controlled by the negative anomaly,the wind direction is abnormal in the east wind,and the convection activity is weak under the control of the anticyclonic circulation.The diagnostic analysis of the drought in 1994 showed that from July18 th to 28 th,the basin was controlled by anticyclone,the convection was weak,and the water vapor conveyor belt from the Indian Ocean was cut off,which led to the occurrence of summer drought in 1994.(2)Circulation pattern and characteristics of summer drought.Based on SOM,the500 h Pa height field in the HRB was divided into 12 circulation patterns.According to the SAPI drought index corresponding to each circulation pattern,the circulation pattern was divided into drought and wet circulation types.It was concluded from the change of frequency that drought type was negatively correlated with SAPI.The drought type showed a significant downward trend.Changes of the summer drought were dominated by thermodynamic changes in comparison to the dynamic changes.Each circulation type mainly took its own circulation type and its adjacent circulation type as the main transfer objects.In order to get the mutual transformation of the two types of circulation to happen,the intermediate circulation type should be used as the medium.In terms of duration,the average duration and the longest duration of A and C drought pattern are the longest.In recent years,the drought type that affected the occurrence of drought in the HRB appeared in a short duration.(3)Attribution analysis of the summer drought changes.In the model evaluation,the models performed well in the occurrence frequency,duration and the transition among the circulation patterns,but performed poorly in the simulation of the wet circulation type.In terms of attribution analysis,the signals of human activities were detected from the average occurrence frequency,average and maximum duration of years.When the occurrence frequency of the drought circulation type is less than 23 times,the probability of the drought circulation type is reduced by human activities,while the probability is opposite when the occurrence frequency is greater than 23 times.The study period was divided into two periods for the attribution analysis of the frequency of drought type.The results showed that the frequency change of drought-types in the two periods of 1966-1985 and 1995-2014 could be attributed to human activities,and in the previous period,human activities tended to increase the total drought frequency and reduce the return period.In the latter period,human activities tended to reduce the total frequency of drought circulation type.