Comparison Of Sudden-onset And Slow-onset Drought Characteristics, Detection And Attribution In The Middle And Lower Reaches Of The Yangtze Rive

Based on Precipitation data of CN05.1 and soil moisture and evapotranspiration data of ERA5,the climatic characteristics of soil moisture and soil drought events in the middle and lower reaches of the Yangtze River during 1979-2020 were analyzed.Based on the speed of drought development,the flash drought and slowly-evolving drought in the study area were identified.The temporal and spatial distribution characteristics of the two types of drought were analyzed and the reasons for the difference in development speed were diagnosed.The evolution process of regional flash drought event and slowly-evolving drought event were compared and analyzed.Finally,CMIP6 experiment data was used to detect the attribution analysis of extreme flash drought events and slowly-evolving drought events,in order to study the impact of human activities on the two types of droughts in the study area.The main conclusions are as follows:(1)The spatial distribution pattern of soil moisture in the middle and lower reaches of the Yangtze River was wet in the south and dry in the north during the whole year and four seasons during 1979-2020.In terms of time characteristics,the average soil moisture of the whole year and spring,summer and autumn showed a significant downward trend.The average frequency,duration and intensity of drought all showed an increasing and strengthening trend,and the drought situation was aggravated.The northwest of Hunan,northern Anhui,Jiangsu and Zhejiang developed slowly,and the development time was more than 4 pentads.Drought in the rest of the region developed faster.(2)Compared with the slowly-evolving drought,the flash drought occurred more frequently,lasted longer and had stronger drought intensity during 1979-2020.The frequency of slowly-evolving drought increased significantly,while the duration and intensity of flash drought increased and intensified more obviously than that of slowly-evolving drought.During the development of flash drought,the net divergence of water vapor was stronger,which strengthened surface evapotranspiration and reduced precipitation,resulting in the rapid development of drought.In the development stage of slowly-evolving drought,the net divergence of water vapor was weak,which made the drought development slower.In the development stage of the flash drought event in 2019,the position of the Western Pacific subtropical high was westward,the wind field divergence in the arid region was strong,water vapor output was more,and the net divergence of water vapor in the atmospheric column was strong,resulting in the rapid development of the drought.However,in the development stage of slowly-evolving drought event in 2010/2011,the divergence effect of wind field was weak,and the net divergence of water vapor was weak,which made the development speed slow.(3)Five extreme flash drought events and three extreme slowly-evolving drought events were selected for detection and attribution analysis of human activities.Under the influence of human activities,the risk of flash drought events in 1990,2003,2013,2016 and 2019 increased by about 33±15%,32±17%,40±19%,40±25%and 58±19%.In 1988,1995 and 2010,the risk of slowly-evolving drought events increased by 27± 12%,30±16%and 33±13%.The existence of human activities promoted the occurrence and development of two kinds of extreme droughts in the middle and lower reaches of the Yangtze River,in which the influence of human activities on extreme flash drought events was greater than that on extreme slowly-evolving drought events.