Research On Regional Characteristics And Circulation Anomalies Of Summer Drought In China Based On SPI Index

Based on the daily precipitation data of 2474 stations in China during 1961 and 2018,NCEP/NCAR reanalysis data and sea surface temperature data,and using summer SPI index,the regional characteristics of summer drought in China during past 60 years and the circulation anomalies when typical regional droughts occur were systematically studied,using statistical analysis methods such as REOF analysis through REOF analysis,the results show that:(1)Summer drought in China has significant regional changes.Through REOF analysis,China can be divided into 15 regions based on summer drought inter-annual variability represented by the SPI index,which basically cover most of China except Tibet and northern Xinjiang.Each region can well characterize the inter-annual variation of local summer drought,and all have periodic variation characteristics of multiple time scales,but their main cycles and variation trends are different.Except for the Hetao and Jianghuai regions,of which the summer SPI changes are significantly negatively correlated,and the other regions are independent of each other.There is a drying trend in summer at most of the north and southwestern of China,while there is a trend of humidification in the southeast of China.(2)Southwest China tends to be drying in summer,especially in Yunnan and Southeast Sichuan.For typical dry summers,the water vapor transport is insufficient and the area was controlled by the descending branch of the oblique vertical circulation between the tropical Northwest Pacific and Southwest China,which is the result of the response of the atmosphere to the anomalous heat source in the tropical Northwest Pacific.At the same time,the atmospheric wave disperses energy downstream through the disturbance of the westerly zone,which provides the necessary disturbance energy accumulation for the formation and maintenance of low-level divergence and high-level convergence in Southwest China,causing the maintenance of drought.(3)After the 21st century,the trend of drought in Hetao area is increasing.The input of wave disturbance energy at 200 hPa and the forcing of SSTA from the east of Philippines are important reasons for the maintenance of drought.Between the Hetao area and the Jianghuai area,there is a north-south drought and flood antiphase oscillation phenomenon,and the water vapor channel associated with this phenomenon mainly comes from the western Pacific.In addition,The phenomenon is closely related to the atmospheric EAP type teleconnection.In the dry-north and wet-south year(the wet-north and dry-south year),it shows the EAP-type teleconnection negative phase(positive phase)configuration,which has an equivalent barotropic structure.Besides,it is also related to NAO/AO and Nino3 index in the previous spring and the same summer.