Evaluation And Projection Of The Summer Quasistationary Circulation Systems Over East Asia

Based on the improved closed-circluation system index(CSI),changes of the summer western North Pacific subtropical high(WNPSH),the South Asian high(SAH)and the Indian Low(IL)during 1961 to 2019 are analyzed.Attention is paid to the relationship between the three quasi-stationary circulation systems(QSCS)and East China precipitation.Moreover,the reliability of CSI is verified by comparing with other traditional indices.Subsequently,the mean state and tendency of the three QSCS from 31 state-of-the-art climate models from Phase 5 of the Coupled Model Intercomparison Project(CMIP5)are evaluated comprehensively.Furthermore,impacts of simutated QSCS on precipitation simulation over East China are discussed.Finally,changes of the three QSCS are further investigated when global warming reaches to 1.5?,2?,3 ? and 4?.Causes for the weakening and eastward retreat of the WNPSH are diagnosed,and impacts of QSCS changes on future precipitation are further analyzed.Main conclusions are drawn as follows:(1)Compared with the traditional index defined by the China Meteorological Administration(CMAI)and the index based on the eddy geopotential height(HEI),CSI can overcome the interference of the geopotential height rising under the global warming background,and reduce the influence of systemtic biases of CMIP5 models and those related to zonal inhomogeneous warming.CSI shows superiority in describing changes of QSCS under the global change.(2)The intensity of WNPSH shows an inter-decadal change during 1961 to 2019,with significant negative correlation with PDO.The intensity and location of WNPSH and IL have no significant trend,while the SAH remarkably moves westward.Each QSCS has a close connection with the East Asian precipitation.The location of WNPSH and SAH is the key factor affecting the precipitation in the southern part of China.When WNPSH and SAH zonally approach(move away)from each other,there is more(less)precipitation in the SYR and less(more)precipitation in the South China.(3)Although there is an underestimation of intensity,models can basically reproduce the mean state of the three QSCS.Models generally show a westward shift of WNPSH compared to NCEP reanalysis.Overall,CCSM4,CNRM-CM5,CESM1-CAM5 and Nor ESM1-M are identified as high-skill models.High-skill models show a better simulation of precipitations in East China,with 21.3% decrease of dry biases in Southeast China.Only 7 of 31 CMIP5 models can simulate the observed tendency of WNPSH,whereas most of them have distinct underestimation.Trends in WNPSH intensity and SAH latitudinal position extert an important influence on the simulation of precipitation trend in North China and in south of the YR.(4)The selected best-skill models consistently show that WNPSH would weaken and move eastward,SAH would weaken and move southwestward,and IL would strengthen and move westward in response to global warming levels evaluated at 1.5?,2?,3 ? and 4?.Moreover,with temperature increasing,changes of QSCS also gradually increase.The Local meridional circulaiton would decrease while the meridional temperature gradient diminishes.Meanwhile,the EL Nino-like warming in the equatorial region diminishes the Walker circulation,which makes the convection over the Maritime Continent and the local meridional circulation,both,decrease.The decreased meridional temperature gradient also diminishes the westerlies in the north of WNPSH.The weakened Walker circulation attenuates the eastlies in the tropics,results in cyclonic circulation anomalies over the north western Pacific.Furthermore,the east-and down-ward heating center,leads to the positive abnormal vorticity generation in the WNPSH.All these conditions make WNPSH weaken and move eastward in future warm climate.(5)In the future,with WNPSH moves eastward,there is an abnormal southlies extending from the South China Sea to the North China,which is conducive to transporting water vapor to the North and Northeast China,leading to significant regional precipitation increase.In addition,WNPSH moves away from SAH,leading to upward motion anomalies in Southeast China.Moreover,stronger westerly winds is conducive to water vapor transport to Southeast China.This circulation condition results in more precipitation over Southeast China in response to global warming levels at 3 ? and 4?.