Study On South Asian High Variations Based On Multiple Sets Of Definitions And The Evaluation For Multi-models Simulation

Based on four sets of definitions,the location,intensity and area of South Asian High variations during 1979 – 2019 are compared.Multiple SAH-associated variables are analyzed to understand the variations for SAH.Furthermore,attentions are paid to the relationship between the SAH indices and the sea surface temperature(SST),and between the South Asian High indices and the precipitation over China.Finally,29 state-of-the-art climate models from Phase 6 of the Coupled Model Intercomparison Project(CMIP6)are evaluated based on quantitative indices,and are compared with the CMIP5 models.Main conclusions are drawn as follows:(1)The location of the South Asian High have an inter-annual variation characteristic,without significant trend and decadal abrupt in the center indices,whereas the intensity and the area index have significant trend and decadal abrupt.When the South Asia high locates more east(west)than usual,the meridional position is southward(northward).When it is stronger(weaker),the area of the South Asian High is larger(smaller).Based on the maximum geopotential height F1 and the closed-circulation-system index F3,the South Asian High intensity is closely connected with the zonal mean value,which indicates that the increasing trend in those two definitions may be forced by the apparent increment of the geopotential height under global warming.However,after removing the parallel circle geopotential height,the significant weakening trends are observed by the eddy geopotential height definition F2 and the changed-isobar closed-circulation-system index F4.The variation characteristics based on F2 and F4 coincide well with those in the eddy geopotential height field and the relative vorticity,indicating that removing the uniform rise in geopotential height is more appropriate to present the long-term variation in the South Asian High.(2)The latitude index of the South Asian High has a significant negative correlation with the sea surface temperature(SST)of the Middle East Pacific and the North Indian Ocean,and a significant positive correlation with the sea surface temperature of the NorthPacific,while the correlation between the longitude index and sea surface temperature is opposite.When the SST in the equatorial Middle East Pacific is high(low),the SST in the Indian Ocean is abnormally warm(low),the location of the South Asia High is eastward(westward)and southward(northward)than usual.The latitudinal index of the South Asia High has a significant positive correlation with the precipitation in North China and north northeast China,and a significant negative correlation with the precipitation in the middle and lower reaches of the Yangtze River.When the South Asia high is northward,the positive anomaly center of geopotential height is over the northeast of North China and the Sea of Japan,and the corresponding high-level circulation shows abnormal divergence,which is conducive to the rise movement.There is a southerly wind anomaly at 850 hPa in eastern China,which is conducive to the northward transportation of water vapor and the increase of precipitation in North China.The cyclonic circulation anomaly is found in the lower layer of the middle and lower reaches of the Yangtze River,and the convergence anomaly at the high level is not conducive to precipitation.When the South Asia high is southward,the circulation situation is opposite.(3)There is a significant negative correlation between the longitude indices of the South Asia high and that of the Northwest Pacific subtropical high on the interannual scale based on the changed-isobar closed-circulation-system index F4.When the South Asia high is opposite to the Northwest Pacific subtropical high,South China is controlled by the high pressure and the vertical velocity is abnormal,which is not conducive to the formation of precipitation.The south wind anomaly in Southeast China is conducive to the northward transport of water vapor,and the vertical movement in the middle and lower reaches of the Yangtze River is abnormal,corresponding to more precipitation.When the South Asia high and the Northwest Pacific subtropical high are backward,the precipitation in the middle and lower reaches of the Yangtze River is less,and the South China is opposite.(4)CMIP6 models can basically reproduce the climate characteristics of 200 hPa geopotential height field,and the inter-model spread is smaller than that of CMIP5 models.However,the simulated 200 hPa geopotential height still has systematic negative deviation.The changed-isobar closed-circulation-system index F4 can overcome the influence of the systematic negative deviation.Based on this method,it is found that CMIP6 models simulate the South Asian High eastward significantly.Most of the models simulate the South Asia High to the south,with the intensity of the simulation is weak and the area of the South Asia high is underestimated.The CESM2 models of NCEP/NCAR center in the United States have a superiority to simulate the intensity,area,longitude and latitude of the South Asia High and 200 hPa geopotential height.NESM3 model from Nanjing University of Information Science and Technology has a better performace for simulating the climate state of South Asia high,ranking the ninth,and has a good representation of the positive correlation between the intensity and area index of the South Asia High and the large-scale precipitation in eastern China.The CAMS-CSM1-0 model from the Chinese Academy of Meteorological Sciences has a nice simulation on the relationship between the longitude and latitude index of the South Asia high and precipitation.