Evaluation Of East Asian Meiyu From CMIP6/AMIP Simulations

The East Asian summer monsoon advances to the Jianghuai basin in China,southern Japan,and the Korean Peninsula in June and July,which is called "Meiyu" in China.Meiyu is one of the important components of the East Asian summer monsoon system,and abnormal Meiyu precipitation will bring severe droughts and floods to the middle and lower reaches of the Yangtze River in China,Korea,and Japan.However,there are still uncertainties in the characterization of the rainfall from high-precision reanalysis data sets and atmospheric models.To improve the accuracy of simulating East Asian Meiyu,this study first examines the characteristics of Meiyu from June 1 to July 31 from 1998 to 2017 using two observational analyses(i.e.,Tropical Rainfall Measuring Mission [TRMM] and Global Precipitation Climatology Project [GPCP])and four reanalysis data sets(i.e.,ModernEra Retrospective Analysis for Research and Applications,Version 2 [MERRA2],ERA5,JRA55,and NCEP/ DOE).In addition,East Asian Meiyu simulated by 35 models from the 6th Coupled Model Intercomparison Project(CMIP6)are systematically evaluated from 1998 to 2014.The main conclusions are listed as follows.(1)In terms of the evaluation of multiple reanalyses in simulating Meiyu,MERRA2 and ERA5 are consistent with TRMM precipitation in terms of both mean and variance patterns,while JRA55(NCEP/DOE)has smaller mean precipitation and variance in the middle and lower reaches of the Yangtze River in China(near southern Japan).For the interannual variability of the precipitation averaged over the Meiyu region,the correlations with TRMM from high to low are ranked by MERRA2,GPCP,JRA55,ERA5,and NCEP/DOE.For the large-scale meteorology conditions for the strong and weak Meiyu years,all reanalyses except NCEP/DOE show that the relative humidity is larger in the strong Meiyu years than that in the weak Meiyu years and relative humidity over Japan and South Korea is larger than that over the lower reaches of the Yangtze River.Thus,TRMM and MERRA2 reanalysis are used as the reference datasets for the investigations of Meiyu.(2)For the evaluation of East Asian Meiyu from CMIP6/AMIP simulations,the spatial distribution of Meiyu in southern Korea and Japan is simulated better than that in the middle and lower reaches of the Yangtze River Basin of China among AMIP models.14 of the 35 AMIP models can simulate the Meiyu precipitation in southern Korea and Japan more realistically,and only 7 models can accurately reproduce the Meiyu precipitation in the middle and lower reaches of the Yangtze River.For the interannual variability of Meiyu,the standard deviation of precipitation simulated by GFDL-CM4 and GFDL-ESM4 is closer according to TRMM,and the standard deviation of 12 models is relatively smaller compared with TRMM.(3)The comprehensive ranking measures(CRM)of models are calculated using the pattern correlation coefficients(PCC)and the interannual variability scores(IVS).The Top 20% and Bottom 20% of the 35 CMIP6/AMIP models are selected based on IVS and CRM,respectively.Besides,the reasons why they perform better and worse are given.In terms of simulating the climatology of Meiyu,the relationships between Meiyu precipitation,and the western Pacific subtropical high and subtropical westerly are explored.The results show that the Top 20% models based on CRM ranking can realistically reproduce the locations of the East Asia-Pacific(EAP)pattern,subtropical westerly,and the western Pacific subtropical high.From the perspective of simulating the interannual variability of Meiyu,the Top 20% models based on IVS ranking can accurately characterize the positive correlations between the averaged Meiyu precipitation and the SST in the equatorial eastern Pacific and the Indian Ocean.In other words,AMIP models with good performance in simulating Meiyu's interannual variabilities can realistically reflect the response of Meiyu precipitation to the El Ni(?)oSouthern Oscillation(ENSO)and the Indian Ocean basin mode(IOBM).