The Impact Of Mesoscale Enhancement On Simulation Of East Asian Summer Monsoon Circulation In An AGCM

In this study,the parameterization of sea surface turbulent heat flux in National Center for Atmospheric Research Community Atmosphere Model version 5(CAM5)is improved by including the contribution of mesoscale convective motion(i.e.,mesoscale enhancement).The effect of mesoscale enhancement on the simulation of the East Asian summer monsoon circulation is further explored with SST-forced experiments that are conducted for the period of January 1979 to December 2015,and the results from 1980 to 2015 were selected.Research indicates:(1)The standard CAM5 exhibits evident biases in simulating the East Asian summer monsoon system.The simulated westerlies at 850 hPa in low latitude of East Asia are too weak,the precipitations in the Bay of Bengal,the South China Sea,and the tropical western Pacific are underestimated,the simulated south Asia and western Pacific subtropical highs are too weak,and the upper and middle troposphere in the low latitudes of East Asia(10°-30°N,70°-160°E)is too cold.By including the mesoscale enhancement in CAM5,these biases are clearly reduced.(2)Further analyses show that the improvements in the East Asian summer monsoon precipitations and circulations can be attributed to the enhancement of convections and the increase in water evaporation in the tropical western Pacific Ocean induced by mesoscale enhancement.Correspondingly,the latent heat due to vapor condensation in the atmosphere also increases,which reduces the cold bias in the mid-upper troposphere.Meanwhile,the geopotential height of the upper-middle-level atmosphere is elevated,which makes the simulated intensity of the south Asian and the western Pacific subtropical highs,two important components of the East Asian summer monsoon circulation,closer to the reality.(3)The simulation of the interannual variation of the summer monsoon and the intensity of high and low-level circulations by CAM5 is underestimated,and the simulation deviation of summer monsoon precipitation in the tropical Indian Ocean and the western Pacific is large.The mesoscale enhancement significantly improved the model’s simulation of the interannual variation of the East Asian summer monsoon circulation.The correlation coefficient between the simulated East Asian summer monsoon index and the observation increased from 0.47 to 0.57.The simulation of the high and low circulation,especially the low-level western Pacific anticyclone,was significantly improved.The simulation of precipitation and tropospheric temperature is also significantly enhanced.(4)The improvement of the interannual variation simulation of the East Asian summer monsoon circulation comes from the enhanced response of the model to ENSO.The introduction of mesoscale enhancement improves the average precipitation and circulation simulation of the East Asian-West Pacific region,while the increase of the average precipitation makes the response of the model to the ENSO-related SST anomaly more reasonable,so the model can reasonably reproduce the interannual variation of the East Asian summer monsoon circulation,which is closer to the observation.