The major features of the East Asian subtropical westerly jet (EASWJ) in the upper troposphere simulated by the two versions of CCSR/NIES/FRCGC climate system model (MIROC_Hires and MIROC_Medres) are examined by analyzing the differences between the coupled model 20 th century simulations and the NCEP/NCAR reanalysis, focusing on the evaluation of the model performances in reproducing the mean EASWJ structures, the seasonal evolution, interannual variability, and the relationship among the

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... seasonal evolution, the meridional temperature gradient and the diabatic heating in the upper troposphere. The mean EASWJ vertical and horizontal structures, the seasonal evolution, and the correspondence of the EASWJ location to the meridional temperature gradient in the upper troposphere are well simulated in the coupled models. The increase in model resolution can improve the simulation of the EASWJ structures, seasonal evolution and interannual variability. However, both coupled models overestimate the EASWJ intensity in winter, and underestimate the jet intensity in summer, relative to the NCEP/NCAR reanalysis. The biases in model EASWJ intensity are found to be associated with biases in meridional temperature gradients in the troposphere, and furthermore with the surface sensible heat flux in summer and convective condensation heating in winter as well as the meridional heat transport gradient. The coupled models simulate well the seasonal evolution of the diabatic heating averaged between 30°N-45°N, and its association with the westerly jet. However, the simulated maximum diabatic heating in summer is located eastward compared with the reanalyzed position, with a relatively weak diabatic heating intensity, especially in MIROC_Hires, while the MIROC_Medres model reproduces relatively strong diabatic heating near 130°E in winter. This study suggests that the condensation latent heating over the western Pacific in winter, the surface sensible heating over the northern side of the Tibetan Plateau in summer and the meridional heat transport gradient determine the EASWJ intensity, location and structure as well as its seasonal evolution. Thus the reasonable reproductions of the meridional heat transport gradient and the surface diabatic heating are the key points for improving the EASWJ simulation by the MIROC model.