Simulations of the East Asian subtropical westerly jet by LASG/IAP AGCMs

Performances of two LASG/IAP (State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics) Atmospheric General Circulation Models (AGCMs), namely GAMIL and SAMIL, in simulating the major characteristics of the East Asian subtropical westerly jet (EASWJ) in the upper troposphere are examined in this paper. The mean vertical and horizontal structures and the correspondence of the EASWJ location to the meridional temperature gradient in the upper troposphere are well simulated by two models. However, both models underestimate the EASWJ intensity in winter and summer, and are unable to simulate the bimodal distribution of the major EASWJ centers in mid-summer, relative to the observation, especially for the SAMIL model. The biases in the simulated EASWJ intensity are found to be associated with the biases of the meridional temperature gradients in the troposphere, and furthermore with the surface sensible heat flux and condensation latent heating. The models capture the major characteristics of the seasonal evolution of the diabatic heating rate averaged between 30°-45° N, and its association with the westerly jet. However, the simulated maximum diabatic heating rate in summer is located westward in comparison with the observed position, with a relatively strong diabatic heating intensity, especially in GAMIL. The biases in simulating the diabatic heating fields lead to the biases in simulating the temperature distribution in the upper troposphere, which may further affect the EASWJ simulations. Therefore, it is necessary to improve the simulation of the meridional temperature gradient as well as the diabatic heating field in the troposphere for the improvement of the EASWJ simulation by GAMIL and SAMIL models.