Evaluation and Bias Analysis for the Performance of IAP AGCM4.0 in Simulating the East Asian Subtropical Westerly Jet

The performance of IAP Atmospheric General Circulation Model Version 4 (IAP AGCM4.0) in reproducing the observed features of the East Asian Subtropical Westerly Jet (EASWJ) is examined by analyzing the differences between model simulations and JRA-25 reanalysis data during 1979-2008, the possible reasons for the bias in model performance are analyzed, and the impact of convective parameterization schemes in the model simulations has been further investigated. The results show that the model can reproduce the climatological characteristics of the EASWJ reasonably well, including the spatial structure and the seasonal meridional displacement. However, the model also shows deficiencies in simulating the intensity and location of the EASWJ, with weaker magnitude in both wintertime and summertime, and the model simulated position of the EASWJ is located southward during wintertime and northward during summertime when compared with observations. Analysis of the results demonstrates that IAP AGCM4.0 can capture the seasonal evolution of the EASWJ well, including the northward jump during summer months. Based on Empirical Orthogonal Function (EOF) decomposition of the zonal wind at 200 hPa, both for the observation and model simulation, it is found that IAP AGCM4.0 can reproduce well the spatial structure of the first leading mode from the observation; however, the model fails to capture the interannual variation of the meridional displacement of the EASWJ. Based on the analysis of surface sensible heat fluxes and mid-upper troposphere Meridional Temperature Difference (MTD), it is found that the model's biases in terms of the simulated surface sensible heat fluxes in the southeastern Arabian Peninsula, northwestern Arabian Sea and northern India cause a weaker intensity and northward shift of MTD. The deficiency of IAP AGCM4.0 in reproducing the interannual variation of MTD leads to the biases in the simulation of the EASWJ. Furthermore, different convective parameterization schemes can influence the simulation of MTD, which will in turn directly affect simulation of the EASWJ.