Evaluation of the Simulation Capability of CAS FGOALS-f3-L for the Second Mode of Interannual Variability of the East Asian Summer Monsoon Based on the Theory of Potential Vorticity Circulation

The accurate simulation of East Asian summer monsoon (EASM) interannual variability remains a challenge for state-of-the-art climate models. Using the recently proposed potential vorticity circulation (PVC) theory, we found that the second mode of the multivariate empirical orthogonal function for the observed EASM exhibits a close relationship with the first modes of cross-equatorial PVC (CEPVC) and the cross-tropopause PVC (CUPVC). We evaluated the simulation capability of the second mode of the EASM in July via the Flexible Global Ocean-Atmosphere-Land System Model, Finite-volume version 3, Low resolution (FGOALS-f3-L) historical experiment and observational analysis and analyzed the potential causes of model bias. Results indicated that the model fails to reproduce the EASM meridional dipole pattern of geopotential height and precipitation, a deficiency closely related to CUPVC simulation bias. Further analysis suggested that the accurate simulation of water vapor flux convergence and divergence, which are related to CUPVC variations, is critical for accurately capturing the second EASM mode. Results also indicated that improving the simulation accuracy of CUPVC related to the South Asian High phenomenon can enhance the simulation capability of the CAS FGOALS-f3-L model for EASM interannual variability.