Evaluation of Coupled Model FGOALS in Simulating the Annual Cycle of Tropical Pacific Rainfall and SST

This study evaluated the ability of four versions of flexible global ocean–atmosphere–land system model (FGOALS-g2, FGOALS-s2, FGOALS-g3, FGOALS-f3-L) developed at the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics (LASG/IAP), Chinese Academy of Sciences in simulating the climatology and seasonal cycle of the tropical Pacific precipitation and sea surface temperature (SST). This paper compared coupled and atmospheric model intercomparison project (AMIP) simulations in terms of atmosphere-ocean feedback mechanisms and heat budget analysis to investigate the formation of precipitation and SST biases. Results exhibited notable improvements in FGOALS-g3 and FGOALS-f3-L compared with previous versions, such as a reduction of 50% of root-mean-square error (RMSE)of SST, decreasing from above 2°C in FGOALS-g2 and FGOALS-s2 to 1°C in FGOALS-f3-L. However, there are negligible improvements in precipitation. FGOALS-f3-L reproduces the climatology mean of SST, the meridional asymmetry of SST, and the precipitation in the East Pacific well because of the reduction of errors in the mean state of shortwave radiation and its reasonable representations of ocean dynamic heat transport and surface shortwave radiation feedback; hence, FGOALS-f3-L shows better improvement in the aforementioned sectors than FGOALS-g3. Furthermore, excessive rainfall biases in the northern side of the equator become more severe than those of the previous models, which is greater than 4 mm d⁻¹. These overestimated convections bring latent heat, which enhances the latitudinal diabatic heat gradient and strengthens the cross-equatorial south wind. Errors in the seasonal cycle of wind actually cancel out part of the warm biases of SST, resulting from the overestimation of shortwave radiation. Similar error sources existed in the simulated seasonal cycle, characterized by an improved annual cycle but a weaker overall amplitude in FGOALS-g3 and FGOALS-f3-L. Improved annual signal of SST results from the stronger amplitude of the annual cycle of the meridional wind and latent heat flux. Generally, errors in the seasonal cycle of wind and latent heat are bigger but cancel out the annual biases in the shortwave radiation flux. Therefore, the annual signal in SST seems to be enhanced than that of former versions. Meanwhile, the false semi-annual cycle of SST in FGOALS-g2 and FGOALS-s2 mostly results from the semi-annual cycle of the latent heat flux.