Representation of the Mean Atlantic Subtropical Cells in CMIP6 Models

The Atlantic Subtropical Cells (STCs) consist of poleward Ekman transport in the surface layer, subduction in the subtropics, and equatorward transport in the thermocline layer that largely compensates the surface Ekman divergence and closes the STCs via equatorial upwelling. As a result, the STCs play an important role in connecting the tropical and subtropical Atlantic Ocean, in terms of heat, freshwater, oxygen, and nutrients exchange. However, their representation in state-of-the-art coupled models has not been systematically evaluated. In this study, we investigate the performance of the Coupled Model Intercomparison Project Phase 6 climate models in simulating the Atlantic STCs. Comparing model results with observations, we first present the simulated mean state with respect to ensembles of the key components participating in the STC loop, that is, the meridional Ekman and geostrophic flow across 10 degrees N and 10 degrees S, and the Equatorial Undercurrent (EUC) at 23 degrees W. We find that the model ensemble reveals biases toward weak Southern Hemisphere Ekman transport and interior geostrophic transports, as well as a weak EUC. We then investigate the large inter-model spread of these key components and find that models with strong Ekman divergence between 10 degrees N and 10 degrees S tend to have strong mixed layer and thermocline interior convergence and strong EUC. The inter-model spread of the EUC strength is primarily associated with the intensity of the southeasterly trade winds in the models. Since the trade-wind-induced poleward Ekman transports are regarded as the drivers of the STCs, our results highlight the necessity to improve skills of coupled models to simulate the Southern Hemisphere atmospheric forcing.