Resolution convergence and sensitivity studies with North Atlantic circulation models. Part I: The western boundary current system

The fidelity of numerical simulations of the general circulation of the North Atlantic Ocean in basin- to global-scale models have improved considerably in the last several years. This improvement appears to represent a regime shift in the dynamics of the simulated flow as the horizontal grid spacing decreases to around 10 km. Nevertheless, some significant biases in the simulated circulation and substantial uncertainties about the robustness of these results with respect to parameterization choices remain. A growing collection of simulations obtained with the POP primitive equation model allow us to investigate the convergence properties and sensitivity of high resolution numerical simulations of the North Atlantic, with particular attention given to Gulf Stream separation and the subsequent path of the North Atlantic Current into the Northwest Corner. Increases in resolution and reductions in dissipation both contribute to the improvements in the circulation seen in recent studies. We find that our highest resolution eddy-resolving simulations retain an appreciable sensitivity to the closure scheme. Our most realistic simulations of the Gulf Stream are not obtained at the lowest levels of dissipation, while the simulation of the North Atlantic Current continues to improve as dissipation is reduced to near the numerical stability limit. In consequence, there is a limited range of parameter space where both aspects of the simulated circulation can be brought into agreement with observations. This experience gained with the comparatively affordable regional North Atlantic model is now being used to configure the next generation of ocean climate models.