Coaction of Top and Bottom Drags in Gulf Stream Dynamics

Bottom drag and top drag-associated with the ocean current feedback (CFB) to the atmosphere-are the energy dissipation processes in the oceanic boundary layers. Both are instrumental in regulating western boundary currents, such as the Gulf Stream (GS). However, the sensitivity of model results to bottom drag parameterization has not yet been considered in simulations where top drag is present. So far, in the absence of top drag, the bottom drag coefficient turns out to be a very sensitive parameter of ocean models. Here, we re-examine this sensitivity when both top and bottom drag are considered in eddy-rich ocean simulations of the North Atlantic basin. The top drag is addressed with a parameterization that considers the wind adjustment to CFB. In agreement with previous studies, we confirm that top drag has a large control on GS dynamics by reducing mesoscale activity. Next, we show that the energy sink due to bottom drag increases when top drag is neglected, but that this increase only partially compensates for the absence of top drag. As a result, the GS representation is too sensitive to bottom drag. Finally, we propose a simple scale-aware parameterization of the subgrid-scale topographic effect on bottom drag. Our results suggest that for a realistic representation of GS dynamics and energy pathways, top and bottom drags must be considered together. Future studies should include processes missing from this study, such as submesoscale dynamics and the surface gravity waves at the ocean-atmosphere interface.