Slippery Bottom Boundary Layers: The Loss of Energy From the General Circulation by Bottom Drag

Bottom drag is believed to be one of the key mechanisms that remove kinetic energy from the ocean's general circulation. However, large uncertainty still remains in global estimates of bottom drag dissipation. One significant source of uncertainty comes from the velocity structures near the bottom where the combination of sloping topography and stratification can reduce the mean flow magnitude, and thus the bottom drag dissipation. Using high-resolution numerical simulations, we demonstrate that previous estimates of bottom drag dissipation are biased high because they neglect velocity shear in the bottom boundary layer. The estimated bottom drag dissipation associated with geostrophic flows over the continental slopes is at least 56% smaller compared with prior estimates made using total velocities outside the near-bottom layer. The diagnostics suggest the necessity of resolving the bottom boundary layer structures in coarse-resolution ocean models and observations in order to close the global kinetic energy budget.

Automated summary

Bottom drag is a key mechanism for dissipating kinetic energy from the ocean's general circulation, but there are significant uncertainties in global estimates of this process. The velocity structures and characteristics of the bottom boundary layer can affect the estimates of bottom drag dissipation, and prior estimates may have been overestimated. Resolving these issues is crucial for closing the global kinetic energy budget.