Sack Lunch Seminar (SLS)

Barotropic turbulence above topography: form stress and eddy saturation

Wind is an important driver of large-scale ocean currents, imparting momentum into the ocean at the sea surface. This force is almost entirely balanced by topographic form stress (that is the correlation of bottom pressure and topographic slope). The direct effect of bottom or skin friction in turbulent boundary layers is almost negligible for the momentum balance. We use a one-layer barotropic model to study the effect of a random monoscale bottom topography on beta-plane geostrophic turbulence. The model forcing is a uniform steady wind stress that produces both a uniform large-scale flow and smaller-scale macroturbulence. The macroturbulence is characterized by both standing and transient eddies and the large-scale flow is retarded by a combination of bottom drag and domain-averaged topographic form stress produced by the standing eddies.

A main control parameter is the ratio of beta to the root mean square gradient of the topographic potential vorticity (PV). We derive asymptotic scaling laws for the strength of the large-scale flow in the limiting cases of weak and strong forcing. If beta is comparable to, or larger than, the topographic PV gradient there is an “eddy saturation” regime in which the large-scale flow is insensitive to large changes in the wind stress. We show that eddy saturation requires strong transient eddies that act effectively as PV diffusion. This diffusion does not decrease the strength of the standing eddies but it does increase the topographic form stress by enhancing the correlation between topographic slope and the standing eddy pressure field.