A NUMERICAL STUDY OF CIRCULATION DRIVEN BY MIXING OVER A SUBMARINE BANK

A primitive equation model is applied to study the spin-up of a linear ly stratified, rotating fluid over an isolated topographic bank. The m odel has vertical eddy mixing coefficients that decay away from the bo ttom over a specified e-folding scale. No external flows are imposed, and a circulation develops due solely to diffusion over the sea bed. V ertical mixing, coupled with the condition of zero diffusive flux of h eat through the sea floor, leads to a distortion of isothermal surface s near the bottom. The associated radial pressure gradients drive a ra dial-overturning circulation with upslope flow just above the bottom a nd downslope hows at greater height. Coriolis forces on the radial flo ws accelerate a vertically-sheared azimuthal (alongslope) circulation. Near the bottom the azimuthal motion is cyclonic (upwelling favourabl e), while outside the boundary layer, the motion is anticyclonic. Sens itivity experiments show that this pattern is robust and maintained ev en with constant mixing coefficients. Attention is given to the drivin g mechanism for the depth-averaged azimuthal motion. An analysis of th e relative angular momentum balance determines that the torque associa ted with bottom stresses drives the anticyclonic depth-averaged flow. In terms of vorticity, the anticyclonic vortex over the bank arises du e to the curl of bottom stress divided by the depth. A parameter sensi tivity study indicates that the depth-averaged flow is relatively inse nsitive to variations in the bottom drag coefficient. (C) 1998 Elsevie r Science Ltd. All rights reserved.