Bottom pressure torque and the vorticity balance from observations in Drake Passage

The vorticity balance of the Antarctic Circumpolar Current in Drake Passage is examined using 4 years of observations from current-and pressure-recording inverted echo sounders. The time-varying vorticity, planetary and relative vorticity advection, and bottom pressure torque are calculated in a two-dimensional array in the eddy-rich Polar Frontal Zone (PFZ). Bottom pressure torque is also estimated at sites across Drake Passage. Mean and eddy nonlinear relative vorticity advection terms dominate over linear advection in the local (50-km scale) vorticity budget in the PFZ, and are balanced to first order by the divergence of horizontal velocity. Most of this divergence comes from the ageostrophic gradient flow, which also provides a second-order adjustment to the geostrophic relative vorticity advection. Bottom pressure torque is approximately one-third the size of the local depth-integrated divergence. Although the cDrake velocity fields exhibit significant turning with depth throughout Drake Passage even in the mean, surface vorticity advection provides a reasonable representation of the depth-integrated vorticity balance. Observed near-bottom currents are strongly topographically steered, and bottom pressure torques grow large where strong near-bottom flows cross steep topography at small angles. Upslope flow over the northern continental slope dominates the bottom pressure torque in cDrake, and the mean across this Drake Passage transect, 3 to 4x10(-9) m s(-2), exceeds the mean wind stress curl by a factor of 15-20.