Abstract
AbstractThe 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 m s−2, exceeds the mean wind stress curl by a factor of 15–20.
Highlights
The broad outlines of the momentum balance in the Antarctic Circumpolar Current (ACC) were described by Munk and Palmen [1951]: momentum input fairly uniformly over the course of the ACC by the wind stress is balanced principally by bottom form stress
We investigate the vorticity balance of the ACC in Drake Passage using a 4-year time series of measurements by current and pressure recording inverted echo sounders (CPIES) deployed across Drake Passage as part of the cDrake project [Chereskin et al, 2012], as well as a 9-year time series of shipboard acoustic Doppler current profiler (SADCP) velocities in Drake Passage, Southern Ocean State Estimate (SOSE), and 21 years of satellite altimeter fields
Local Vorticity Balance in the Polar Frontal Zone (PFZ) in Drake Passage The time-varying and time-mean local vorticity balances in the PFZ are dominated by mean relative vorticity advection and horizontal velocity divergence, including significant ageostrophic contributions
Summary
The broad outlines of the momentum balance in the Antarctic Circumpolar Current (ACC) were described by Munk and Palmen [1951]: momentum input fairly uniformly over the course of the ACC by the wind stress is balanced principally by bottom form stress. Hughes and de Cuevas [2001] confirmed that a numerical model zonally integrated barotropic vorticity balance is dominated by wind stress curl and bottom pressure torque, with locally large nonlinear terms. Hughes [2005] used the mean surface dynamic topography of Niiler et al [2003] and an assumption of equivalent-barotropic vertical structure to examine the mean nonlinear vorticity balance of the ACC He described two regimes: a wave-like one in regions of smooth bathymetry in which the surface planetary and mean relative vorticity advection balanced (with a wavelength consistent with stationary Rossby waves with small net depth-integrated divergence); and in regions of enhanced bathymetry (including Drake Passage) a regime in which planetary vorticity advection dominated over relative vorticity advection.
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