Does Topographic Form Stress Impede the Antarctic Slope Current?

Topographic form stress plays a central role in constraining the transport of the Antarctic Circumpolar Current (ACC), and thus the rate of exchange between the major ocean basins. Topographic form stress generation has been linked to the formation of standing Rossby waves in the ACC’s retrograde flow (i.e., flow opposite to the direction of Rossby wave propagation). However, it is unclear whether topographic form stress similarly retards prograde (i.e., in the same direction as Rossby wave propagation) flows such as the Antarctic Slope Current (ASC), as such flows do not arrest planetary waves. In this study, we investigate the momentum balance and barotropic transport of wind-driven prograde and retrograde zonal channel flows impeded by varying topographic ridges and bottom friction, using a suite of high-resolution process-oriented simulations. Consistent with previous studies, in a flat channel spanned by a central ridge that partially blocks barotropic potential vorticity (f/H) contours, we find that the momentum balance of wind-driven retrograde flows is consistently closed by topographic form stress. In contrast, momentum input by prograde winds is primarily balanced by bottom friction provided that the planetary potential vorticity contours remain unblocked; prograde wind stress along zonally obstructed planetary potential vorticity contours is instead balanced by topographic form stress. Experiments with a geometry resembling a continental shelf and slope yield similar results for retrograde flows. However, despite f/H contours being unobstructed in this case, we find that topographic form stress can still dominate the momentum balance of prograde flows when the continental slope undergoes sufficiently large meridional excursions. Our findings imply that the stratification and transport of prograde flows, such as the ASC and the West Greenland Current, may be expected to be more sensitive to changes in wind forcing than retrograde flows, such as the ACC and the subtropical eastern boundary currents, because prograde momentum balance is at least partially dependent on bottom friction, and thus the speed of the barotropic flow. Processes influenced by prograde ocean flows, for example the water mass formation near the Antarctic coastline, may be expected to be more susceptible to climate shifts.