Topographic Rectification of Tidal Currents on the Sides of Georges Bank

TLDR

Rectification of M2 tidal currents on Georges Bank’s sloping sides, driven by continuity, Coriolis effects, and bottom friction, is predicted to contribute significantly to its year‑round clockwise circulation, with non‑linear interactions between mean and tidal currents becoming important where the cross‑isobath tidal excursion and topographic length scale are comparable. The study extends Huthnance’s depth‑averaged theory to incorporate mean‑current tidal‑current interaction, spatially varying bottom friction, and rotary tidal currents. Using depth‑averaged theory, the authors predict mean Eulerian and Lagrangian currents, cross‑isobath sea‑surface slopes, and current speeds for half‑sinusoid bottom topography on Georges Bank’s sides, accounting for mean‑current tidal‑current interaction, variable bottom friction, and rotary tidal currents. The analysis shows that the ratio of cross‑isobath tidal excursion to topographic length scale controls the nonlinearity of the Coriolis mechanism, that Stokes velocities reduce mean Lagrangian currents to about two‑thirds of Eulerian values, and that the predicted clockwise flow, strongest on the northwestern and northern sides, matches observed speeds and is spread over the Bank’s top by mean‑current tidal‑current interaction.

Abstract

The rectification of M2 tidal currents on the sloping sides of Georges Bank is predicted to make an important year-round contribution to its observed mean clockwise circulation. A rectification mechanism involving continuity and Coriolis effects, but regulated by bottom friction (Huthnance, 1973), is operative. Huthnance's (1973) depth-averaged theory for the along-isobath mean Eulerian current associated with this mechanism and with a second, purely frictional, mechanism is extended to include mean current-tidal current interaction, spatially-varying bottom friction and rotary tidal currents. The ratio of cross-isobath tidal excursion Le to topographic length scale L is found to be an important nondimensional parameter in determining the degree of nonlinearity of the Coriolis mechanism. A significant Stokes velocity is associated with both rectification processes, so that, for the Coriolis mechanism, the mean Lagrangian current is only about two-thirds of the mean Eulerian current. On the sides of Georges Bank, Le and L are of the same order, and the rectification is sufficiently non-linear that interaction of the mean current with the tidal current is important. The mean Eulerian and Lagrangian currents, and the cross-isobath mean sea surface slopes, are predicted for half-sinusoid representations of bottom topography on the northwestern, northern and open ocean sides of the Bank. The mean flow is clockwise and concentrated over the edge of the Bank, but smeared out onto the top of the Bank by the mean current-tidal current interaction. The predicted current speeds, which are greatest on the northwestern and northern sides, are of the same order as those observed.