Observations and a Model of Undertow over the Inner Continental Shelf
Journal of Physical Oceanography
Onshore volume transport (Stokes drift) due to surface gravity waves propagating toward the beach is known to result in a compensating Eulerian offshore flow in the surfzone referred to as undertow. Observed offshore flows indicate wave-driven undertow extends well offshore of the surfzone, over the inner shelves of Martha's Vineyard, Massachusetts and North Carolina. Theoretical estimates of the wave-driven offshore transport from linear wave theory and observed wave characteristics account
... eristics account for 50% or more of the observed offshore transport variance in water depths between 5 m and 12 m, and reproduce the observed dependence on wave height and water depth. During weak winds, wave-driven cross-shelf velocity profiles over the inner shelf have maximum offshore flow (1-6 cm s −1 ) and vertical shear near the surface, and weak flow and shear in the lower half of the water column. The observed offshore flow profiles do not resemble the profiles observed within the surfzone, which are parabolic with maximum flow at mid-depth. Instead, the vertical structure is similar to the Stokes drift velocity profile but with the opposite direction. This vertical structure is consistent with a dynamical balance between the Coriolis force associated with the offshore flow and an along-shelf "Hasselmann wave stress" due to the influence of the Earth's rotation on surface gravity waves. The close agreement between the observed and modeled profiles provides compelling evidence for the importance of the Hasselmann wave stress in forcing oceanic flows. Summer profiles are more vertically sheared than either winter profiles or model profiles, for reasons that remain unclear.