Observations and a model of undertow over the inner continental shelf

Journal of Physical Oceanography
By: , and 



Onshore volume transport (Stokes drift) due to surface gravity waves propagating toward the beach can result in a compensating Eulerian offshore flow in the surf zone referred to as undertow. Observed offshore flows indicate that wave-driven undertow extends well offshore of the surf zone, 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 for 50% or more of the observed offshore transport variance in water depths between 5 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 parabolic profiles with maximum flow at middepth observed within the surf zone. 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.

Publication type Article
Publication Subtype Journal Article
Title Observations and a model of undertow over the inner continental shelf
Series title Journal of Physical Oceanography
DOI 10.1175/2008JPO3986.1
Volume 38
Year Published 2008
Language English
Publisher American Meteorological Society
Contributing office(s) St. Petersburg Coastal and Marine Science Center
Description 17 p.
First page 2341
Last page 2357
Country United States
State Massachusetts, North Carolina
Other Geospatial Martha's Vineyard
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