National Oceanic and
Atmospheric Administration
United States Department of Commerce


 

FY 1984

Topographic effects of the Alaskan Stream on shelf currents

Lagerloef, G.S.E., and G.A. Cannon

NOAA Tech. Memo. ERL PMEL-53, NTIS: PB84-205137, 139 pp (1984)


Current meter observations from a deep trough on the Kodiak Island shelf show that the flow is largely barotropic and follows depth contours around the trough to form a cyclonic vortex. This year-round feature is interpreted as a Taylor-Proudman column and is reproduced with a numerical potential vorticity model. When the velocity scale is appropriately small (~5 cm s), the lowest order balance approaches = 0, and the flow is constrained to follow isobaths in good agreement with the data. As a consequence, stronger currents accompany steeper topography as streamlines which follow isobaths converge. This flow pattern is sustained by the predominantly southwestward regional mean shelf flow, which is shown here to be driven by the Alaskan Stream, an adjacent oceanic boundary current over the continental slope. A linear parabolic vorticity equation is solved with boundary conditions for the geostrophic shelf break current and no wind forcing. These create a vorticity across the shelf which is balanced by vortex stretching of cross-isobath flow, resulting in an along-shelf current increasing seaward from zero at the coast and a pressure gradient sloping downward along-shelf in the flow direction. The addition of wind forcing through a coastal boundary condition modifies these results by adding a near shore current to account for the observed seasonal and shorter period variability on the Kodiak shelf. The largest responses occurred when storms brought along-shelf winds to a considerable length of the northern Gulf of Alaska coastline, regardless of the locally measured winds near Kodiak. The relevance of these various results is made clear by comparing the Kodiak shelf with other well-studied shelf domains. Major features of the theory presented here for boundary current forcing modified by wind stress are consistent with the South Atlantic Bight and the Gulf Stream and may be applicable to the Mid Atlantic Bight as well.




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