National Oceanic and
Atmospheric Administration
United States Department of Commerce


 

FY 1985

Theoretical and observed profiles of tidal currents at two sites on the southeastern Bering Sea shelf

Mofjeld, H.O., J.D. Schumacher, and D.J. Pashinski

NOAA Tech. Memo. ERL PMEL-62, NTIS: PB86-105236/XAB, 60 pp (1984)


A semi-analytic theory for vertical profiles of tidal currents on the continental shelf is presented in which the vertical eddy viscosity is obtained with a high-resolution, Level II turbulence closure model. Each tidal constituent is assumed to be a free, shallow water wave propagating on an unstratified shelf of constant depth. The eddy viscosity is a time-dependent composite of contributions from the major tidal constituents. The theoretical profiles have been fit to M2 and K1 current harmonic constants observed at two sites on the Southeastern Bering Sea Shelf. At the coastal station BBL1 (56°19'N, 161°33'W; 63 m depth) off the Alaska Peninsula occupied during 15-30 May 1981 in a Kelvin wave regime with rectilinear tidal currents, the fit of M2 and K1 theoretical profiles reproduces the general features of the tidal currents. The thick bottom boundary layers observed at BBL1 require a large apparent bottom roughness (1.0 cm) which may be due to strong surface swell and/or bedforms. The predicted eddy viscosity has a maximum of 400 cm/s located at a height of 20 m above the bottom. At the mid-shelf station BBL2 (57°37'N, 167°45'W; 69 m depth) occupied during 28 July-5 August 1982 in a Sverdrup wave regime with rotatory tidal currents, the fit to the thin boundary layers observed for M2 and K1 reveals a small apparent bottom roughness (0.001 cm), possibly due to calm weather and/or the lack of bedforms. The theory overestimates slightly the width of the M2 ellipses but predicts the K1 width and the perpendicular orientation of the M2 and K1 ellipses. The predicted eddy viscosity at BBL2 has a maximum of 250 cm/s at a height of 25 m. The theory provides estimates of residual tidal currents under very restrictive assumptions. For the coastal Kelvin waves propagating along the Alaska Peninsula, the residual tidal current (sum of 01, K1, N2 and M2) is due almost entirely to Stokes drift and produces a transport of 2(10) m/s toward Bristol Bay. For the Sverdrup waves in the mid-shelf regime, the magnitudes of the theoretical residual currents are a factor of 1/20 smaller than the coastal currents although bottom topography (not in the theory) can generate much stronger residual currents.




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