Short-Period Variations in a Great Lakes Coastal Current by Aerial Photogrammetry

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  • 1 Marine Studies Center, The University of Wisconsin, Madison 53706
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Abstract

Measurements of the surface velocity structure off the Keweenaw Peninsula of Lake Superior were obtained in 1971 and 1972, using aerial photography to track surface drift cards. Variations in the current structure are described at 9-min intervals, over a 45-min period of one experiment, using streamlines and isotachs extending across the entire coastal region. Speed contour irregularities and eddies of about 100 m diameter can be traced in some of the aerial sequences. Speed fluctuations of 25% of the mean flow occur frequently. The horizontal divergence and relative vorticity structure for each sequence is also calculated; magnitudes of each are up to three times that of the local Coriolis parameter. Both inshore and offshore countercurrents are observed.

The region of anticyclonic shear is typically twice as wide as the cyclonic shear region. Cross-stream velocity gradients are about three times larger than those measured in the Gulf Stream. Rossby numbers range from 0.5 to 0.8, and inertial accelerations appear to be larger than local accelerations at least 25% of the time. Horizontal eddy viscosity coefficients range from ±103 to ±105 cm2 s−1. Geostrophic calculations based on bathythermograph sections, airborne radiometer flights and meteorological data are also discussed.

Abstract

Measurements of the surface velocity structure off the Keweenaw Peninsula of Lake Superior were obtained in 1971 and 1972, using aerial photography to track surface drift cards. Variations in the current structure are described at 9-min intervals, over a 45-min period of one experiment, using streamlines and isotachs extending across the entire coastal region. Speed contour irregularities and eddies of about 100 m diameter can be traced in some of the aerial sequences. Speed fluctuations of 25% of the mean flow occur frequently. The horizontal divergence and relative vorticity structure for each sequence is also calculated; magnitudes of each are up to three times that of the local Coriolis parameter. Both inshore and offshore countercurrents are observed.

The region of anticyclonic shear is typically twice as wide as the cyclonic shear region. Cross-stream velocity gradients are about three times larger than those measured in the Gulf Stream. Rossby numbers range from 0.5 to 0.8, and inertial accelerations appear to be larger than local accelerations at least 25% of the time. Horizontal eddy viscosity coefficients range from ±103 to ±105 cm2 s−1. Geostrophic calculations based on bathythermograph sections, airborne radiometer flights and meteorological data are also discussed.

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