Acoustically and visually tracked drogue measurements of nearsurface water velocities in Lake Huron, plus observations of a coastal upwelling

Abstract

During July and August of 1980 our research group measured nearsurface water velocities near the eastern coast of Lake Huron by tracking drogues using acoustic travel time and compass sighting techniques. The velocity fields appeared to consist of two components. These have been termed: a sub-current, which varied slowly with depth (compared to the deepest drogue depth of 5.2 m) and, in most cases, was apparently in geostrophic balance with the cross shore pressure gradient; and, a surface layer-current (defined by the relative velocity from deeper to shallower drogues) which decayed rapidly with depth and was directed nearly parallel with the wind and waves. There was no discernable relationship between wind speed and relative velocity. There was, however, a direct dependence of relative velocity with estimated surface roughness, suggesting that Stokes drift may have been primarily responsible for the shear. The magnitudes of the observed relative velocities were approximately equal to Stokes drift magnitudes calculated from representative wave energy spectra. Also reported are measurements of current and temperature structure made prior to and following a coastal upwelling.