Bottom stress generation and sediment transport over the shelf and slope off of Lake Superior's Keweenaw peninsula

Abstract

Data from near-bottom instruments reveal that the mechanisms responsible for generating bottom stresses and resuspending sediment over the shelf and slope off of Lake Superior's Keweenaw peninsula exhibit distinct seasonal variations. Notably, near-bottom flows over the slope are persistently weak (<10 cm s−1) during summer but frequently attain high speeds, in excess of 20 cm s−1, in autumn and winter. During the intense storms of autumn and winter the generation of bottom stress is enhanced by the action of near-bottom orbital velocities due to surface waves. Even at 90-m depth, orbital velocities can increase bottom stress by a factor of up to 20% during storms. Where the seasonal thermocline intersects the lake floor, bottom stress is also considerably enhanced, often by more than a factor of 2, by high-frequency motions in the internal wave band. Over the Keweenaw slope, sediment resuspension is largely confined to autumn and winter episodes of high bottom stress. Our analysis indicates that this resuspended material tends to be carried offshore, a phenomenon that is partly due to the coincidence of the direction of the buoyancy-driven component of the Keweenaw Current with downwelling favorable alongshore winds. As a result of this coincidence, currents and bottom stresses tend to be greater during periods of downwelling, as opposed to upwelling, circulation. A potential challenge to modeling storm-driven resuspension in the study region is indicated by observations that the minimum stress required for resuspension may vary significantly with time over the autumn and winter.