Coastal Upwelling/Downwelling Cycles in Southern Lake Superior

Abstract

Extensive current meter, hydrographic and wind data were collected in the region of the Keweenaw Current, a strong coastal jet, during the late spring and early summer regime of Lake Superior. These data were compared with surface wind and pressure patterns over the region with the following conclusions: 1) Longshore winds associated with migrating low-pressure systems generate onshore Ekman transport and coastal downwelling. The coastal downwelling results in an intensification of the onshore internal pressure gradient, and consequently the coastal jet. Speeds of over 75 cm s−1 are both observed and calculated, suggesting the Keweenaw Current, at these times, is in geostrophic equilibrium. 2) Longshore winds associated with migrating high-pressure systems generate offshore Ekman transport and coastal upwelling. Upwelling velocities of 13-50 m day−1, an offshore surface transport layer of 10 m, a near-shore deceleration of the Keweenaw Current and current direction fluctuations of near- and sub-inertial period are found associated with the upwelling. The period of passage of highs (upwelling) and lows (downwelling) in July is about 4–6 days. This gives the Keweenaw Current the appearance of a pulsating coastal jet of period 4–6 days and amplitude about 60 cm s−1.