Studies of Bottom Ekman Layer Processes and Mid-Lake Upwelling in the Laurentian Great Lakes

Profiles of horizontal current velocity have been measured over long time intervals across bottom Ekman layers in Lake Michigan. Measurements at offshore locations in water depths greater than 100 m have disclosed current speeds exceeding 25 cm s"1 at 1 m above the lake floor during winter storms when the lake water is unstratified. Counterclockwise veering of the current velocity vector occurs as the bottom is approached, with the veering being most rapid close to the bottom. Measurements of currents during winter and spring have shown prevailing cyclonic circulation at all water depths. This is true of the other deep Great Lakes as well. Mass conservation calculations with the current meter data yield upwelling velocities on the order of 1 m day'1 in the center of the lake during winter. Water temperature data have been obtained from thermistor strings attached to meteorological buoys moored in the center of the lake by the National Data Buoy Center, Bay St. Louis, Mississippi. Simulations of the temperature data with four different one-dimensional mixed layer models were all premature in timing the onset of stratification. Additional simulations of the temperature data were made with a constant upwelling velocity imposed through late spring. Results of the new simulations were significantly improved over those neglecting upwelling. These results emphasize the importance of Ekman pumping in retarding the start of stratification in large lake basins.