Knut Aagaard (UW), Elise Ralph (UMN, Duluth)

Corresponding author: Knut Aagaard (


Superior is the world's largest lake, and its low mean annual temperature is near that of the maximum density for freshwater.  Of particular interest are the combined effects of wind and thermal forcing in a freshwater regime sufficiently large that rotational effects are also dynamically important.  As part of an interdisciplinary program to quantify the role of coastal currents and thermal fronts in mediating cross-margin transport in Lake Superior, we are therefore conducting a three-year process- and rate-oriented field study near the Keweenaw Penninsula, where the currents are exceptionally strong and large responses to forcing might be expected.  Nine instrumented moorings are being deployed each year, and these time series are supplemented by ship-borne hydrographic and ADCP sections. We find evidence that the annual cycle in wind forcing results in major changes in the longshore mode of circulation, and that the annual cycle of thermal forcing drives a vertical overturning cell that switches direction seasonally.  We find little, if any, evidence of significant turbulent cross-frontal fluxes.

Record-length mean velocity for each instrument moored within the Keweenaw Current, together with the regional bathymetry.  The current records variously extend from 1-3 years.

Temperature and alongshore (u, positive nominally eastward) and offshore (v) current speed measured at mooring sites E1, E2, and E4  during 1998-2001 (cf., Figure 1 for mooring locations).  The offshore component is defined by being zero averaged over the record.

Monthly mean alongshore current speed composited for the instruments on moorings O1 and O2, H1 and H2, and E1 and E2.  The alongshore component of the wind is also shown, based on the observed wind at Passage Island, MI, at the eastern end of Isle Royal.

Monthly mean velocity components (alongshore and offshore) composited for the three years 1998-2001 and for the various sections.  The monthly means from the one-year record at E4 are shown separately.

Cross-shore displacement vs. time in 1999 year-days (top panel) and cross-shore displacement vs. alongshore displacement (bottom panel) during 1999-2000 at nominally 20 m depth at moorings H1 and H2.  In the bottom panel the year-day is indicated by a dot every three days.


We gratefully acknowledge financial support for this work from the National Science Foundation (NSF).

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