Lake Superior: A cold freshwater
Ocean
Mooring
Measurements from 1998-2001
Knut Aagaard (UW), Elise Ralph (UMN,
Duluth)
Corresponding author: Knut Aagaard (aagaard@apl.washington.edu)
OVERVIEW
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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