Space Grant Consortium - University of Wisconsin - Green Bay

land stations over the lake [Schwab and Bedford, 1994; Hsu, 1986]. The concentration of
atmospheric CO2 above the lake was assumed equal to concentrations at the nearby WLEF tall
tower and was provided at monthly resolution by Ankur Desai [Desai et al., 2008].
Results
Lake Circulation. Few observations of large-scale circulation of Lake Superior exits. Most
current observations are restricted to the Keweenaw Current, which is not adequately captured at
such a coarse resolution. The mean circulation of Lake Superior is unknown, but the the gyres in
the modeled circulation agree with the previous modeling study of Lam [1978]. We expect year-
to-year variability in flow driven by changes in wind direction and speed, so it is interesting that
most of the modeled flows also agree with observed winter and summertime integrated flow
from one year during the late 1960s [Beletsky et al., 1999] (Figure 2).
Figure 2. Current observations from Beletsky et al., [1999] (left) during summer of 1967 (top) and winter of 1966-
1967 (bottom). Model column-integrated currents (right) during summer (top) and winter (bottom). Bold arrows on
model depict major flow directions in common between the model (2001) and observations (1967).
Lake Temperature. The physical model captures the seasonal cycle of lake surface temperature
reasonably well (Figure 3a). The seasonal cycle of temperature in the deepest portion of the lake
is 2 C too small in the model, and the seasonal cycle along the southern coast is 2 C too large.
Model surface temperatures are lower than satellite temperatures in the open lake during
summer, but the satellite detects skin temperature and the model predicts temperature for the top
5 meters. Phytoplankton need light and nutrients for growth. Stratification keep phytoplankton
from mixing out of the sunlight, and lake surface temperatures reach 4 C and begin stratifying at
the three NOAA buoys at the observed time (Figure 3b). Model mixed layer depths also agree
with EPA temperature profiles in August (not shown).
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