A COLD FRESHWATER OCEAN
Aagaard (UW), Elise Ralph (UMN, Duluth)
author: Knut Aagaard (email@example.com)
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.
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.
alongshore component of the wind is also shown, based on the observed wind
at Passage Island, MI, at the eastern end of Isle Royal.
We gratefully acknowledge financial support
for this work from the National Science Foundation (NSF).
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.
the bottom panel the year-day is indicated by a dot every three days.
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