Observations of near-inertial energy in Lake Superior

Jay Austin

Limnol. Oceanogr., 58(2), 2013, 715-728 | DOI: 10.4319/lo.2013.58.2.0715

ABSTRACT: Directly measured velocity data collected in Lake Superior between 2008 and 2011 show that currents in the open waters of the lake are dominated by near-inertial energy. The near-inertial signal is composed almost entirely of clockwise rotation, with vertical structure dominated by the first baroclinic mode, where waters above and below the thermocline are roughly 180° out of phase with each other. The strength of the oscillations is strongly related to the strength of the stratification; in periods of the year when the water column is well-mixed (typically late autumn and late spring) the near-inertial signal is very weak; when stratification exists, near-inertial oscillations can occur. Combining the velocity amplitudes with an estimate of the thermocline displacement allows estimation of the dominant direction and horizontal wavelength of the near-inertial field, showing that horizontal wavelengths are on the order of 50–100 km, and the direction of the waves veers counter-clockwise over the course of the season with a period of ∼ 1 month. Observations of backscatter suggest that inertial oscillations may be responsible for re-suspension of bottom sediments, which could have significant ecological consequences.

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