Poincaré wave-induced shear instability in Lake Michigan

Cross-thermocline mixing is an important process during the stratified period in the Great Lakes, especially in the lake interiors. Recent evidence suggests that basin-scale mixing in the world's largest lakes may actually occur primarily in the lake interiors. Several years of velocity and temperature data are examined from Lake Michigan in order to examine processes that are potentially responsible for cross-thermocline mixing. Near-inertial internal Poincaré waves are shown to thoroughly dominate surface currents and thermocline shear in Lake Michigan's deeper waters. Empirical orthogonal function decomposition of the observed velocity fields show that a vertical mode 1 Poincaré wave is dominant and responsible for most of the observed thermocline shear. Calculated Richardson numbers, observed microstructure, and stability analysis suggest that these waves do cause cross-thermocline mixing, and that high-resolution epxperiments are necessary to better quantify mixing in the interior of Lake Michigan.