Temperature Fine-Structure of Lake Michigan Hypolimnion

High-resolution temperature profiles from the hypolimnion of Lake Michigan in June and August 1978 showed the frequent occurrence of intrusive layers of thickness 10 m or less. Anomalously warm layers$(\DeltaT \lesssim 0.1^\circ C)$seemed to extend horizontally for$\gtrsim 0.5 km$and to persist at one location for$\thickapprox 0.5$day. Since temperatures were above maximum density values, these layers would have been hydrostatically unstable were it not for observed stabilizing gradients of dissolved silica. Interleaving of the layers, shear-induced turbulence, and stirring at the bottom resulted in various small-scale temperature structures. After an intense coastal upwelling event, a bottom layer 5-m thick was observed along the 100-m isobath in which temperatures and silica concentrations were nearly uniform and in which a value of at least 0.1 cm2· s-1 was calculated for the vertical eddy diffusivity.