A year of internal Poincaré waves in southern Lake Michigan

A unique set of full year, deep water observations from the middle of Lake Michigan's southern basin are analyzed to quantify the seasonal variability of the dominant near‐inertial internal Poincaré wave. At this mid‐lake location, the Poincaré wave is seen to describe more than 80% of the observed surface current variability for much of the year, with characteristic near‐inertial frequency and clockwise‐rotating velocities. The dominance of the near‐inertial seiche on the flow decreases with depth. The wave persists during the “stratified period,” roughly May through late December, and is supported by as few as 1–2 degrees of thermal stratification over 150 m; only after complete water column mixing does the wave go dormant for January through April. The strongest Poincaré wave activity is seen to correspond to the period of strongest summer thermal stratification (August), in spite of the relatively weak winds at this time. A simple inertial slab model optimized with linear friction is shown to capture the seasonal variability of the near‐inertial energy at this location reasonably well. The vertical structure of the wave shows good agreement with that calculated with a standard normal modes formulation, which is in turn used to characterize the potential shear and mixing caused by the wave. Late‐spring and summer events of elevated Poincaré wave activity are shown to generate sufficiently strong shear with persistent periods of sub‐1 Richardson numbers within the thermocline, suggesting that the near‐inertial seiche is likely generating thermocline instabilities in the lake's interior. Key Points Near‐inertial waves dominate summer offshore currents in Lake Michigan A simple slab model re‐creates the seasonal variation of near‐inertial energy The cross‐thermocline shear from near‐inertial waves can be generate mixing