This project examined the physics of "near-inertial internal waves", which play an important role in the circulation and mixing patterns of Lake Michigan.  These waves induce strong currents in the surface waters at the center of large lakes, with a regular period of 17-18 hrs for Lake Michigan.  The project focused on producing a better understanding of how and when these waves are generated, and how they affect the transport of material across the lake and down into its interior from the lake surface.  A wide range of data were collected as part of the project.  These data included water velocity and temperature measurements from several locations in Lake Michigan, including the deepest part of Lake Michigan's southern basin (150m depth), which are rare data because of the difficult in obtaining data from this far offshore location.  Additionally, data were collected during a 5 day-long research cruise to the center of Lake Michigan's southern basin, during which GPS-tracked drifters and a large dye patch were released and subsequently tracked in order to quantify rates of material spreading and dispersion.  Additionally, turbulence was quantified during this research cruise by repeatedly lowering a turbulence profiler from a small boat.  In addition to these physical measurements, computer simulations were performed to simulate how these waves are generated by wind, and how their influence varies across the lake and through the seasons.

Analysis showed that these waves induce strong surface currents in the interior surface waters of Lake Michigan, as strong as 50 cm/s.  These currents were shown to persist for most of the year, from March through December, and disappear when the lake becomes thermally homogeneous during winter.  The lateral structure of these currents is mostly consistent, but the waves induce opposite motions above and below the thermocline - which separates warm surface waters from colder botom waters -  that is fairly well predicted by theory.  When these waves are created by wind bursts, they last for about one week.

The waves were analyzed in order to determine how they affect the movement of materials throughout the lake.  The analysis highlighted that these waves greatly enhance the lateral spreading of material near the lake surface, in the interior of the lake, for times immediately following the release of the substance.  This enhancement is due to the vertical shear associated with the wave-induced motions, which has stronger near-surface velocities that act to convey surface-trapped material faster than the water below, like a series of conveyer belts moving at different speeds ("shear flow dispersion").  After about 18 hours, the waves have diminishing effects on lateral transport.  A remarkable data set collected as part of the project tracked floating drifters for nearly 3 months, and highlighted not only the dispersion characteristics associated with near-inertial waves, but also lake-wide circulation patterns that have never before been seen.  

The effect of these waves on vertical mixing does not appear to be dramatic, disproving one of the primary hypotheses of the project.  While the turbulence data has not yet been fully analyzed, very little mixing was observed in the surface mixed layer and thermocline at the center of Lake Michigan, in spite of the large currents there associated with the near-inertial waves.  This is likely due to the influence of thermal stratification, which renders turbulent mixing less effective as water parcels tend to settle back to an equilibrium location that matches their temperature.  This data will continue to be analyzed in order to finally quantify these effects.

The project had important broader impacts for society.  Most directly, the project supported several graduate students, two of whom wrote Ph.D. dissertations on the project's scientific work.  Additionally, the project supported several undergraduate researchers, two of whom are now completing Ph.D. degrees in engineering (1 female).  Additionally, the project supported the laboratory development of the principal investigator, who was an early career scientist, by supporting equipment, support personnel, and the development of analysis techniques.  Importantly, as part of the project, a K-12 science educator workshop was held.  During this workshop, local science educators were taught concepts and ideas related to the physical, geological, and biological processes of Lake Michigan.  The educators were also shown how to utilize different data sets in their teaching, and worked to create lesson plans that incorporated their learning into classrooms.   

Last Modified: 09/02/2016
Modified by: Cary Troy