Observations and modeling of nearshore-offshore exchange processes in Lake Superior

Abstract

Lake Superior's nearshore areas, including embayments, are warmer and more productive than the colder central lake basins. They are important habitat for a variety of species, and are also where human activities are concentrated. The distinction between nearshore and offshore areas is maintained by the lake's mean circulation, which is primarily shore-parallel in the nearshore. Exchange between nearshore and offshore areas occurs where and when this general pattern is disrupted, and affects important nearshore water quality parameters including concentrations of nutrients, pollutants and heat. This dissertation reports the results of three studies focused on Lake Superior nearshore exchange processes conducted with satellite data and numerical modeling. In the first study, satellite Synthetic Aperture Radar (SAR) images of Lake Superior were examined, and eddies were identified and characterized. Eddies potentially contribute to exchange between nearshore and offshore areas of the lake because they typically form in turbulent flows, and long-lived eddies directly transport water from place to place. Forty one eddies were identified and basic statistics including their location, size, sense of rotation and distance to shore were compiled. The SAR images are of limited spatial extent, and each image only included a portion of the entire lake surface. The images reviewed covered the ice free months of 1992-1998. Comparison with satellite images of lake surface temperature showed the eddies were located within the region of high thermal gradient that characterizes the summer coastal circulation. The second study was a sensitivity analysis conducted with a numerical model of lake hydrodynamics. Idealized model domains of varying bottom slopes characteristic of Lake Superior were used to simulate the expansion of stratification from nearshore areas where it first occurs to the entire lake basin. The expansion was quantified by the advance of the 4 °C temperature front, known as the thermal bar, from the perimeter of the idealized basins to the center. The thermal bar is known to inhibit exchange between nearshore and offshore areas. Forcing for the simulations was varied systematically to predict the time scales over which thermal bars are expected in Lake Superior. The third study used a realistically configured model and virtual tracers to characterize exchange between areas where water depth is less than 100 m and deeper parts of the lake. Two series of model simulations were conducted. The first covered the ten year period 2003-2012 and utilized an age tracer that showed the rate of exchange varied annually corresponding to the lake's cycle of vertical mixing and stratification. A second series of simulations covered the summer of 2004 and used passive tracers that showed offshore dispersal of nearshore water increased in late summer with the offshore expansion of stratification. The model resolved eddies at locations that included those reported in the first study and provided additional details about their contribution to exchange between the nearshore and offshore areas of the lake. Appendix 1 is a report of observations of the thermal bar from the south shore of Lake Superior in the spring of 2012. Appendix 2 is a summary of procedures for creating a nested grid configuration in the Regional Ocean Modeling System (ROMS). Supplemental movie 1 illustrates the seasonal cycle of nearshore to offshore exchange using the Age tracer of the third study described above. Supplemental movie 2 shows eddy locations and surface temperature anomalies in the simulations of the third study described above.