Transport by Intrusions Generated by Boundary Mixing

Iowa State University, Ames IA

Investigators

Abstract

OCE-0647253 Field observations over the last decade suggest that mixing near basin boundaries exceeds that in the interior by an order of magnitude or more and largely determines the basin-averaged mixing rate. In the ocean, boundary mixing has been attributed to critical reflection of internal waves on topography and internal waves generated by internal tides flowing over rough topography. In lakes, boundary mixing is due to critical reflection and turbulence generated by bottom currents from seiches. Although much recent work has focused on boundary mixing processes, there has been little work on the ultimate fate of the mixed fluid. In this study, field experiments will be conducted to determine how fluid mixed at the boundary of a stratified water body is transported to the interior. Experiments will be conducted in two lakes in Iowa: Ada Hayden Lake, near Iowa State University and West Okoboji Lake, which is larger and deeper. The objectives of the proposed work are to 1) classify the response of lakes to forcing according to the properties of the resulting internal wave field and mean currents, 2) identify intrusions and relate the turbulence and mixing to the forcing, and 3) to track the intrusions as they evolve. The lakes differ in size and one hypothesis that will be tested is that the magnitudes of potential energy change and energy supplied for mixing depend on the size of the lake but the ratio of these rates (called the mixing efficiency) will be larger on the sloping sides of the lake than along the flat bottom. Measurements of currents and temperature will be made and the spectra of internal waves will be calculated. Intrusions will be tracked with a fluorescent dye and microstructure measurements will be made throughout the dye patch. Broader impacts of the proposed work include the training of graduate and undergraduate students, outreach to a rural school near Iowa State University, and outreach based out of the Iowa Lakeside Laboratory. This will a new summer class on physical limnology at the Lakeside Laboratory as well as non-credit courses for local residents and K-12 schools. Graduate students in the PI's research group will both conduct the experiments for the proposed research and participate in the outreach activities. Requested equipment will enhance the infrastructure for research. The PI's research group has broadened participation of underrepresented groups and disseminated results to the public through its past activities. Results from this project will affect and connect oceanography, limnology, and engineering; in particular, the outreach will increase awareness of local and national water quality issues.