Collaborative Research: The Role of Wind in Estuarine Dynamics

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Intellectual Merit: The physical workings of an estuary depend on the outcome of a contest between forces acting to stratify its waters, and forces acting to destroy this stratification. For six decades, the destructive force has been identified as the tides, which generate turbulence through ebb and flow over the rough bottom. In recent years, a new player has emerged: wind blowing over the estuary. Research has shown wind to be an effective mixer, even to the point of de-stratifying the water column. More recently, it has been found that this mixing takes place, not solely as a one-dimensional vertical process, but also through straining of the density structure in the longitudinal and lateral direction of the estuary. The driving hypothesis for this project is that, in addition to driving strong circulations along and across the estuary, it can also be a major contributor to the mixing providing the energy for the classical two-layer estuarine circulation, at least in estuaries with long fetches and weaker circulation. This project has been designed to investigate the role of wind in estuarine dynamics and to test these hypotheses. In order to examine this role, the response of the estuary to an applied wind stress must be described in three-dimensional detail, detail that has not been provided heretofore. In addition, this response must be clearly identified, separate from the underlying circulation of interest?the slower, steadier estuarine flow. Finally, this description must be scalable to the broad class of estuaries. Achieving this detail and understanding requires an extensive and intensive program of observation and analysis in combination with numerical models that have the ability to separate, dissect, integrate, and scale the multiple circulation and mixing components. The observational program in the Chesapeake Bay will involve an intensive array of instrumentation, the most novel of which is a high-resolution tower equipped with velocimeters and temperature-salinity recorders that will enable direct measurements of the stress profile. A dense array of buoys will be instrumented with meteorological sensors to measure local structure in the wind field. Finally, a scheme of multi-ship, towed-vehicle sampling is designed to provide detailed spatial pictures of the density structure with a sufficiently high repetition rate to resolve changes over a tidal cycle. A partnership with the meteorologists responsible for weather forecasting in the Chesapeake Bay region will aid both the observation and analysis of this complex, interactive system.Broader Impacts: The circulation and enclosed nature of estuaries make them highly productive fisheries, fisheries threatened by the effects of human alteration of the landscape. Estuaries worldwide are in various states of degradation, the chief cause of which is excess nutrients delivered from agricultural runoff and municipal sewage. These nutrients over-enrich the waters and lead to oxygen sags in the lower layers, sags that deprive living resources of the ability to use this valuable habitat. In estuaries such as Chesapeake Bay and Long Island Sound, these sags can proceed to hypoxia and even anoxia, the total depletion of dissolved oxygen. Costly programs are planned and underway to restore the health of our nation's estuaries. These management programs will need to rely on an accurate description of the physics of estuarine circulation if they are to be successful. In addition, the public will need to be informed of both the problem and the efforts toward solutions if they are to provide the political and economic support necessary. To that end, this research project will engage with education and outreach efforts, especially with the COSEE Coastal Trends program and with the Horn Point Laboratory Scientist-Educator Program, and will involve a scientist-educator, who will head a team of undergraduates to develop a teaching module related to the role of estuarine circulation (including wind mixing) in the Chesapeake Bay's Dead Zone. Finally, this proposal will provide training to two graduate students pursuing PhD degrees and a Postdoctoral fellow.