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.

Project Report

There is increasing knowledge that the presence of Langmuir circulation (LC) plays a fundamental roll in mixing near the ocean’s surface. LC usually manifests itself at the ocean surface as windrows, where surface convergence of water leads to long wind-aligned rows of seaweed, bubbles and other visible flotsam. These windrows result from the underlying circulation pattern that consists of pairs of counter-rotating vorticies. Despite the increasing acknowledgement that LC plays a fundamental role in surface mixed layer dynamics, there are relatively few detailed field measurements that fully characterize the turbulent mixing of LC. Of the field studies that provide high quality measurements of LC, none have been conducted in an estuarine environment. Given the presence of both strong stratification and strong tidal shears, estuarine environments are unlikely locations for LC to play an important roll in surface mixed layer process. However, one of the most striking findings of this project is that strong coherent circulations consistent with LC are commonly observed in Chesapeake Bay, and when present, dominate the mixing in the surface mixed layer. Measurements made through this project clearly demonstrate that under strong winds and waves, the observed low frequency (<1/20 Hz) vertical motions are characterized by: 1) strong coherence over most of the water column; 2) negative vertical velocity skewness indicative of strong/narrow downwelling and weak/broad upwelling; 3) strong negative correlations with the low frequency horizontal velocity in the direction of wave propagation. The orientation of the regions of surface convergence inferred from the observations are closely aligned with the dominant direction of wave propagation, which often deviates significantly from the wind direction. The inferred horizontal spacing between downwelling zones is generally consistent with the depth of the surface mixed layer (aspect ratio ~ 1), but shows considerable scatter and a lognormal distribution consistent with surface convergence that occurs randomly in both time and space. Tidal currents, vertical density stratification and the surface heat flux all modulate the intensity and coherence of the observed circulations. While strong tidal flows inhibit the development of LC, the surface heat flux can either inhibit or enhance the observed circulation depending on whether the heat flux is stabilizing or destabilizing. The circulations we observe appear to be highly variable in time and space, inconsistent with the traditionally assumed alongwind-uniform windrows. The LC observed in Chesapeake Bay is more analogous to coherent turbulence than the traditionally assumed 2-dimensional wind-aligned pair of counter rotating vorticies. Consistent with recent results from Large Eddy Simulations, our results suggeset that wave breaking seeds the flow with vertical vorticity that is tilted over by the stokes drift shear, initiating a coherent instability consistent with LC. The intensity of the observed circulation is strongly dependent upon the surface wave height, which is strongly related to both wave breaking and the stokes drift shear in fetch-limited environments like Chesapeake Bay. When LC is present, our observations suggest that it is the dominant mechanism for transporting momentum and other materials near the ocean surface. Direct measurements of the mixing in Chesapeake Bay suggests that eddies with physical scales of ~10m are responsible for most the turbulent flux. Because of the effectiveness of mixing by LC, the near surface velocity shear (the vertical gradient in velocity) is up to an order of magnitude smaller than expected based on traditional theory. Mixing by LC is not included in any of the numerical models that are used to simulate circulation in Chesapeake Bay and other coastal and estuarine systems, yet the results from this study suggest the surface mixing is dominated by its presence.

National Science Foundation (NSF)
Division of Ocean Sciences (OCE)
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Eric C. Itsweire
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Woods Hole Oceanographic Institution
Woods Hole
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