THE EFFECTS OF SURFACE GRAVITY WAVES ON AIR-SEA MOMENTUM TRANSFER AND VERTICAL MIXING IN A FETCH-LIMITED, ESTUARINE ENVIRONMENT
Fisher, Alexander William
Sanford, Lawrence P.
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Surface gravity waves are the principal pathway through which momentum and energy are transferred from the atmosphere to the ocean. Recent studies have contributed to a growing recognition that wind events can be of leading-order importance for mixing and circulation in estuaries, yet the specific nature of air-sea momentum transfer in coastal environments remains relatively understudied. As part of a collaborative investigation of wind-driven estuarine physics, this dissertation addresses the role that surface gravity waves play in the transfer of momentum from the air to the oceanic surface boundary layer in a fetch-limited, estuarine environment. Using a combination of direct field observations and numerical simulations, the role of surface gravity waves in structuring momentum transfer and vertical mixing were examined for a range of wind, wave, and stratification conditions. Results indicate that inclusion of surface gravity waves in bulk parameterizations of wind stress reduced bias to below 5% for nearly all observed wind speeds and that up to 20% of wind stress variability within Chesapeake Bay was directly attributable to surface wave variability. Furthermore, the 10-meter neutral drag coefficient was shown to vary spatially by more than a factor of two over the extent of Chesapeake Bay as a result of combined wind and wave variability. Anisotropic fetch-limitation resulted in dominant wind-waves that were commonly and persistently misaligned with local wind forcing. Direct observations of stress above and below the water surface demonstrated that, within the oceanic surface layer, stress was more aligned with wave forcing than wind forcing. Accounting for the surface wave field was needed to close the local momentum budget between the atmosphere and the mean flow. Directly observed turbulent profiles showed that breaking waves dominated the transfer of momentum and energy and resulted in a three-layer turbulent response consisting of a wave transport layer, surface log layer, and stratified bottom boundary layer. Comparisons to commonly employed second-moment turbulence closures suggest that the presence of breaking waves homogenized the surface layer to a greater extent than predicted by present parameterizations of turbulent kinetic energy transport away from a source at the surface.