Coastal Ocean Reynolds Stresses: Using New Methods with Long-Term Observations to Investigate Exchange and Evaluate Model Closures

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Over the continental shelf, turbulence plays a primary role in distributing momentum through the water column. In the inner shelf, where surface and bottom boundary layers overlap, the magnitude and vertical structure of turbulent momentum flux (Reynolds stress) is thought to have a controlling influence on the circulation and exchange. The vertical structure of Reynolds stress in the coastal ocean is not well known, due in part to the difficulty in making unbiased estimates of stress in wavy environments. Recently developed analysis techniques have documented ways to account for wave-induced biases, allowing direct observations of coastal ocean stresses using the velocity profiles of commonly-deployed acoustic Doppler current profilers (ADCPs). This work proposes to use these methods to estimate the stresses present in a more than 10 year-long record of velocity profiles from the Martha's Vineyard Coastal Observatory (MVCO). Estimated stresses will be utilized, along with velocity, hydrographic, and wind observations, to examine how inner-shelf processes interact to determine the across-shelf exchange due to along-shelf winds, across-shelf winds, and surface gravity waves and relate variations in momentum transfer due to these processes to variations in across-shelf exchange. These estimates will be compared to predictions using common turbulence closure models for similar forcing conditions in both simple one- and two-dimensional or complex three-dimensional numerical model simulations. Intellectual Merit: In situ observations of turbulent stress profiles, traditionally the least accessible term in the momentum equations, are central to our ability to explain and predict coastal ocean dynamics. Using the long-term dataset, this work will separate the incremental effects of wind or wave forcing on Reynolds stresses, examining the dependence of the across-shelf circulation on the stress. Comparing these observations to predictions from numerical models will provide a powerful new tool to evaluate closure methods, enabling more accurate predictions of how stress-causing processes interact to drive circulation when such measurements are not available. The primary data product of the analysis, stress profiles estimated from MVCO's long-term velocity dataset, will be archived and served on the Observatory's website, and therefore available as a community data product for future research. Broader Impacts: The mechanisms that control the across-shelf exchange of water masses and nutrients control the productivity of the coastal ocean as well as the dispersal or retention of planktonic larvae and pollutants. The additional knowledge of turbulent stresses and their use in evaluating model parameterizations offer, potentially, a more precise understanding of the dynamics of the inner shelf and across-shelf exchange then has previously been possible, allowing exchange-dependent processes to be studied and modeled with increased accuracy. Results will be disseminated directly into the classroom via teaching and ongoing outreach programs.