Ocean Science www.ocean-sci.net 1812-0784 1812-0792 5 2 2009 10.5194/os-5-115-2009 http://www.ocean-sci.net/5/115/2009/ http://www.ocean-sci.net/5/115/2009/os-5-115-2009.html http://www.ocean-sci.net/5/115/2009/os-5-115-2009.pdf 115 139 2009-05-14 Ekman layers in the Southern Ocean: spectral models and observations, vertical viscosity and boundary layer depth S. Elipot ship@pol.ac.uk S. T. Gille Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0230, USA Scripps Institution of Oceanography and Department of Mechanical and Aerospace Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0230, USA now at: Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool L3 5DA, UK Spectral characteristics of the oceanic boundary-layer response to wind stress forcing are assessed by comparing surface drifter observations from the Southern Ocean to a suite of idealized models that parameterize the vertical flux of horizontal momentum using a first-order turbulence closure scheme. The models vary in their representation of vertical viscosity and boundary conditions. Each is used to derive a theoretical transfer function for the spectral linear response of the ocean to wind stress. <br></br> The transfer functions are evaluated using observational data. The ageostrophic component of near-surface velocity is computed by subtracting altimeter-derived geostrophic velocities from observed drifter velocities (nominally drogued to represent motions at 15-m depth). Then the transfer function is computed to link these ageostrophic velocities to observed wind stresses. The traditional Ekman model, with infinite depth and constant vertical viscosity is among the worst of the models considered in this study. The model that most successfully describes the variability in the drifter data has a shallow layer of depth O(30–50 m), in which the viscosity is constant and O(100–1000 m² s^&minus;1), with a no-slip bottom boundary condition. The second best model has a vertical viscosity with a surface value O(200 m² s^&minus;1), which increases linearly with depth at a rate O(0.1–1 cm s^&minus;1) and a no-slip boundary condition at the base of the boundary layer of depth O(10³ m). 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