Ekman layers in the Southern Ocean: spectral models and observations, vertical viscosity and boundary layer depth

S Elipot,S T Gille

doi:10.5194/os-5-115-2009

Abstract

Abstract. 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. 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−1), with a no-slip bottom boundary condition. The second best model has a vertical viscosity with a surface value O(200 m2 s−1), which increases linearly with depth at a rate O(0.1–1 cm s−1) and a no-slip boundary condition at the base of the boundary layer of depth O(103 m). The best model shows little latitudinal or seasonal variability, and there is no obvious link to wind stress or climatological mixed-layer depth. In contrast, in the second best model, the linear coefficient and the boundary layer depth seem to covary with wind stress. The depth of the boundary layer for this model is found to be unphysically large at some latitudes and seasons, possibly a consequence of the inability of Ekman models to remove energy from the system by other means than shear-induced dissipation. However, the Ekman depth scale appears to scale like the climatological mixed-layer depth.

Highlights

The Southern Ocean is believed to be a primary location of surface ocean mixing as a result of wind energy input, and this is of relevance for the global oceanic circulation (Wunsch and Ferrari, 2004). Large et al (1997) stressed that observations of mixing processes from this region are needed to constrain general circulation models
This study focuses on mixing processes that occur in the oceanic boundary layer (OBL) and that are linked to the wind-forced input of momentum to the upper ocean
We found that the phase of the transfer function depended where · is the expected value operation over an ensemble linearly on frequency, which corresponds to a constant time of time series segments of length T and ·∗ is the complex lag between the wind stress and drifter data

Summary

Introduction

The Southern Ocean is believed to be a primary location of surface ocean mixing as a result of wind energy input, and this is of relevance for the global oceanic circulation (Wunsch and Ferrari, 2004). Large et al (1997) stressed that observations of mixing processes from this region are needed to constrain general circulation models. The Southern Ocean is believed to be a primary location of surface ocean mixing as a result of wind energy input, and this is of relevance for the global oceanic circulation (Wunsch and Ferrari, 2004). Large et al (1997) stressed that observations of mixing processes from this region are needed to constrain general circulation models. A number of recent studies have evaluated mixing processes in the Southern Ocean, both in the deep ocean We still lack observations of near-surface mixing on large scales. This study focuses on mixing processes that occur in the oceanic boundary layer (OBL) and that are linked to the wind-forced input of momentum to the upper ocean. Most of our understanding of the ocean’s response to wind forcing at the local scale has been framed in terms of Ekman (1905) theory, usually used to assess ocean response

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Conclusion