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

[摘要] Spectral characteristics of the oceanicboundary-layer response to wind stress forcing are assessed bycomparing surface drifter observations from the Southern Ocean to asuite of idealized models that parameterize the vertical flux ofhorizontal momentum using a first-order turbulence closure scheme. Themodels vary in their representation of vertical viscosity and boundaryconditions. Each is used to derive a theoretical transfer functionfor 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 subtractingaltimeter-derived geostrophic velocities from observed drifter velocities (nominallydrogued to represent motions at 15-m depth). Then the transfer function iscomputed to link these ageostrophic velocities to observed wind stresses.The traditional Ekman model, with infinite depth and constant verticalviscosity is among the worst of the models considered in this study.The model that most successfully describes the variability in the drifterdata has a shallow layer of depth O(30–50 m), in which theviscosity is constant and O(100–1000 m² s⁻¹), with a no-slip bottom boundarycondition.The second best model has a vertical viscosity with a surface valueO(200 m² s⁻¹), which increaseslinearly with depth at a rate O(0.1–1 cm s⁻¹) and a no-slipboundary condition at the base of the boundary layer of depth O(10³ m).The best model shows little latitudinal orseasonal variability, and there is no obvious link to wind stress orclimatological mixed-layer depth. In contrast, in the second best model,the linear coefficient and the boundarylayer depth seem to covary with wind stress.The depth of the boundary layer for this model is found to be unphysically largeat some latitudes and seasons, possiblya consequence of the inability of Ekman models to removeenergy from the system by other means than shear-induced dissipation.However, the Ekman depth scale appears to scale like theclimatological mixed-layer depth.

[学科分类] 海洋学与技术