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Oceanographic Factors
Published in Ronald C. Chaney, Marine Geology and Geotechnology of the South China Sea and Taiwan Strait, 2020
Under ideal conditions, the Ekman spiral results in a net transport (Ekman transport) of wind-driven water in the affected water column (Ekman Layer), which is 90° to the right from the wind direction in the northern hemisphere and to the left in the southern hemisphere. In shallow water it will be somewhat less than 90° because of the restricting influence of the ocean bottom.
Surface currents in operational oceanography: Key applications, mechanisms, and methods
Published in Journal of Operational Oceanography, 2023
Johannes Röhrs, Graig Sutherland, Gus Jeans, Michael Bedington, Ann Kristin Sperrevik, Knut-Frode Dagestad, Yvonne Gusdal, Cecilie Mauritzen, Andrew Dale, Joseph H. LaCasce
A classical view of the wind-driven current are steady-state solutions of the ocean surface boundary layer (OSBL). In Ekman's seminal solution, there is a balance between the vertical shear stress and the Coriolis force in the boundary layer, with wind stress providing the upper boundary condition (Ekman 1905). In Ekman's solution, the (ageostrophic) surface current is 45 to the right of the wind in the northern hemisphere, and the velocities decay with depth, turning to the right in an ‘Ekman spiral’ (indicated in the lower portion of Figure 1). Allowing for more realistic vertical mixing which varies with depth and stratification, alters the deflection and the decay, but not the qualitative picture (Lentz 2001).
Nonlinear wind-drift ocean currents in arctic regions
Published in Geophysical & Astrophysical Fluid Dynamics, 2022
The presented Lagrangian analysis provides an explicit solution to the nonlinear governing equations for the leading-order dynamics of arctic wind-drift flows in deep regions outside the Amundsen Basin (so that we may rely on the f-plane approximation, avoiding the North Pole, where the meridians converge). The nonlinear solution consists of inertial oscillations superimposed on a mean wind-drift current (the classical Ekman spiral). The revealed structure of the solution enables us to study in some detail the generally very intricate coupling between the surface wind, the sea-ice motion and the surface current.
The Ekman spiral for piecewise-constant eddy viscosity
Published in Applicable Analysis, 2022
As already mentioned, at non-equatorial latitudes the oceanic flow below a depth of about 100 metres or so is, to a good degree of approximation, in geostrophic balance. However, closer to the surface, wind-generated turbulence in the water carries the momentum of the wind down to the interior, thus inducing currents (superimposed to the underlying geostrophic flow) where the force balance is between the Coriolis force and the frictional forces due to the wind. The layer where these frictional effects are significant is called the Ekman layer and is named after the Swedish scientist who first formulated and analysed a mathematical model describing the behaviour of wind-generated steady surface currents. In his pioneering work [8], published in 1905, Ekman modelled the frictional effects by introducing an (in general depth-dependent) eddy viscosity coefficient (see the next section); in the simplest setting of constant eddy viscosity, he came to the following conclusions: The induced surface current is deflected to the right of the direction of the generating wind by an angle of 45 in the Northern Hemisphere (to the left in the Southern Hemisphere).With increasing depth, the velocity of the current decays in magnitude and is further deflected in clockwise (anticlockwise) direction in the Northern (Southern) Hemisphere, following a spiral (now called an Ekman spiral).The net mass transport of the current is directed at right angle to the right (left) of the direction of the wind in the Northern (Southern) Hemisphere.