 # Coriolis–Stokes force

In fluid dynamics, the Coriolis–Stokes force is a forcing of the mean flow in a rotating fluid due to interaction of the Coriolis effect and wave-induced Stokes drift. This force acts on water independently of the wind stress.

This force is named after Gaspard-Gustave Coriolis and George Gabriel Stokes, two nineteenth-century scientists. Important initial studies into the effects of the Earth's rotation on the wave motion – and the resulting forcing effects on the mean ocean circulation – were done by Ursell & Deacon (1950), Hasselmann (1970) and Pollard (1970).

The Coriolis–Stokes forcing on the mean circulation in an Eulerian reference frame was first given by Hasselmann (1970):

$\rho {\boldsymbol {f}}\times {\boldsymbol {u}}_{S},$ to be added to the common Coriolis forcing $\rho {\boldsymbol {f}}\times {\boldsymbol {u}}.$ Here ${\boldsymbol {u}}$ is the mean flow velocity in an Eulerian reference frame and ${\boldsymbol {u}}_{S}$ is the Stokes drift velocity – provided both are horizontal velocities (perpendicular to ${\hat {\boldsymbol {z}}}$ ). Further $\rho$ is the fluid density, $\times$ is the cross product operator, ${\boldsymbol {f}}=f{\hat {\boldsymbol {z}}}$ where $f=2\Omega \sin \phi$ is the Coriolis parameter (with $\Omega$ the Earth's rotation angular speed and $\sin \phi$ the sine of the latitude) and ${\hat {\boldsymbol {z}}}$ is the unit vector in the vertical upward direction (opposing the Earth's gravity).

Since the Stokes drift velocity ${\boldsymbol {u}}_{S}$ is in the wave propagation direction, and ${\boldsymbol {f}}$ is in the vertical direction, the Coriolis–Stokes forcing is perpendicular to the wave propagation direction (i.e. in the direction parallel to the wave crests). In deep water the Stokes drift velocity is ${\boldsymbol {u}}_{S}={\boldsymbol {c}}\,(ka)^{2}\exp(2kz)$ with ${\boldsymbol {c}}$ the wave's phase velocity, $k$ the wavenumber, $a$ the wave amplitude and $z$ the vertical coordinate (positive in the upward direction opposing the gravitational acceleration).

## See also

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