For a pure wave motion in fluid dynamics, the **Stokes drift velocity** is the average velocity when following a specific fluid parcel as it travels with the fluid flow. For instance, a particle floating at the free surface of water waves, experiences a net Stokes drift velocity in the direction of wave propagation.

More generally, the Stokes drift velocity is the difference between the average Lagrangian flow velocity of a fluid parcel, and the average Eulerian flow velocity of the fluid at a fixed position. This nonlinear phenomenon is named after George Gabriel Stokes, who derived expressions for this drift in his 1847 study of water waves.

The **Stokes drift** is the difference in end positions, after a predefined amount of time (usually one wave period), as derived from a description in the Lagrangian and Eulerian coordinates. The end position in the Lagrangian description is obtained by following a specific fluid parcel during the time interval. The corresponding end position in the Eulerian description is obtained by integrating the flow velocity at a fixed position—equal to the initial position in the Lagrangian description—during the same time interval.

The Stokes drift velocity equals the Stokes drift divided by the considered time interval. Often, the Stokes drift velocity is loosely referred to as Stokes drift. Stokes drift may occur in all instances of oscillatory flow which are inhomogeneous in space. For instance in water waves, tides and atmospheric waves.

In the Lagrangian description, fluid parcels may drift far from their initial positions. As a result, the unambiguous definition of an average Lagrangian velocity and Stokes drift velocity, which can be attributed to a certain fixed position, is by no means a trivial task. However, such an unambiguous description is provided by the *Generalized Lagrangian Mean* (GLM) theory of Andrews and McIntyre in 1978.^{[2]}

The Stokes drift is important for the mass transfer of all kind of materials and organisms by oscillatory flows. Further the Stokes drift is important for the generation of Langmuir circulations.^{[3]}
For nonlinear and periodic water waves, accurate results on the Stokes drift have been computed and tabulated.^{[4]}

The Lagrangian motion of a fluid parcel with position vector *x = ξ(α,t)* in the Eulerian coordinates is given by:

where *∂ ξ / ∂t* is the partial derivative of

is the Lagrangian position vector of a fluid parcel,**ξ**(**α**,t)is the Eulerian velocity,**u**(**x**,t)is the position vector in the Eulerian coordinate system,**x**is the position vector in the Lagrangian coordinate system,**α***t*is the time.

Often, the Lagrangian coordinates * α* are chosen to coincide with the Eulerian coordinates

But also other ways of labeling the fluid parcels are possible.

If the average value of a quantity is denoted by an overbar, then the average Eulerian velocity vector * ū_{E}* and average Lagrangian velocity vector

Different definitions of the average may be used, depending on the subject of study, see ergodic theory:

- time average,
- space average,
- ensemble average and
- phase average.

The Stokes drift velocity * ū_{S}* is defined as the difference between the average Eulerian velocity and the average Lagrangian velocity:

In many situations, the mapping of average quantities from some Eulerian position * x* to a corresponding Lagrangian position

For the Eulerian velocity as a monochromatic wave of any nature in a continuous medium: one readily obtains by the perturbation theory – with as a small parameter – for the particle position

Here the last term describes the Stokes drift velocity ^{[7]}

See also: Airy wave theory and Stokes wave |

The Stokes drift was formulated for water waves by George Gabriel Stokes in 1847. For simplicity, the case of infinite-deep water is considered, with linear wave propagation of a sinusoidal wave on the free surface of a fluid layer:^{[8]}

where

*η*is the elevation of the free surface in the*z*-direction (meters),*a*is the wave amplitude (meters),*k*is the wave number:*k = 2π / λ*(radians per meter),*ω*is the angular frequency:*ω = 2π / T*(radians per second),*x*is the horizontal coordinate and the wave propagation direction (meters),*z*is the vertical coordinate, with the positive*z*direction pointing out of the fluid layer (meters),*λ*is the wave length (meters), and*T*is the wave period (seconds).

As derived below, the horizontal component *ū _{S}*(

As can be seen, the Stokes drift velocity *ū _{S}* is a nonlinear quantity in terms of the wave amplitude

It is assumed that the waves are of infinitesimal amplitude and the free surface oscillates around the mean level *z = 0*. The waves propagate under the action of gravity, with a constant acceleration vector by gravity (pointing downward in the negative *z*-direction). Further the fluid is assumed to be inviscid^{[10]} and incompressible, with a constant mass density. The fluid flow is irrotational. At infinite depth, the fluid is taken to be at rest.

Now the flow may be represented by a velocity potential *φ*, satisfying the Laplace equation and^{[8]}

In order to have non-trivial solutions for this eigenvalue problem, the wave length and wave period may not be chosen arbitrarily, but must satisfy the deep-water dispersion relation:^{[11]}

with *g* the acceleration by gravity in (*m / s ^{2}*). Within the framework of linear theory, the horizontal and vertical components,

The horizontal component *ū _{S}* of the Stokes drift velocity is estimated by using a Taylor expansion around