= Cross-wave Stokes drift
{title2=$\langle(\boldsymbol\xi_1\cdot\nabla)\mathbf u_2+(\boldsymbol\xi_2\cdot\nabla)\mathbf u_1\rangle$}
Two <plane internal gravity waves> can each have zero <Stokes drift> but give a nonzero drift through interference. The displacement of one wave need not be perpendicular to the other wave's <wave vector>. For upward waves with wave vectors $(\kappa,-\kappa)$ and $(-\kappa,-\kappa)$ and vertical amplitudes $W$, the cross-wave drift is $(0,-2\kappa W^2\cos(2\kappa x)/\omega)$ with $\omega=N/\sqrt2$. It has zero spatially averaged vertical flux. It cannot on its own satisfy an impermeable oscillating boundary: the second-order <Eulerian mean velocity> supplies the necessary normal correction there.
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