Western boundary current

A western boundary current is a narrow, intense return flow that closes a broad wind-driven interior circulation. The beta effect selects the western side for this frictional boundary layer.

The Stommel boundary layer closes a Sverdrup interior using linear drag. Balancing beta-effect advection against drag gives widthfor the normalization in which quasi-geostrophic drag is $-\gamma {\nabla }^2\psi$.

A Munk boundary layer closes a Sverdrup balance interior using lateral viscosity. At a zonal wall, balancing $\beta {\psi }_x$ with $\nu {\nabla }^4\psi$ gives the width $(\nu /\beta {)}^{1/3}$.