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Henry Stommel explained a mechanism for the western intensification of wind-driven ocean currents. His 1948 basin model combined wind stress, a latitude-dependent Coriolis parameter and linear bottom friction; the resulting circulation concentrated streamlines on the western side.

The result connects a global transport balance with a narrow boundary current. Friction is essential to closing the circulation even when it is weak in much of the interior, illustrating why a small local effect can control the structure of an entire flow.^[ref]

An ocean current is organized water motion that transports mass, heat, salt and other properties through the ocean. Surface winds, pressure gradients, buoyancy changes, Earth's rotation and friction all contribute, with their relative importance depending on scale and depth.

A steady large-scale interior flow can approximately balance a horizontal pressure gradient against the Coriolis force, while boundary currents require additional dynamics. Henry Stommel showed that the latitude dependence of rotation, combined with friction in a wind-driven basin, can concentrate the return flow on the western side. The circulation is therefore not simply water moving in the direction of the local wind.^[ref]

The Stommel boundary layer is a frictional western boundary current that returns the transport carried by a broad wind-driven Sverdrup balance interior. In a flat-bottomed, linear beta plane model, use streamfunction $\psi$ with $(u,v)=(-{\psi }_y,{\psi }_x)$ and dimensional linear drag acceleration $-r\mathbf{u}$, where $r>0$ has units of inverse time. The steady vorticity equation iswhere $\beta =df/dy>0$ and $F$ is the wind-stress-curl forcing per unit mass and depth.

For a narrow boundary correction near the western wall $x=0$, the leading balance isIts decaying solution is ${\psi }_{\mathrm{b}}=C(y)e^{-x/{\delta }_S}$, withThe dimensions are length because $[r]=T^{-1}$ and $[\beta ]=L^{-1}{T}^{-1}$. If a dimensionless drag parameter $\gamma$ is defined by $r=\gamma f_0$ for positive reference Coriolis parameter $f_0$, the same width is $\gamma f_0/\beta$. For a dimensional drag term $-\gamma {\nabla }^2\psi$ in the vorticity equation, the width instead reads $\gamma /\beta$.

The exponentially decaying correction can enforce the western no-normal-flow boundary condition while leaving the interior Sverdrup balance approximately unchanged. The underlying linear wind-driven model and the role of latitude-dependent rotation were established by Henry Stommel. ^[ref]