Longshore pressure gradients caused by offshore wind

Observations of currents 12 km south of the Long Island coast show that strong offshore winds could generate considerable longshore nontidal flow well below any surface Ekman drift. A momentum balance calculation in the longshore direction shows a surface level gradient of order 10−6 to be the proximate cause of the longshore flow. A very simple model of the observed phenomena is a sloping plane beach acted upon by cross‐shore wind, varying sinusoidally in the longshore direction. With bottom friction parameterized by a linear law, a parabolic equation is found to govern steady state flow, expressing a balance of vorticity tendencies due to cross‐isobath flow, curl of bottom stress, and any forcing. Calculated solutions for variable cross‐shore wind show a trapped pressure field on the inner shelf which controls the transition between an essentially frictionless momentum balance on the outer shelf to frictionally dominated flow at the shore. Realistic estimates of the parameters entering the theory suggest that the longshore gradients associated with the trapped inner shelf field are of the correct order of magnitude to explain the generation of longshore flow by a system of cross‐shore winds.