Wind-driven pressure and flow around an island

Abstract

Observations on the south-western shelf of the island of Puerto Rico indicate that alongshore flows near the surface and on the shallow part of the insular shelf are primarily driven by the alongshore trade winds. The above flow has the tendency to reverse when the wind stress drops and observations of near-bottom currents on the deep outer shelf indicate flow counter to the wind stress. These observations indicate an alongshore pressure gradient that tends to oppose the wind stress. An analytical reduced-gravity model is presented describing the idealized wind-driven pressure and flow fields around a cylindrical island under low-frequency wind stress. The model shows that the offshore wind-driven flow around the island sets up pressure gradients in the same direction as observed at the study site. We calculate pressure gradients from a wind-driven reduced-gravity model of the offshore flow. Using these as a boundary condition for a barotropic model of wind-driven flow on the shallow shelf, we obtain steady-state pressure gradients on the insular shelf that are comparable to those found from observations. The model developed could also be used to study other phenomena. For example, in the limit that the coastal radius goes to zero (while the radius to the shelf edge is finite) the solutions for a sea-mount are obtained. The solutions suggest that pressure gradients over the sea-mount could be important in driving bottom flow in a direction opposite that of the wind. Also, the alongshore pressure gradients at the island shelf edge are independent of the parameters describing the island's size and may be useful in describing sea-surface set-up along generalized coastlines.