Islands in Zonal Flow*

The impact of a meridional gradient in sea surface temperature (warm toward the equator, cold toward the pole) on the circulation around an island is investigated. The upper-ocean eastward geostrophic flow that balances such a meridional gradient is blocked where the isotherms intersect boundaries. In the case in which the boundaries represent either the eastern or western side of a planetary-scale island, circulation integrals around the island show that some of this eastward transport will flow around both the equatorward and poleward tips of the island. There is also a net downwelling along the western side of the island and a net upwelling along the eastern side of the island. An analytic model of the eastern and western boundary currents is used together with a circulation integral to estimate the fraction of the eastward transport that flows around the equatorward and poleward tips of the island and the net upwelling/downwelling on either side of the island. Calculations with a primitive equation numerical model are in close agreement with the theory. A simple closed-form analytic solution for the transport around the island tips is derived in the limit of strong buoyancy forcing. It is found that, over a wide range of parameter space, a significant fraction of the eastward transport in the upper ocean circulates around the tips of the island from the western basin into the eastern basin.