A Modified Sverdrup Model of the Atlantic and Caribbean Circulation

Abstract

An analytical model of the mean wind-driven circulation of the North Atlantic and Caribbean Sea is constructed based on linear dynamics and assumed existence of a level of no motion above all topography. The circulation around each island is calculated using the island rule, which is extended to describe an arbitrary length chain of overlapping islands. Frictional effects in the intervening straits are included by assuming a linear dependence on strait transport. Asymptotic expansions in the limit of strong and weak friction show that the transport streamfunction on an island boundary is dependent on wind stress over latitudes spanning the whole length of the island chain and spanning just immediately adjacent islands, respectively. The powerfulness of the method in enabling the wind stress bands, which determine a particular strait transport, to be readily identified, is demonstrated by a brief explanation of transport similarities and differences in earlier numerical models forced by various climatological wind stress products. In the absence of frictional effects outside western boundary layers, some weaker strait transports are in the wrong direction (e.g., Santaren Channel) and others are too large (e.g., Old Bahama Channel). Also, there is no western boundary current to the east of Abaco Island. Including frictional effects in the straits enables many of these discrepancies to be resolved. Sensitivity in strait transport to friction parameter is explored for the Caribbean island chain. Transport reversal in the minor passages around the Bahama Banks and Windward Passage as the friction parameter increased is noted. The separation latitude of the western boundary currents on Cuba's east coast moves southward as the friction parameter increases from zero, so making the Great Inagua Passage transport a better proxy for the Windward Passage transport. Major discrepancies with observations, namely, eastward instead of westward flow in Grenada Passage, a southward instead of northward Guyana Current, and hence a Caribbean circulation and Florida Current fed wholly by water masses of North Atlantic origin, cannot be resolved. However, they are simply overcome by extending the model to three layers with the wind-driven and upper limb of the thermohaline circulation confined to the top layer, and the lower limb of the thermohaline circulation to the bottom layer. If it is assumed that over the latitudes of the Caribbean there is no significant upwelling/downwelling between the layers, then the thermohaline-driven circulation is effectively a western boundary current, and all of the results for the analytical wind-driven-only model carry over, but with the value of the upper-layer transport streamfunction on the boundary of the American continent set to the magnitude of the thermohaline circulation rather than that on Africa. Exploration of strait transport sensitivity to friction parameter gives that realistic transports through the passages of the Windward Islands are only obtained if the friction coefficient in these passages is an order of magnitude larger than that in the western passages. Windward Passage transport reverses from south to north for a smaller value of the friction parameter than in the absence of the thermohaline circulation; Anegada and Mona Passages are robust inflow passages for the Caribbean Sea. South Atlantic water masses enter the Caribbean Sea through the passages from Grenada Passage to Martinique Passage. As the friction coefficient in the Windward Islands passages increases from zero, South Atlantic water mass is partially deflected northward along the outer arc of the islands and enters the Caribbean Sea through the passages up to Anegada Passage. The model suggests that for realistic friction parameters, South Atlantic water masses are unlikely to be found in the more western passages, or in the western boundary current skirting the edge of the Bahama Banks.