Impact of interbasin exchange on the Atlantic overturning circulation

The thermohaline exchange between the Atlantic and the Southern Ocean is an alyzed using a dataset based on WOCE hydrographic data. It is shown that th e salt and heat transports brought about by the South Atlantic subtropical gyre play an essential role in the Atlantic heat and salt budgets. It is fo und that on average the exported North Atlantic Deep Water (NADW) is freshe r than the return flows (basically composed of thermocline and intermediate water), indicating that the overturning circulation (OC) exports freshwate r from the Atlantic. The sensitivity of the OC to interbasin fluxes of heat and salt is studied in a 2D model, representing the Atlantic between 60 degrees N and 30 degree s S. The model is forced by mixed boundary conditions at the surface, and b y realistic fluxes of heat and salt at its 30 degrees S boundary. The model circulation turns out to be very sensitive to net buoyancy fluxes through the surface. Both net surface cooling and net surface saltening are sources of potential energy and impact positively on the circulation strength. The vertical distributions of the lateral fluxes tend to stabilize the stratif ication, and, as they extract potential energy from the system, tend to wea ken the flow. These results imply that a change in the composition of the N ADW return transports, whether by a change in the ratio thermocline/interme diate water, or by a change in their thermohaline characteristics, might in fluence the Atlantic OC considerably. It is also shown that the circulation is much more sensitive to changes in the shape of the lateral buoyancy nux than to changes in the shape of the s urface buoyancy flux, as the latter does not explicitly impact on the poten tial energy of the system. It is concluded that interocean fluxes of heal a nd salt are important for the strength and operation of the Atlantic thermo haline circulation, and should be correctly represented in models that are used for climate sensitivity studies.