Impact of Interbasin Exchange on the Atlantic Overturning Circulation

Abstract

The thermohaline exchange between the Atlantic and the Southern Ocean is analyzed, using a data set based on WOCE hydrographic data. It is shown that the 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 found that on average the exported North Atlantic Deep Water (NADW) is fresher than the return flows (basically composed of thermocline and intermediate water), indicating that the overturning circulation (OC) exports freshwater 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 • N and 30 • S. The model is forced by mixed boundary conditions at the surface, and by realistic fluxes of heat and salt at its 30 • 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 stratification, and, as they extract potential energy from the system, tend to weaken the flow. These results imply that a change in the composition of the NADW return transports, whether by a change in the ratio thermocline/intermediate water, or by a change in their thermohaline characteristics, might influence the Atlantic OC considerably.It is also shown that the circulation is much more sensitive to changes in the shape of the lateral buoyancy flux than to changes in the shape of the surface buoyancy flux, as the latter does not explicitly impact on the potential energy of the system. It is concluded that interocean fluxes of heat and salt are important for the strength and operation of the Atlantic thermohaline circulation, and should be correctly represented in models that are used for climate sensitivity studies.

On the stability of the Atlantic meridional overturning circulation

“…Based on observational data, Weijer (38) found that the real AMOC exports freshwater from the Atlantic across 30°S (at a rate of Ϸ0.3 Sv); this indicates it may be bistable. An analysis of nine climate models in contrast found that all but one import freshwater via the AMOC (Weber et al in ref.…”

Three decades of research on the greater Agulhas Current

“…The stability of the northern Agulhas Current has subsequently been studied and explained as being a consequence of a very steep continental slope (De Ruijter et al, 1999). The only place along its path where this does not hold is at the Natal Bight, a wider part of the shelf in a coastal offset north of Durban (see Fig.…”

“…Based on a palaeo-climatological investigation it has even recently been demonstrated (Peeters et al, 2004) that this inter-ocean leakage has been particularly strong at the end of each of the last five glacial periods, but that it has never been absent entirely (Rau et al, 2002). Modelling studies by Weijer et al (1999Weijer et al ( , 2001Weijer et al ( , 2002 have in addition shown that a diminution in the exchange at the Agulhas retroflection leads inexorably to a slowing of the Atlantic Ocean's thermohaline overturning. The importance of this process of ring shedding at the Agulhas retroflection has thus been demonstrated during the past decade as have the mechanisms that control it.…”

On the Physics of the Agulhas Current: Steady Retroflection Regimes

“…For example, in the POCM model (Stammer et al, 1994), the Agulhas has a realistic transport of 66 Sv and it indeed retroflects near 20 • E (De Ruijter et al, 1999). In these models, several other factors contribute to the retroflection, in particular vortex stretching due to the deformation of isopycnals .…”