Interbasin transport of the meridional overturning circulation

The meridional overturning circulation (MOC) is important in setting high latitude oceanographic conditions in both the northern and southern hemispheres: in particular, it transports heat northward in the Atlantic. In this work, the MOC is studied in a domain of simplified geometry comprising two basins connected by a circumpolar channel in the southernmost region. Flow is forced at the surface by longitude-independent wind-stress, freshwater flux and fast temperature relaxation to a prescribed profile. The only asymmetry between the two basins is that one is twice as wide as the other. Two states are compared, one with sinking in the wide basin and one with sinking in the narrow basin. In both cases, sinking is compensated by upwelling everywhere else, including the passive basin. Despite the greater area of the wide basin, the residual overturning transport is found to be the same, regardless of the location of sinking. However, the interbasin transport is larger when sinking occurs in the narrow basin. The interbasin transport is geostrophically balanced, and maintained by a difference in the depth of the thermocline at the eastern boundaries of each basin. Gnanadesikan's (1999) model for the upper branch of the MOC is extended to include two basins connected by a re-entrant channel, and is used to illustrate the basic properties of the solution in this geometry: the layer containing the intermediate water is shallower in the active basin than it is in the passive basin, and this difference geostrophically balances an interbasin-exchange flow from the passive to the active basin. This exchange flow is larger when sinking occurs in the narrow basin, while the net residual overturning is approximately the same regardless of the sinking location. A visualization of the horizontal structure of the upper branch of the MOC shows that both the gyres and the meridional flow are important in determining the flow field in the upper layer.