Interbasin Transport of the Meridional Overturning Circulation

AbstractThe meridional overturning circulation (MOC) is studied in an idealized domain with 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 prescribed profiles. The only longitudinal asymmetry is that one basin is twice as wide as the other. Two states, a preferred one with sinking in the narrow basin and an asymmetrically forced one with sinking in the wide basin, are compared. 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 the same regardless of the location of sinking. The two basins exchange flow at their southern edge by a geostrophic transport balanced by the difference in the depth of isopycnals at the eastern boundaries of each basin. Gnanadesikan’s model for the upper branch of the MOC is extended to include two basins connected by a reentrant channel and is used to illustrate the basic properties of the flow: the layer containing the surface and intermediate water is shallower in the active basin than in the passive basin, and this difference geostrophically balances an exchange flow from the passive to the active basin. The exchange flow is larger when sinking occurs in the narrow basin. 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