Warm-Route versus Cold-Route Interbasin Exchange in the Meridional Overturning Circulation

The interbasin exchange of the meridional overturning circulation (MOC) is studied in an idealized domain with two basins connected by a circumpolar channel in the southernmost region. Gnanadesikan's conceptual model for the upper branch of the MOC is extended to include two basins of different widths connected by a reentrant channel at the southern edge and separated by two continents of different meridional extents. Its analysis illustrates the basic processes of interbasin flow exchange either through the connection at the southern tip of the long continent (cold route) or through the connection at the southern tip of the short continent (warm route). A cold-route exchange occurs when the short continent is poleward of the latitude separating the subpolar and subtropical gyre in the Southern Hemisphere (the zero Ekman pumping line); otherwise, there is warm-route exchange. The predictions of the conceptual model are compared to primitive equation computations in a domain with the same idealized geometry forced by wind stress, surface temperature relaxation, and surface salinity flux. Visualizations of the horizontal structure of the upper branch of the MOC illustrate the cold and warm routes of interbasin exchange flows. Diagnostics of the primitive equation computations show that the warm-route exchange flow is responsible for a substantial salinification of the basin where sinking occurs. This salinification is larger when the interbasin exchange is via the warm route, and it is more pronounced when the warm-route exchange flows from the wide to the narrow basin.