"Warm-route versus cold-route interbasin exchange in the meridional overturning circulation: why is the Atlantic saltier than the Pacific"
Among the processes attributed to the higher salinity of the Atlantic Ocean relative to the Pacific Ocean, several are associated with the Atlantic sinking of the meridional overturning circulation (MOC), and the absence of an equivalent overturning in the Pacific. Ocean-only general circulation computations in an idealized domain with two basins connected by a circumpolar channel in the southernmost region focus on two important asymmetries preferring the Atlantic as the site for sinking: its narrower width and a connection with the Indo-Pacific at a subtropical latitude (the tip of South Africa) and at a subpolar latitude (the tip of South America). These computations, together with a simple conceptual model for the upper branch of the MOC illustrate the basic processes of interbasin exchange either through the connection at the subpolar latitude of the long continent (``cold route'') or through the connection at the subtropical latitude of the short continent (``warm route''). A cold-route exchange occurs when the short continent is poleward of the latitude separating the sub-polar and sub-tropical gyre (the zero wind-stress curl line) in the southern hemisphere, 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. A visualization of the horizontal structure of the upper branch of the MOC illustrates 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. [Background Reading]