Size Matters: Another Reason Why the Atlantic is Saltier than the Pacific

The surface salinity in the North Atlantic relative to the North Pacific controls the position of the sinking branch of the meridional overturning circulation (MOC): the surface of the North Atlantic has higher salinity, so deep water formation occurs there rather than in the North Pacific. Here it is shown that in a 3-D primitive equation model of two basins connected by a re-entrant channel, there is a preference for sinking in the narrower of the two basins even under zonally-uniform surface forcing. This preference is linked to the details of the velocity and salinity fields in the "sinking" basin. The southward western boundary current of the subpolar gyre has higher transport in the wide basin than in the narrow basin. It overwhelms the northward western boundary current associated with the MOC for wide-basin sinking, so fresh water is brought from the very north of the domain southward and forms a pool on the western boundary in the wide basin. The fresh pool suppresses local convection and spreads eastward, leading to low salinities in the north of the wide basin for wide-basin sinking. This pool of fresh water is much less prominent in the narrow basin for narrow-basin sinking, where the northward MOC western boundary current overcomes the southward subpolar gyre western boundary current, bringing salty water from lower latitudes northward and enabling convection.