The global meridional overturning circulation in an idealized two-basin model
This dissertation investigates the influence of ocean basin geometry on the stratification, circulation and salinity of the global ocean. This problem is studied using an idealized-geometry primitive-equation ocean-only model, in which two basins of different widths are connected by a re-entrant channel at the southern edge of the domain.
In addition to the primitive-equation model, we develop a conceptual model of the meridional overturning circulation (MOC). The conceptual model shows that if there is no deep-water formed in the wide basin, the northward Ekman transport into that basin must be returned southward by a geostrophic current. This geostrophic current flows southwards from the wide basin into the channel, and northwards from the channel into the narrow basin, balanced by the difference in the depth of isopycnals at the eastern boundaries of each basin. The predictions of the conceptual model are confirmed in our primitive-equation model.
Under zonally uniform wind stress, temperature relaxation and freshwater flux, the primitive-equation model shows a preference for sinking in the narrower basin (which represents the Atlantic). The velocity field above a mid-depth isopycnal is a superposition of the wind-driven gyres and the MOC's upper branch in a western boundary current. This velocity field transports salt northward more efficiently in a narrower basin, causing the preference for narrow-basin sinking.
Characterizing the flow in the upper branch of the MOC using a pseudostreamfunction, we divide the salt transport into two components: transport by flow along open streamlines (which represents the warm-route transport) and transport by the gyres. We find that for larger interbasin flow, the northward salt transport along open streamlines in the sinking basin increases. We conclude that increasing the warm-route transport would bring more salt into the north of the Atlantic basin.
In the same idealized geometry, we explore the connection between the mid-depth and abyssal cells of the MOC. A ROC budget is used to visualize the pathway of the residual circulation: 40% of deep water formed in the north of the narrow basin is recycled through the abyss before returning to the upper ocean of the narrow basin.