From small whirls to the global ocean: how ocean eddies affect the Atlantic Meridional Overturning Circulation.
In the North Atlantic, intense net downward motions connect the upper and lower branches of the Atlantic Meridional Overturning Circulation (AMOC). The classical view of an “ocean conveyor belt” with northward surface currents and southward return currents transporting convectively-formed waters from the subpolar North Atlantic Ocean to other ocean basins suggests a tight relation between convection and sinking. However, convection is a mixing process that hardly results in a net vertical mass transport. In fact, theory predicts that sinking in the ocean is limited to a narrow band along the perimeter of the basin, where ageostrophic processes play a role.
At first glance, this seems to imply that deep convection in the interior of marginal seas is irrelevant for the sinking process and in turn for the AMOC. This is obviously at odds with the fact that dense water masses formed in the marginal seas of the North Atlantic Ocean (Labrador Sea, Irminger Sea and Nordic Seas) can be traced in the deeper parts of the world’s oceans.
This apparent contradiction is addressed by considering the role of eddies shed from the fast boundary currents of the subpolar gyre, as they provide a possible connection between the convection regions and the boundaries of the basin. Based on a combination of simulations with highly versatile idealized ocean models (well-suited for extensive process studies) and a realistic global-scale ocean simulation, a complex three-dimensional view on sinking, water mass transformation and overturning in the North Atlantic Ocean is revealed, involving the boundary current, the ocean interior and interactions with the eddy field.