Continuity Constraints on the Atlantic Meridional Overturning Circulation

In the subtropics, the Atlantic meridional overturning circulation (MOC) has the same strength and variability whether measured in depth- or density-space. Two different continuity budgets must therefore be satisfied north of the subtropics, one via diapycnal volume transport and the other via downward volume transport. However, as water can get denser without getting deeper (and vice versa), it is unclear why the integrated effect of these processes, the MOC, should have the same strength and variability in both depth- and density-space, provided one integrates these terms sufficiently far south (e.g. to 26 °N). Previous work has investigated the surface buoyancy forcing and mixing processes which drive diapycnal volume transport. Here, we use a suite of observational products and new analyses in a vorticity framework to study the magnitude and distribution of the various terms responsible for vertical volume transport, and gain further insight by also evaluating these terms using VIKING20X model output. We conclude that bottom Ekman transport and advection curl around the boundaries of the subpolar gyre, particularly around Greenland, are dominant drivers of downward vertical transport and hence crucial for closing MOC streamlines in depth-space, with much of the variability also projecting onto the MOC in density-space. As these processes are “spun-up” by the sub-polar gyre yet project onto the overturning, our results offer new insights into the coupling between the overturning and gyre circulations.