We quantify the volume transport and watermass transformation rates of the global ocean circulation using the Estimating the Circulation and Climate of the Ocean version 4 release 4 (ECCOv4r4) reanalysis product. Our results support large rates of intercell exchange between the mid-depth and abyssal cells, in agreement with modern theory and observations. However, the present-day circulation in ECCO cannot be interpreted as a near-equilibrium solution. A dominant portion of the apparent diapycnal transport of watermasses within the deep ocean is not associated with irreversible watermass transformation. Instead up- and down-welling is associated with isopycnal volume changes, reflecting trends in the deep ocean density structure. Our results reveal disagreement between ECCO’s representation of the overturning circulation and associated watermass transformations, and prevailing equilibrium theories of the overturning circulation.

We quantify the volume transport and watermass transformation rates of the global ocean circulation using data from the Estimating the Circulation and Climate of the Ocean version 4 release 3 (ECCOv4r3) reanalysis product. Our results support large rates of intercell exchange between the mid-depth and abyssal cells, in agreement with modern theory and observations. However, the present-day circulation in ECCO cannot be interpreted as a near-equilibrium solution. Instead, a dominant portion of the apparent diapycnal transport of watermasses within the deep ocean is associated with isopycnal volume change, rather than diabatic processes, reflecting trends in the deep ocean density structure. Our results imply two possibilities: either such trends in ECCOv4r3 are unrealistic, implying that ECCO’s representation of the overturning circulation and watermass transformations are inconsistent, or the trends in ECCOv4r3 are realistic and equilibrium theories of the overturning circulation cannot be applied to the present-day ocean.