The global overturning circulation and the importance of non-equilibrium
effects in ECCOv4r3
Abstract
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.