We quantify the volume transport and watermass transformation rates of the global overturning circulation using the Estimating the Circulation and Climate of the Ocean version 4 release 4 (ECCOv4r4) reanalysis product. The ECCO solution shows large rates of intercell exchange between the mid-depth and abyssal cells, consistent with other recent inferences. About 10 Sv of North Atlantic deep water enters the abyssal cell in the Southern Ocean and is balanced by a similar amount of apparrent diapycnal upwelling in the Indo-Pacific. However, much of the upwelling in ECCO's deep ocean is not associated with irreversible watermass transformations, as typically assumed in theoretical models. Instead, a dominant portion of the abyssal circulation in ECCO is associated with isopycnal volume tendencies, reflecting a deep ocean in a state of change and a circulation in which transient tendencies play a leading role in the watermass budget. These volume tendencies are particularly prominent in the Indo-Pacific, where ECCO depicts a cooling and densifying deep ocean with relatively little mixing-driven upwelling, in disagreement with recent observations of deep Indo-Pacific warming trends. Although abyssal ocean observations are insufficient to exclude the trends modeled by ECCO, we note that ECCO's parameterized diapycnal mixing in the abyssal ocean is much smaller than observational studies suggest and may lead to an under-representation of Antarctic Bottom Water consumption in the abyssal ocean. Whether or not ECCO's tendencies are realistic, they are a key part of its abyssal circulation and hence need to be taken into consideration when interpreting the ECCO solution. We analyze results taken from the Estimating the Circulation and Climate of the Ocean (ECCO) state estimate in order to investigate the internal structure and watermass budget of the global ocean's large-scale circulation. The ECCO solution supports the modern view of an interconnected global ocean with substantial exchange between the overturning circulation of the Atlantic and that of the Indo-Pacific via the Southern Ocean. However, our investigation also reveals that the density structure of much of the deep ocean in the ECCO product is in a state of change, and that these changes play a key role in the watermass budget of the circulation. These results reveal disagreement between the model's representation of the deep ocean and the prevailing theoretical depictions of the ocean's large-scale circulation, which generally assume that the circulation is in a steady state. Disagreement between ECCO's deep ocean mixing rates and independent estimates indicate that the trends in ECCO may be biased, but deep ocean observations are insufficient to conclusively infer the true trends. The meridional overturning circulation (MOC) in Estimating the Circulation and Climate of the Ocean version 4 release 4 (ECCOv4r4) exhibits substantial linkage between the mid-depth and abyssal cellsTransient isopycnal volume change plays a key role in the watermass budget of the MOC in ECCOECCO's transient interior state must be taken into account when interpreting its climatological state
