Low-frequency changes in Southern Ocean ventilation quantified using an Atlantic sector model: A boundary-propagator approach

The ventilation of the Southern Ocean, i.e., the exchange between the Southern Ocean surface and the interior, plays a key role in regulating Earth’s climate. In recent decades, surface westerly winds have been observed to intensify and shift poleward, resulting in changes to the SO circulation and ventilation. This has important implications for the ocean’s capacity to absorb carbon and heat from the atmosphere. While recent inverse modelling of repeat hydrographies has provided some insights into ventilation changes in terms of changes in CFCs, these studies have so far been limited to analyzing differences between two snapshots in time. Here we use an idealized Atlantic sector forward model to quantify inter-annual to decadal variability in fundamental ventilation metrics, such as the water-mass fraction last ventilated some time during the last τ years, as a function of τ. We do this by explicitly computing the boundary-propagator Green function, G, that partitions water locally according to where and when it was last ventilated. The boundary propagators were computed with respect to surface regions that ventilate bottom, intermediate, and mode waters. Suitable integrals of G quantify the timescales and pathways of ventilation, and how these change on interannual to decadal timescales. We also consider the impact of the ocean’s changing ventilation on anthropogenic carbon. To investigate the changing interplay between eddies and mean circulation, the calculations were performed for both coarse (1°x1°) and eddy permitting (1/4°x1/4°) resolutions.