Subantarctic mode water: distribution and circulation

The subduction and export of subantarctic mode water (SAMW) as part of the overturning circulation play an important role in global heat, freshwater, carbon and nutrient budgets. Here, the spatial distribution and export of SAMW is investigated using Argo profiles and a climatology. SAMWis identified by a dynamical tracer: a minimum in potential vorticity. We have found that SAMW consists of several modes with distinct properties in each oceanic basin. This conflicts with the previous view of SAMW as a continuous water mass that gradually cools and freshens to the east. The circulation paths of SAMW were determined using (modified) Montgomery streamlines on the density surfaces corresponding with potential vorticity minima. The distribution of the potential vorticity minima revealed “hotspots” where the different SAMW modes subduct north of the Subantarctic Front. The subducted SAMWs follow narrow export pathways into the subtropical gyres influenced by topography. The export of warmer, saltier modes in these “hotspots” contributes to the circumpolar evolution of mode water properties toward cooler, fresher and denser modes in the east.