On the fate of North Atlantic deep waters in the Southern Ocean

Dense waters formed in the North Atlantic (NADW) propagate southward into the Southern Ocean. There, a portion upwells to the surface in a south-eastward spiral (Tamsitt et al. 2017), a portion is entrained into the northward flowing Antarctic Bottom water (AABW), and a portion propagates northward at mid-depths of the Indian and Pacific Oceans. Combined, these processes result in a gradual eastward dilution of NADW at its core density around the Southern Ocean. However, no consensus has been reached on the dilution rate of these waters and how it impacts global ocean ventilation. Here, we use historical  observations to track the dilution of NADW around the Southern Ocean. We find a persistent  maximum at mid-densities, corresponding to the NADW core, which erodes eastward from the Atlantic sector. Furthermore, we evaluate the dilution of an artificial North Atlantic dye in three models of ocean transports: a 1° global configuration of the Nucleus for European Modelling of the Ocean (NEMO), version 2 of the Ocean Circulation Inverse Model (OCIM), and the Total Matrix Intercomparison (TMI). The erosion of the North Atlantic dye maximum around the Southern Ocean varies markedly across models, and is largest in TMI. The available  data point to an overly rapid erosion in TMI, but remain too scarce in the Pacific sector to place strong constraints on the dilution rate of the NADW core. A zonal circumpolar transect of salinity and  measurements, together with characterisation of inter-laboratory offsets, would greatly help to constrain the fate of NADW.