Role of diapycnal mixing in the circulation and tracer distribution in the Atlantic Ocean

We investigate the importance of diapycnal mixing in the Atlantic Ocean, and in particular in affecting the dense southward flowing water masses of North Atlantic origin, and the distribution of the tracers carried within. Diapycnal mixing is here quantified through a combination of estimates inferred from different observational sources, namely Argo floats, CTD casts combined with microstructure profiles, bulk estimates from an inverse model and observationally-based theoretical estimates of the mixing generated by internal tides and lee waves. Using observations and model results, we find that while mixing’s contribution to conversion of the North Atlantic Deep Waters into lighter northward-flowing waters is of secondary importance, it plays a primary role in the distribution of tracers carried by AMOC from the North Atlantic to the Southern Ocean. We argue that mixing along the path of AMOC has a control over the density classes within which tracers of North Atlantic origin upwell in the Southern Ocean, with crucial implications for tracer ventilation timescales.