| Summary: | Natural and anthropogenic heat and carbon, as well as other climatically and biologically important tracers, enter the deep global overturning circulation mainly during the formation of dense waters at the high latitudes around Antarctica and in the North Atlantic. Through the transport, sequestration and upwelling of tracers, the global overturning circulation contributes to the regulation and variability of our climate system. This thesis investigates the role of diapycnal (cross-density) mixing in regulating the deep ocean overturning circulation and the distribution of tracers carried within it. Diapycnal mixing is here quantified through a combination of pre-existing estimates inferred from different observational sources, namely Argo floats, CTD casts combined with microstructure profiles, bulk estimates from an inverse model and observation-based theoretical estimates of the mixing generated by internal tides and lee waves. Firstly, it is shown that the upwelling of abyssal waters is the residual of large near-boundary upwelling and interior downwelling, the patterns of which are highly sensitive to the specifics of the energy pathways from tides and geostrophic motions to internal waves and thereafter to mixing and dissipation. Secondly, we focus on the Atlantic Ocean, where we investigate the role of mixing in the closure of the Atlantic meridional overturning circulation. Diapycnal mixing is shown to contribute only marginally to the conversion of the dense waters formed in the North Atlantic into lighter northward-flowing waters within the Atlantic basin, but it plays a crucial role in the distribution of tracers carried from the North Atlantic to the Southern Ocean. The impact of mixing on tracer distributions is analysed both from observational mixing estimates and from pre-existing numerical simulations of a tracer release experiment in a quasi-realistic Atlantic sector model. As a tracer is carried southward by the Atlantic overturning circulation, we quantify its exchange between different water ...
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