Impact of physically relevant and numerically induced diapycnal mixing and meso-scale dissipation on meridional mass and tracer transports in the Southern Ocean

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The Meridional Overturning Circulation (MOC) in the Southern Ocean (SO) is composed of two limbs, the Upper Circumpolar Deep Water (UCDW) flows polewards and upwards across the Antarctic Circumpolar Current (ACC), upwells to the surface at the poleward flank of the ACC and then returns equatorwards as a near surface current. This upper limb is known to be largely adiabatic. In contrast to that, the lower limb of the MOC formed by the Lower Circumpolar Deep Water (LCDW) upwells closer to Antarctica, cools down substantially near the surface and convects down at high turbulent mixing to form the Antarctic Bottom Water (AABW). While the adiabatic upper limb is driven by an imbalance of a northward Ekman transport and an opposing meso-scale eddy transport, the dynamics of the lower limb is more complex due to the additional significance of diapycnal mixing. Thus, a good quantification of these dynamics requires a correct representation of both small-scale (diapycnal) and meso-scale (lateral) eddy mixing. On the other hand, it has long been known that in particular in the SO, large numerical mixing and dissipation in ocean circulation models due to the discretisation of the advection terms obscures the representation of diapycnal mixing and thus strongly limits the predictability of ocean models. It is therefore the aim of this project to use and to further develop a novel analysis tools for numerical mixing and dissipation to quantify effective mixing and dissipation, given by the sum of explicitly parameterised and numerically induced values. By estimating realistic effective mixing and dissipation rates, using our combined expertise of numerical mathematics and small-scale turbulence parameterisation and large-scale high-resolution modelling, we are able to the first time to assess the sensitivity of realistic models of the SO to diapycnal mixing and numerical dissipation and to understand the interplay between meso-scale (lateral) and small-scale (diapycnal) eddy mixing on the lower limb of the MOC.