The Impact of Turbulence and Convection on Transport in the Southern Ocean

Taimoor Sohail, University of New South Wales, Mathematics and Statistics, Sydney, NSW, Australia, Catherine Ann Vreugdenhil, University of Melbourne, Melbourne, Australia, Bishakhdatta Gayen, University of Melbourne, Mechanical Engineering, Melbourne, Australia and Andrew M Hogg, Australian National University and ARC Centre of Excellence for Climate Extremes, Research School of Earth Sciences, Canberra, ACT, Australia
Abstract:
Volume transport in the Southern Ocean is quantified using an idealised, turbulence-resolving direct numerical simulation. The behaviour of the overturning circulation and zonal transport in the presence of turbulence, as well as its sensitivity to increasing wind stress, is investigated. We find that, with the presence of turbulent convection, a rich baroclinic eddy field develops in the simulation, characterised by a high fluctuating kinetic energy. With increasing wind stress, this increasingly energetic eddy field modifies the meridional stratification and results in a zonal transport that is eddy saturated. The abyssal overturning (and by extension, Antarctic Bottom Water formation) strengthens with enhanced turbulence, whilst the lower overturning contracts in both spatial extent and magnitude. The dominant drivers of the abyssal overturning are fluctuating flow features, while mean flow (specifically, mean Ekman transport due to surface winds) plays the primary role in the upper overturning. The strong coupling between Ekman transport and the upper overturning drives a linearly increasing response of the upper overturning to increasing winds. The abyssal cell also strengthens with increasing winds, driven by enhanced diapycnal mixing and fluctuating flow features. Therefore, it appears that the sensitivity of the abyssal overturning to winds is highly dependent on the degree to which turbulence is characterised. These findings emphasize the importance of resolving turbulence when investigating volume transport in the Southern Ocean.
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