Turbulence and Convection in Southern Ocean Circulation
Circulation in the Southern Ocean is a key component of the global climate system as it controls the exchange of water masses between the Atlantic, Pacific and Indian oceans, thereby modulating heat and carbon uptake and biological activity. In the Southern Ocean, surface winds and buoyancy produce...
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| Format: | Doctoral or Postdoctoral Thesis |
| Language: | English |
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| Online Access: | http://hdl.handle.net/1885/204889 https://doi.org/10.25911/5ee74e69cf188 https://openresearch-repository.anu.edu.au/bitstream/1885/204889/3/TaimoorSohail_PhDThesis.pdf.jpg |
| Summary: | Circulation in the Southern Ocean is a key component of the global climate system as it controls the exchange of water masses between the Atlantic, Pacific and Indian oceans, thereby modulating heat and carbon uptake and biological activity. In the Southern Ocean, surface winds and buoyancy produce a circulation characterised by processes spanning a wide range of length and time scales, from large-scale overturning and zonal flow to small-scale turbulence and convection. A major source of uncertainty in numerical studies of the Southern Ocean lies in their characterisation of convection. Convection, an advective process triggered by a destabilising buoyancy gradient, can manifest over a small area (for instance, as open-ocean convection) or broad region (such as along the Antarctic coast). Convection in the ocean is a turbulent process, and is not resolved by large-scale ocean models. In addition, it can be difficult to capture field observations of convection at a sufficiently high resolution. As a consequence, the impact of convection on Southern Ocean circulation remains uncertain. In this thesis, we use a turbulence-resolving Direct Numerical Simulation (DNS) to investigate the influence of turbulent convection on Southern Ocean circulation. Our DNS model resolves scales of motion down to the turbulent grid-scale. This allows us to explore the effect of wind stress and convection on energetics and volume transport in the region. The DNS also facilitates understanding problems with the characterisation of small-scale flows in large-scale ocean models. We find that energy fluxes in the model are modified by turbulence, convection and surface winds. Diapycnal mixing is highest where turbulent convection is active, and dissipation is highest in the viscous sub-layer underlying surface wind stress. Both mixing and dissipation increase with increasing winds, and dissipation exhibits a higher sensitivity to winds than mixing. Dissipation is smaller than mixing in the wind stress regime relevant to the Southern ... |
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