MSE Research Project Database

Effects of convection on the circumpolar circulation and overturning of the Southern Ocean


Project Leader: Bishakhdatta Gayen
Student: Mr Taimoor Sohail (ANU)
Collaborators: Prof Ross W. Griffiths (ANU) Prof Andy Hogg (ANU)
Sponsors: ARC
Primary Contact: Bishakhdatta Gayen (bishakhdatta.gayen@unimelb.edu.au)
Keywords: computational fluid dynamics
Disciplines: Mechanical Engineering
Domains:

Convection is one of the major modes of heat transfer in fluids and is especially important given the large scales of most geophysical flows. Heat transfer by natural convection plays a significant role in the structure of Earth's atmosphere, oceans and mantle. Flow in a horizontal layer of fluid heated uniformly from below and cooled from above—Rayleigh-Be´nard convection—is an idealized problem from which much can be learned about the nature of convective flow and heat transfer in physics, astrophysics and geophysics. On the other hand, the flows resulting from a horizontal difference in temperature or heat (buoyancy) flux at a single horizontal boundary of a fluid — ‘horizontal convection’— is a basic model for thermally driven ocean circulation. 

The circulation in the Southern Ocean (SO) which is driven by meridional surface buoyancy gradient and persistent westerly winds, connects each of  the major ocean basins. The surface buoyancy fluxes (due to cooling) drive convection and lead to the formation of dense water on the Antarctic continental shelf, which is exported as a  plume of downwelling water along the continental slope, and which mixes to become Antarctic Bottom Water. The circulation in SO thereby controls the global ocean density stratification and catalyses strong upwelling that results in the outgassing of ancient carbon dioxide. The Southern Ocean insulates Antarctica from the warm subtropical water to the north, resulting in lower temperatures around (and on) the coldest continent.  The Southern Ocean is thought to be responsible for around 40% of the heat draw-down and a comparable fraction of the CO2 draw-down from the atmosphere to the ocean associated with anthropogenic climate warming Therefore, knowledge of these processes is essential to understand their consequences for global climate.

Numerical simulation is a great tool to study circulation and convective process in the Sourthen Ocean. Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) are used to obtain solutions under both laboratory and ocean conditions that can be compared with experimental data, and they provide a complete view of the mechanical energy budget for  horizontal convection under strong buoyancy forcing (high Rayleigh numbers) and mechanical forcing (surface wind stress). Large Eddy Simulation (LES) will potentially enable the study of convection to extend into the asymptotic (‘ultimate') regime at very large Rayleigh numbers. The simulation of convection  is combined with other oceanic phenomena, including at a relatively small scale, the melting if Antarctic ice shelves and glacier tongues that involves convection at the ice front, or at much larger scales with mesoscale eddies, Rossby waves and wind-driven circulations. We have developed simple model (see figure) to study the fluid dynamics processes in the Sourthern Ocean.  

Overview of problem
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