HR: 15:00h
AN: OS52L-05    [PDF]
TI: The Meridional Overturning Circulation: Some Insights From Simple Models
AU: * Mullarney, J C
EM: julia.mullarney@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AU: Griffiths, R W
EM: ross.griffiths@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AU: Hughes, G O
EM: graham.hughes@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AB: The convection driven by differential heating at a horizontal boundary is a useful conceptual model of the thermally-driven component of the oceanic circulation. `Horizontal convection' was originally proposed by Stommel (1961) and Rossby (1965) as a simple model of the overturning circulation in a single basin. We further develop their model and examine flows with larger Rayleigh numbers and smaller aspect ratio. A heat flux is imposed over one half of the base of a long shallow tank while the other half of the base is cooled, forming a representation of a pole to equator circulation in a basin with meridional boundaries. The forcing is applied at the base of the tank and the results inverted for discussion of the oceanographic implications. As in previous studies, the resulting flow is highly asymmetric. A stable boundary layer analogous to the ocean thermocline forms over the cooled part of the base. The flow within this `surface' layer is towards the `polar' end of the basin, where extremely localised `deep convection' occurs as a plume rising through the full depth of the tank and ventilating the interior. A novel aspect of the circulation is the presence of a layer of unsteady three-dimensional convection embedded within the steady two-dimensional flow. This convectively mixed layer forms near to the onset of the heating and deepens towards the heated end of the tank. The thermocline is eroded from beneath and the mixed layer feeds directly into the turbulent plume. A simple encroachment model describes the evolution of the depth of the mixed layer. The plume outflow at the top of the box involves large overturning eddy structures in a weak interior stratification. Two-dimensional numerical solutions are consistent with the experiments, although they do not accurately reproduce the convection in the mixed layer. We conclude that a vigorous overturning circulation can be driven by a thermal buoyancy flux applied at a single horizontal boundary (such as the ocean surface) and this overturning involves a convective mixed layer and localised turbulent downwelling.
DE: 4203 Analytical modeling
DE: 4532 General circulation
DE: 4568 Turbulence, diffusion, and mixing processes
SC: Ocean Sciences [OS]
MN: 2004 Ocean Sciences Meeting