A very simple conceptual model for the meridional overturning component of the ocean circulation is the convective flow driven by a horizontal surface temperature gradient. The meridional overturning circulation (also referred to as the global thermohaline circulation) carries warm subtropical surface waters to high latitudes, where it cools and sinks. In the present pattern of circulation the dense cold (and sufficiently saline) water sinks in confined regions at high latitudes to form 'Deep' and 'Bottom' waters. There must be a slow, possibly basin-wide, upwelling to the surface to match the poleward mass flux, with the loop closed by an inferred zonal average return flow toward the equator at depths below the thermocline. The density structure and heat transport must depend on the rate of vertical diffusion of heat. In the oceans the diffusion is attributed internal, turbulent mixing processes, which act to increase the potential energy of the water column. The resulting heat flux carried by the overturning circulation has a significant role in the climate system, and changes in this circulation have been implicated as a cause of climate variability on decadal to millennial timescales. However, there is uncertainty in the source of the energy for internal mixing, and a discrepancy between predicted and measured rates of mixing. There is also controversy as to whether the meridional heat flux (i.e. buoyancy) is a significant forcing for the oceans. Some have concluded that the heat flux is simply incidental to a flow forced by surface wind stress and in which the density structure is maintained by mixing energised by winds and tides. We address these issues by examining circulation forced purely by thermal buoyancy.
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