Poleward heat transport in the ocean: A review of a hierarchy of models of increasing resolution

The large-scale transport of heat and carbon by the ocean circulation play an important role in the Earth's climate. Progress in developing realistic models of this process is reviewed. Sufficient numerical experiments have been carried out to indicate the role of subgrid scale mixing of temperature and salinity in the transport behaviour of the models. The vertical component of subgrid diffusion in the models is essential for determining the amplitude of the thermohaline circulation of the ocean. In the case of simple geometries, poleward heat transport in models is approximately proportional to vertical mixing to the two-thirds power. The horizontal component of the diffusion appears to play almost no role in the poleward transport of heat as long as the value is less than 10³ m²/s. At low values of horizontal diffusion and viscosity, mesoscale eddies are generated spontaneously in the models through baroclinic and barotropic instability. In analogy with the atmosphere one would expect these mesoscale disturbances to play an important role in poleward heat transport. The results of numerical experiments show that this may not be the case. The mesoscale eddies in the models generate mean flows which tend to cancel the eddy fluxes in much the same way that eddy-mean flow compensation occurs for atmospheric disturbances in the lower stratosphere.