The role of mesoscale eddies in the poleward transport of heat by the oceans: A review

The poleward transport of heat by the ocean circulation plays a major role in the global heat balance, but many details of this process remain unclear. In particular it is difficult to determine from observations what role energetic mesoscale eddies play in poleward heat transport. In place of missing long-term buoy measurements over extensive areas, eddy resolving numerical ocean circulation simulations offer some means to gain insight. For ocean models with specified meridional density distributions at the upper boundary it is possible to compare the poleward heat transport in eddy resolving and non-eddy-resolving simulations. While a change of vertical diffusion in models is known to be very important for poleward heat transport, the reduction of horizontal viscosity and diffusion, which allows mesoscale eddies to appear in the simulation, seems to have little effect. A possible explanation appears to be that the models for normal parameter ranges are very weakly driven thermal systems. The time dependent mesoscale eddies appear to set up nearly adiabatic flows in which eddy transport of heat is compensated by induced mean flows which transport heat in the opposite direction. For extremely strong forcing of the density field in the upper ocean this is no longer true.