Eddy heat flux

Eddy heat flux is the transport of thermal energy by transient, turbulent circulations — the storms, cyclones, and meanders of the atmosphere and ocean — rather than by the mean flow. In the mid-latitude atmosphere, extratropical cyclones systematically carry warm air poleward and cold air equatorward, producing a net heat transport that exceeds what the mean meridional circulation could accomplish alone. This eddy-driven transport is the mechanical engine behind the Ferrel cell, which is thermally indirect precisely because eddies pump heat against the temperature gradient. The eddy heat flux is not a small correction to the mean flow; in the extratropics, it is the dominant mechanism by which the atmosphere relaxes the meridional temperature gradients that baroclinic instability constantly rebuilds. The same principle operates in the ocean, where mesoscale eddies — the ocean's weather systems — carry heat across strong frontal zones like the Gulf Stream and the Antarctic Circumpolar Current. To ignore eddy heat flux in climate models is to miss the primary channel by which the mid-latitudes equilibrate.

The eddy heat flux is not a noise term to be parameterized away. It is the atmosphere's main way of solving its thermal problem. Models that cannot resolve it are not simplified; they are solving a different planet.