Eddy advective and diffusive transports of heat and salt in the Southern Ocean

There are two distinct mechanisms by which eddies provide systematic transport of tracer on isopycnals: the advective transport, associated with the slumping of isopycnals, and the diffusive transport, associated with down-gradient diffusion. Depending on the large-scale tracer distribution, eddy advective transport has either the same direction as or opposite direction to eddy diffusive transport. As a consequence, eddy advection and eddy diffusion can reinforce each other for some tracers but oppose each other for other tracers. Using scaling analysis, it is argued that the relative directions of eddy advective and diffusive transports can be determined simply from the relative slopes of tracers and isopycnals. An eddy-resolving (1/12°) global ocean model is used to illustrate the two eddy transport mechanisms for temperature and salinity in the Southern Ocean. Applications to other tracers, such as oxygen, are discussed. The diagnosed eddy diffusivity for temperature (and salinity) is found to be considerably different from the eddy diffusivity for eddy advective transport velocity.