A full-tensor approach to parameterising oceanic eddy tracer transport

Abstract: In numerical ocean models the effects of unresolved mesoscale eddies on tracer distributions are typically represented by a turbulent diffusion closure. Such a closure is characterised by a diffusivity coefficient for which scalars, diagonal tensors and full tensors have been proposed. We opt for the most general full-tensor approach diagnosed from an eddy-resolving tracer simulation with no additional averaging or simplifications. The resulting 'transport tensor' exhibits new levels of complexity and its properties -- such as negative eigenvalues and non-uniqueness -- contrast those of eddy diffusion closures currently implemented. This work suggests a rethink of the role of mesoscale eddies and their representation in numerical models may be required.