Complexity of Mesoscale Eddy Diffusivity in the Ocean

Stirring of water by mesoscale currents (“eddies”) leads to large-scale transport of many important oceanic properties (“tracers”). These eddy-induced transports can be related to the large-scale tracer gradients, using the concept of turbulent diffusion. The concept is widely used to describe these transports in the real ocean and to represent them in climate models. This study focuses on the inherent complexity of the corresponding coefficient tensor (“K-tensor”) and its components, defined here in all its spatio-temporal complexity. Results demonstrate that this comprehensive K-tensor is space-, time-, direction- and tracer-dependent. Using numerical simulations with both idealized and comprehensive models of the Atlantic circulation, we show that these properties lead to upgradient eddy fluxes and the potential importance of all tensor components. The uncovered complexity of the eddy transports calls for reconsideration of how they are estimated in practice, included in the general circulation models and theoretically interpreted.