The Influence of Near-Inertial Wave–Mesoscale Turbulence Interactions on Passive Tracer Fluxes

Joseph G Fitzgerald and Jörn Callies, California Institute of Technology, Pasadena, CA, United States
The dynamics of near-inertial waves (NIWs) interacting with mesoscale eddies has received much recent attention in theoretical, modelling, and observational studies. Although much remains to be understood about these dynamics, previous work has established that NIW–eddy interaction can lead to significant energy exchange between NIWs and eddies and can modify the statistical equilibrium of ocean macroturbulence. To date, less emphasis has been placed on examining how NIW–eddy interactions impact the transport of passive tracers carried out by mesoscale eddies. These eddy fluxes of passive tracers are climatologically important, affecting, for example, the transport of anthropogenic carbon into the ocean’s interior. As NIW–eddy interactions are likely ubiquitous in the ocean, understanding how these interactions influence transport processes is important to our understanding of the climate system.

In this work we investigate how NIW–eddy interactions modify isopycnal tracer fluxes using a forced–dissipated model in which barotropic balanced turbulence interacts with baroclinic NIWs that have an assumed plane wave vertical structure. Tracer fluxes are produced as the eddy field stirs a uniform background tracer gradient. Simulations in which the forcing excites only balanced turbulence are compared with those in which additional forcing is added to excite NIWs at the domain scale. Consistent with previous studies, we find that NIW–eddy interactions modify the balanced turbulence, extracting energy through stimulated imbalance. We also find that the presence of NIWs weakens the tracer fluxes carried by the eddies and reduces the equilibrium level of maintained tracer variance. The coupling between NIWs and eddies thus produces a system for which increasing the energy injection rate of the forcing, by adding NIW forcing, reduces transport. The reduction in tracer transport does not appear to be a direct result of a reduction in eddy energy due to stimulated imbalance, as transport is reduced even when NIW-eddy interactions modify the eddy energy only weakly. We explore the physical mechanisms behind the reduced tracer transport using recently advanced asymptotic models that describe the coupled dynamics of NIWs and balanced turbulence.