Isolating the role of mesoscale eddies in mixing of a passive tracer in an eddy resolving model

Abstract

This study examines the role of mesoscale eddies in distribution of chlorofluorocarbon (CFC) in a numerical model of the North Atlantic. Our main conclusions are based on a comparison of two eddy‐resolving experiments, one with and one without eddy advection of CFC. The CFC distributions in these two experiments exhibit substantial differences within the subpolar gyre. In particular, in the presence of eddies the horizontal distribution of CFC in the upper 1000 m is more uniform and the CFC concentration is lower in the lower 1500 m. Our analysis demonstrates that these basin‐wide differences are caused by the interactions of eddy advection with convection and mean advection in several isolated locations. The corresponding eddy flux divergence terms are spatially non‐uniform in magnitude; their effects are often nonlocal and sometimes work against the spreading of CFC being carried out by the mean advection. These conclusions are further confirmed by a third experiment, in which the eddy advection was replaced by a down‐gradient diffusive parameterization. Our results demonstrate that advection by mesoscale eddies is crucial to the mixing of atmospheric gasses into the ocean, and outline difficulties in representing eddies by the down‐gradient diffusion.