Global Observations of Horizontal Mixing from Argo Float and Surface Drifter Trajectories

• Published in: Journal of geophysical research. Oceans Vol. 123; no. 7; pp. 4560 - 4575
• Main Authors: Roach, Christopher J., Balwada, Dhruv, Speer, Kevin
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-07-2018
• Subjects: Antarctic Circumpolar Current; Argo floats; Boundary currents; Depth; Diffusion coefficients; Diffusivity; Drift; Drifters; Eddies; Eddy current testing; Eddy currents; Eddy diffusion; Eddy diffusivity; Eddy kinetic energy; Equator; Geophysics; isopycnal mixing; Kinetic energy; Lagrange multiplier; Lagrangian methods; Latitude; Mesoscale eddies; Ocean currents; Oceanic tracers; Oceans; Parameterization; Particle analysis; Surface drifters; Tracers; Trajectory analysis; Western boundary currents
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2018JC013750

Mixing by mesoscale eddies in the ocean plays a major role in setting the distribution of oceanic tracers, with important implications for physical and biochemical systems at local to global scales. Roach et al. (2016; https://doi.org/10.1002/2015JC011440) demonstrated that a two‐particle analysis of Argo trajectories produces robust estimates of horizontal mixing in the Southern Ocean. Here we extend this analysis to produce global 1° × 1° maps of eddy diffusivity at the nominal Argo parking depth of 1,000 m. We also applied this methodology to estimate surface eddy diffusivities from Global Drifter Program (GDP) surface drifters. The global mean eddy diffusivity was 543 ± 155 m2/s at 1,000 m and 2637 ± 311 m2/s at the surface, with elevated diffusivities in regions of enhanced eddy kinetic energy, such as western boundary currents and along the Antarctic Circumpolar Current. The eddy kinetic energy at the equator is high at both the surface and depth, but the eddy diffusivity is only enhanced near the surface. At depth the eddy diffusivity is strongly suppressed due to the presence of mean flow. We used our observational estimates to test the validity of an eddy diffusivity parameterization that accounts for mixing suppression in the presence of zonal mean flows. Our results indicated that this parameterization generally agrees with the directly observed eddy diffusivities in the midlatitude and high‐latitude oceans.
Key Points
We applied a two‐particle method to produce global maps of horizontal eddy diffusivity near the surface and at 1,000 m
Our estimated eddy diffusivities show reasonable agreement with previous studies
We tested a mixing globalization including mixing by mean flow and obtained agreement with our estimates of diffusivity