Abstract
Mixing along isopycnals plays an important role in the transport and
uptake of oceanic tracers. Isopycnal mixing is commonly quantified by a
tracer diffusivity. Previous studies have estimated the tracer
diffusivity using the rate of dispersion of surface drifters, subsurface
floats, or numerical particles advected by satellite-derived velocity
fields. This study shows that the diffusivity can be more efficiently
estimated from the dispersion of coherent mesoscale eddies. Coherent
eddies are identified and tracked as the persistent sea surface height
extrema in both a two-layer quasigeostrophic (QG) model and an idealized
primitive equation (PE) model. The Lagrangian diffusivity is estimated
using the tracks of these coherent eddies and compared to the diagnosed
Eulerian diffusivity. It is found that the meridional coherent eddy
diffusivity approaches a stable value within about 20–40 days in both
models. In the QG model, the coherent eddy diffusivity is a good
approximation to the upper-layer tracer diffusivity in a broad range of
flow regimes, except for small values of bottom friction or planetary
vorticity gradient, where long-range correlations between same-sign
eddies become important. In the PE model, the tracer diffusivity has a
complicated vertical structure and the coherent eddy diffusivity is
correlated with the tracer diffusivity at the e-folding depth of the
energy-containing eddies where the intrinsic speed of the coherent
eddies matches the rms eddy velocity. These results suggest that the
oceanic tracer diffusivity at depth can be estimated from the movements
of coherent mesoscale eddies, which are routinely tracked from satellite
observations.