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Global eddy mixing rates inferred from tracer transport by satellite-derived surface currents

John Marshall and Ryan Abernathey
MIT
(Abstract received 04/30/2012 for session D)
ABSTRACT

Global velocities derived from AVISO sea-surface height observations are used to simulate the stirring of passive tracers at the ocean surface by the eddy field in order to quantify the associated mixing rates. The simulations are used to diagnose eddy mixing rates in two ways.

First, the method of Nakamura is applied to a channelized (i.e. zonal) version of the East Pacific running from the ACC across the Equator in to the Northern Hemisphere. Experiments are carried out with and without the mean flow present. We find weak meridional mixing at the surface in the Southern Ocean (O~1000 m2s-1) consistent with previous results), but higher mixing rates (3000−5000 m2s-1) in the tropical ocean. The mixing rates are strongly influenced by the presence of a mean flow, with both suppression and enhancement of mixing occurring at different latitudes, depending on the mean flow direction relative to the phase speed of the eddies.

Second, the Osborn-Cox diagnostic is used to yield global 2-d maps of mixing rates at the sea surface. This agrees qualitatively with the results from the East Pacific. It also reveals high mixing rates (O~104m2s-1) in western-boundary-current regions. The Osborn-Cox diffusivity is sensitive to the choice of tracer, which we attribute to the presence of anisotropic mixing processes. The mean flow is also shown to play a strong role in these regions. Results are compared with other recent estimates of mixing rates using Lagrangian data and inverse methods.