story by Helen Hill 
This month we spotlight work by Ryan Abernathey  who has been using MITgcm to map surface mixing rates globally. Ryan, who is currently a postdoc at Scripps, works with Paola Cessi.
Enlarging and extending the basic method originally developed by Marshal and co-authors in their 2006 paper, Estimates and Implications of Surface Eddy Diffusivity in the Southern Ocean Derived from Tracer Transport, the recently published JGR paper Mixing of Passive Tracers by the Ocean Surface Geostrophic Flow sets out to produce a map of mixing rates everywhere.
Using velocities derived from AVISO sea-surface height observations to simulate the evolution of passive tracers at the ocean surface, the authors carry out simulations to diagnose eddy mixing rates in two ways. In experiments with and without a mean flow they first apply the method of Nakamura to a sector in the East Pacific and then they calculate the Osborn-Cox diffusivity globally in two dimensions. The East Pacific results show weak meridional mixing at the surface in the Southern Ocean (~ 1000 m2s-1), consistent with previous findings), but higher mixing rates (~ 3000-5000 m2s-1) in the tropical ocean. The Osborn-Cox diagnostic provides a truly global picture of mixing rates and agrees qualitatively with the results from the East Pacific. It also shows extremely high mixing rates (~ 10 m2s-1) in western-boundary-current regions. The authors attribute the Osborn-Cox diffusivity’s sensitivity to the tracer initialization, to the presence of anisotropic mixing processes. Mixing rates are shown to be strongly influenced by the presence of a mean flow nearly everywhere, as shown by comparison with an eddy-only calculation. To find out more, contact Ryan .
Latitude tracer stirred with satellite-derived velocities from AVISO – animation credit: R. Abernathey.
Abernathey, R. P., and J. Marshall (2013) Global surface eddy diffusivities derived from satellite altimetry, J. Geophys. Res. Oceans, 118, doi:10.1002/jgrc.20066 .
Marshall, J., E. Shuckburgh, H. Jones, and C. Hill. (2006) Estimates and implications of surface eddy diffusivity in the southern ocean derived from tracer transport, J. Phys. Oceanogr., Vol. 36, No. 9, pages 1806-1821, doi: 10.1175/JPO2949.1