[A-CG40] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[ACG40-06] Isopycnal diffusivity in deep Southern Ocean

★Invited Papers

*Katsuro Katsumata¹ (1.University of Tokyo)

Keywords:Isopycnal diffusivity, Southern Ocean, deep sea

It is well established that eddies play crucial roles in the Southern Ocean circulation dynamics. At scales larger than these eddies, the effect is expressed as eddy diffusion or eddy viscosity. This diffusivity, in particular in the isopycnal or horizontal direction, is difficult to estimate from observational data. At sea surface, a multitude of methods can be applied -- from eddy resolving geostrophic velocity field estimated by satellite altimetry to 'spaghetti diagrams' of surface buoys. Deep into the ocean, the methods are limited -- extrapolating the surface values by means of assumed vertical modes or drift paths from Argo floats. Here two methods to estimate subsurface isopycnal diffusivities from tracers are introduced and employed. One is to use the steady advection-diffusion balance for temperature and salinity. The other is to use a cross section of tracers such as CFCs. When applied on the continental rise in the Australia-Antarctic Basin, the former method yeilds isopycnal diffusivities of -200 to 200 m²s^-1. The latter method applied on a meridional section at 55°E longitude, the isopycnal diffusivity was about 100 m²s^-1. These are small values but not unrealistic as estimates in deep ocean.