UC San Diego

The eddy heat flux generated by the statistically equilibrated baroclinic instability of a uniform, horizontal temperature gradient is studied using a two- mode quasigeostrophic model. An overview of the dependence of the eddy diffusivity of heat D[tau] on the planetary potential vorticity gradient [beta], the bottom friction [kappa], the deformation radius [lambda], the vertical shear of the large-scale flow 2U and the domain size L is provided at 70 numerical simulations with [beta]=0 (f-plane) and 110 simulations with [beta] != 0 ([beta]-plane). Strong, axisymmetric, well-separated baroclinic vortices dominate the equilibrated barotropic vorticity and temperature fields of f-plane turbulence. The heat flux arises from a systematic northward (southward) migration of anti-cyclonic (cyclonic) eddies with warm (cold) fluid trapped in the cores. Zonal jets form spontaneously on the [beta]-plane, and stationary, isotropic, jet-scale eddies align within the strong eastward-flowing regions of the jets. In both studies, the vortices and jets give rise to a strong anti-correlation between the barotropic vorticity [zeta] and the temperature field [tau]. The baroclinic mode is also an important contributor to dissipation by bottom friction and energizes the barotropic mode at scales larger than [lambda]. This in part explains why previous parameterizations for the eddy heat flux based on Kolmogorovian cascade theories are found to be unreliable. In a separate study, temperature and salinity profiles obtained with expendable conductivity, temperature and depth (XCTD) probes within Drake Passage, Southern Ocean are used to analyze the turbulent diapycnal eddy diffusivity [kappa][rho] to a depth of 1000 meters. The Polar Front separates two dynamically different regions with strong, surface-intensified mixing north of the Front. South of the Polar Front mixing is weaker and peaks at a depth of approximately 500 m, near the local temperature maximum. Peak values of [kappa][rho] are found to exceed 10^-3 m^2 s^-1. Wind-driven near-inertial waves, mesoscale eddies and thermohaline intrusions are discussed as possible factors contributing to observed mixing patterns