Upper Ocean Turbulent Processes in the Antarctic Frontal Zones

A number of processes in the Southern Ocean are thought to support high levels of mixing relative to other regions of the global ocean. At the surface, strong winds and storms force the ocean at near-inertial frequencies, generating internal waves that can propagate downward and break. Deep reaching currents associated with density fronts flow over rough topography generating internal lee-waves which radiate energy and provide power for turbulence in the stratified ocean interior. Watermass variability and strong mesoscale activity also precondition the water column for double-diffusive instability.

Due to the remote location and harsh conditions, few direct measurements of turbulence have been collected in the Southern Ocean. Direct measurements of oceanic turbulent parameters were taken upstream of and across Drake Passage, in the region of the Subantarctic and Polar Fronts. Values of turbulent kinetic energy dissipation rate, estimated by microstructure are up to two orders of magnitude lower than previously reported fine-structure derived estimates in the upper 1000m. Despite the prevalence of wind forcing, turbulence driven by near-inertial shear is weak. The dissipation rate of thermal variance is enhanced in the upper 1000m, with the highest values found in northern Drake Passage where water mass variability is the most pronounced. Using the density ratio, evidence for mixing by double-diffusive instability is presented, and implications for the mixing efficiency are also discussed.