(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2016-06)
Merrifield, Sophia T.
The Southern Ocean is one of the most energetic regions of the world ocean due to
intense winds and storm forcing, strong currents in the form of the Antarctic Circumpolar
Current (ACC) interacting with steep topography, and enhanced mesoscale
activity. Consequently, the Southern Ocean is believed to be a hotspot for enhanced
oceanic mixing. Due to the remote location and harsh conditions, few direct measurements
of turbulence have been collected in the Southern Ocean. Previous studies
have used indirect methods based on finestructure observations to suggest that strong
mixing is ubiquitous below the mixed layer. Results from a US/UK field program,
however, showed that enhanced internal wave finestructure and turbulence levels are
not widespread, but limited to frontal zones where strong bottom currents collide
with steep, large amplitude topography.
This thesis studies the processes that support turbulence and mixing in the surface
boundary layer and at intermediate depths in the Drake Passage region. Direct measurements
of turbulence show that previous estimates of mixing rates in the upper
1km are biased high by up to two orders of magnitude. These biases are discussed in
the context of the internal wave environment and enhanced thermohaline finestructure.
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. Double diffusive processes and turbulence both contribute to
buoyancy flux, elevating the effective mixing efficiency above the canonical value of
0.2 in the upper 1km. Despite the prevalence of energetic wind events, turbulence
driven by downward propagating near-inertial wave shear is weak below the mixed
layer. The results of this study inform large-scale modeling efforts through parameterizations
of mixing processes in the highly undersampled Southern Ocean.
