Impacts of Zonal Asymmetry on Southern Ocean Dynamics and Biogeochemistry

Citation

Dove, Lilian Aja (2023) Impacts of Zonal Asymmetry on Southern Ocean Dynamics and Biogeochemistry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qnah-sc58. https://resolver.caltech.edu/CaltechTHESIS:05302023-201452585

Abstract

The Southern Ocean is a key region for the ventilation and formation of intermediate and deep water masses. Interactions of the Southern Ocean’s Antarctic Circumpolar Current (ACC) with bathymetry can result in the diversion and compaction of frontal currents, resulting in standing meanders associated with enhanced mesoscale eddy kinetic energy (EKE) and submesoscale vertical velocities. As a result, standing meanders are thought to shape uptake and sequestration of heat and carbon across the ACC. In this thesis, I use observations from remote sensing and various autonomous underwater vehicles to investigate how physical mechanisms, from the submesoscale to the basin scale, shape the biogeochemical properties and tracer distributions of the Southern Ocean.

Processes at the ocean's submesoscale can play a vital role in exchanging water across the base of the mixed layer, contributing to water mass ventilation. Data from over 20,000 profiles from biogeochemical-Argo floats across the ACC highlight that the high EKE regions associated with standing meanders have relatively reduced apparent oxygen utilization (AOU) values below the base of the mixed layer. This result, as well as larger AOU variance in deep potential density classes, suggests there is enhanced ventilation occurring in standing meanders. Further investigation suggests the observed increased ventilation is due to both along-isopycnal stirring and enhanced exchange across the base of the mixed layer by vertical velocities at the submesoscale, highlighting the importance of standing meanders in shaping temporal and spatial variability of biogeochemical cycles and air-sea exchange.

Observations with horizontal scales of 2-4 kilometers in the standing meander associated with Crozet Plateau show that submesoscale processes are indeed ubiquitous. In this region, processes on the submesoscale to mesoscale spectrum play a role in enhancing surface frontal gradients and heightening tracer variability at depth. A separate field program provided novel observations at the submesoscale in Drake Passage during wintertime. The Polar Front, one of the major fronts of the ACC, is shown to be eddy-suppressing, suggesting that along-isopycnal submesoscale processes contribute to ventilation at the front. These spatial variations in stratification may additionally impact carbon fluxes between the atmosphere, surface mixed layer, and interior ocean. This thesis presents evidence that ventilation is a heterogeneous process across the Southern Ocean, with contributions from processes at physical scales that are undersampled by current observational programs.

Thesis Files