UC San Diego

This dissertation focuses on Subantarctic Mode Water (SAMW) formation. A new hybrid method for finding the mixed layer depth (MLD) is developed that models the general shape of each profile, searches for physical features in the profile, and calculates threshold and gradient MLDs to assemble a suite of possible MLD values. It then analyzes the patterns in the suite to select a final MLD estimate. The algorithm is applied to Argo profiles from the southeast Pacific Ocean, a SAMW formation region. In general, threshold methods find deeper MLDs than the new algorithm and gradient methods produce more anomalous MLDs than the new algorithm. The deepest MLDs in the region typically occur in August and September, routinely reach 500 m, and are found immediately north of the mean Subantarctic Front (SAF). Two hydrographic surveys in the southeast Pacific Ocean and a one-dimensional mixed layer model are used to assess the role of air-sea fluxes in forming the deep SAMW mixed layers. The simulated winter mixed layers generated by five forcing products resemble Argo observations of SAMW. Mixing driven by buoyancy loss and wind forcing is strong enough to deepen the SAMW layers. Wind-driven mixing is central to SAMW formation, as model runs forced with buoyancy forcing alone produce very shallow mixed layers. Air-sea fluxes indirectly influence winter SAMW properties by controlling how deeply the profiles mix. The stratification and heat content of the initial profiles are important in determining the properties of the SAMW and the likelihood of deep mixing. An optimum multiparameter (OMP) analysis is used to estimate the cumulative effect of the cross-frontal exchange, revealing an along-front increase in Polar Frontal Zone (PFZ) water fractional content in the region north of the SAF between the 27.0 and 27.3 kg m⁻³ isopycnals. The fractional content of PFZ water north of the SAF increases by approximately 0.1 for every 15 degrees of longitude in the southeast Pacific. A simple bulk calculation reveals that this magnitude of cross-frontal exchange could cause the downstream evolution of SAMW mixed layer properties