Oxygen uptake and vertical transport during deep convection events in the Labrador Sea and its interannual variability
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Dissolved oxygen (DO) is essential for marine life and biogeochemical cycling. To a first order approximation, DO is determined by the competition between ocean ventilation and biological productivity. Approximately 21% of the atmospheric gases is oxygen, and the waters at the ocean surface are enriched in oxygen. Ventilation occurs through a suite of physical processes that brings the DO-rich surface waters into the interior ocean. This dissertation combines two works that closely examine the ventilation of oxygen in the region of deep water formation, and explore the relationship between air-sea oxygen flux and surface forcing aiming at deepening our understanding of the processes that regulate he DO inventory. Through these analyses we develop a framework to understand the oxygen to ocean heat content (O2-OHC) ratio in the ocean interior. Both works focus on the Labrador Sea and include a theoretical development and its validation using a suite of numerical sensitivity experiments. The first work leads to two main conclusions. 1) Both the duration and the intensity of the winter-time cooling are important to the total O2 uptake for a convective event. Stronger cooling leads to deeper convection and brings oxygen into deeper depths. Longer duration of the cooling period increases the total amount of oxygen uptake over the convective season. 2) The bubble-mediated influx of oxygen can increase oxygen uptake, but part of the contribution is compensated by the weakening the diffusive influx because the air-sea disequilibrium of oxygen is shifted towards supersaturation. The degree of compensation between the diffusive and bubble-mediated gas exchange depends on the relative strength of oceanic vertical mixing and the gas transfer velocity. Strong convective mixing reduces the degree of compensation so that the two components of gas exchange together drive exceptionally strong oceanic oxygen uptake. A numerical model with idealized domain and non-hydrostatic dynamics is used to test the hypotheses in this work. The second work explores what controls the O2-OHC ratio during deep convection. Models of different complexities ranging from 1-D convective adjustment model to a regional ocean circulation model that includes a complex biogeochemical module are used. The bubble injection increases the oxygen flux and the magnitude of the O2-OHC ratio under intense convective events. Longer cooling duration leads to a larger magnitude of the O2-OHC ratio. The pre-conditioning of the vertical gradients in oxygen and temperature are important for the O2-OHC ratio under different climate scenarios. With these two works, we highlight a few key mechanisms that are important to regulate the DO inventory in the ocean interior, but further efforts are needed to understand the global DO variability and to constrain the deoxygenation potential under a warming climate.