Oxygen uptake and vertical transport during deep convection events in the Labrador Sea and its interannual variability

Daoxun Sun
Wednesday, March 11, 2020 - 1:00pm
ES&T 1229
Dr. Taka Ito (Advisor), Dr. Annalisa Bracco (Advisor), Dr. Emanuele Di Lorenzo, Dr. Jie He, and Dr. Curtis Deutsch (U. Washington)

Dissolved oxygen (DO) is essential for marine life and biogeochemical cycling. To a first order, 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 DO. 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 oxygen ventilation 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 the 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 intensity of 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 cooling period duration increases total O2 uptake over the convective season. 2) The bubble-mediated influx of O2 can increase O2 uptake, but part of the contribution is compensated by weakening the diffusive influx because the air-sea disequilibrium of oxygen is shifted towards supersaturation. The degree of compensation between 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 these hypotheses.

The second work explores what controls the O2-OHC ratio during deep convection. Models of different complexities ranging from 1-D convective adjustment to regional ocean circulation models that includes a complex biogeochemical module are used. The bubble injection increases O2 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 DO 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.