Evaluating Physical and Biogeochemical Controls on Ocean Oxygen, Nitrate, and Carbon in Coupled Biogeochemical Ocean Circulation Models

Abstract

The balance of ocean physics and the photosynthetic production and respiratory consumption of O2 dynamically links oxygen, organic carbon, and nitrate in the ocean. This thesis uses coupled biogeochemical ocean circulation models to explore the physical and biogeochemical controls on the ocean O2 cycle and its connections to surface nitrate and organic carbon in climate sensitive regions where these mechanisms have not previously been fully characterized. In the North Pacific Oxygen Deficient Zone (Chapter 2), I demonstrate that the physical supply of O2 is controlled by three processes: the injection of O2 along its poleward edge by zonal jets, an indirect O2 supply from Tropical Instability Waves that induce a meridional O2 supply that is observable as high [O2] water impinges on the equatorward Oxygen Deficient Zone boundary and a coastal diffusive seasonal O2 flux. In the North Atlantic, I leverage historical hydrographic data and a hindcast configuration of the Community Earth System Model to show that the historical variability in O2 that exceeds that expected from temperature-dependent solubility is explained by the enhancement of growth rates at higher temperatures. Finally, using a high-resolution model of the tropical Pacific (Chapter 4), I demonstrate that a net organic carbon sequestration of 3 mol C m-2 yr-1 is narrowly constrained by empirical nitrate data. I further show that there are significant regional differences in the fraction of respired carbon production in the tropical Pacific: greater than 5 times more integrated respiration occurs below the depth horizon where water parcels reach the surface in ≤2 years in the eastern equatorial Pacific compared to the western equatorial Pacific.