Seasonal Oxygen Supersaturation and Air-Sea Fluxes from Profiling Floats in the Pacific

The Pacific Ocean is a heterogeneous basin that includes regions of strong CO₂ fluxes to and from the atmosphere. The Kuroshio Extension (KE) is a current associated with the largest CO₂ flux into the Pacific Ocean, which extends across the Pacific basin between the subarctic and subtropical regions. The relative importance of the biological and physical processes controlling this sink is uncertain. The stoichiometric relationship between O₂ and dissolved inorganic carbon during photosynthesis and respiration may allow in situ O₂ measurements to help determine the processes driving this large CO₂ flux.

In this study, we used Argo profiling floats with modified oxygen sensors to estimate O₂ fluxes in several areas of the Pacific. In situ air calibrations of these sensors allowed us to accurately measure air-sea O₂ differences, which largely control the flux of O₂ to and from the atmosphere. In this way, we determine air-sea O₂ fluxes from profiling floats, which previously did not measure O₂ accurately enough to make these calculations. To characterize different areas within the KE, we separated O₂ measurements from floats into 3 regions based on geographical position and temperature-salinity relationships: North KE, Central KE, and South KE. We then used these regions and floats in the Alaska Gyre and subtropical South Pacific gyre to develop seasonal climatologies of ΔO₂ and air-sea flux.

Mean annual air-sea oxygen fluxes (positive fluxes represent addition of O₂ to the ocean) were calculated for the Alaska Gyre of -0.3 mol m^-2 yr^-1 (2012-2015), for the northern KE, central KE, and southern KE (2013-2015) of 6.8, 10.5, and 0.5 mol m^-2 yr^-1, respectively, and for the south subtropical Pacific (2014-2015) of 0.6 mol m^-2 yr^-1. The air-sea flux due to bubbles was greater than 50% of the total flux for winter months and essential for determining the magnitude and, in some cases, direction of the cumulative mean annual flux. Increases in solubility due to wintertime cooling coupled with a deepening mixed layer were responsible for ~50% of the mean annual O₂ flux in the KE. We can use the seasonal cycles in regional fluxes to determine the relative importance of summertime biological production, advection, and water mass formation in the KE and annual net community production in the Alaska Gyre, south KE, and south subtropical Pacific.