Biological and physical controls on the oxygen cycle in the Kuroshio Extension from an array of profiling floats

Abstract

The Kuroshio Extension (KE) is a current associated with the largest CO2 flux into the Pacific Ocean, a broad region of uptake that 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. Because oxygen is stoichiometrically linked to changes in dissolved inorganic carbon due to photosynthesis and respiration and subject to many of the same physical drivers as the CO2 flux, in situ oxygen measurements help to determine the processes driving this large CO2 flux. We analyzed data from eight Argo profiling floats equipped with oxygen sensors to estimate oxygen fluxes in the upper ocean of the KE region (approximate bounds: 25°N to 45°N, 135°E to 155°E). In situ air calibrations of these sensors allowed us to accurately measure air-sea oxygen differences, which largely control the flux of oxygen to and from the atmosphere. To characterize distinct biogeographical regions in the Kuroshio Extension and to accommodate seasonal north-south shifts in the location of the regional boundaries, we averaged oxygen measurements from different floats along isopycnal surfaces into 3 regions based on temperature-salinity relationships: North KE, Central KE, and South KE. Using these regional concentration time series, we determined the physical fluxes using an upper ocean layered model and calculated the residual oxygen flux, a combination of column-integrated net community production and physical processes unexplained by the model. The annual oxygen budget is largely a balance of air-sea exchange and the residual oxygen term. Residual oxygen fluxes are -5.4 ± 1.1, -5.9 ± 0.1, and 2.1 ± 2.2 mol O2 m-2 yr-1 (where negative is a loss from the upper ocean) for the North, Central, and South KE regions, respectively. The North and Central KE are regions of mode water formation, which balances the large air-sea fluxes into the ocean. The South KE oxygen residual indicates a biologically produced flux to the atmosphere in two out of three years that agrees with previous estimates of subtropical annual net community production (ANCP) but exhibits high interannual variability. This study suggests that physical processes are the primary drivers for the annual uptake of the gases oxygen and carbon dioxide in the Central KE region where the annual CO2 uptake is strongest.