Estimating net community production in the Southern Ocean based on atmospheric potential oxygen and satellite ocean color data

Abstract

The seasonal cycle of atmospheric potential oxygen (APO ∼ O2 + 1.1 CO2) reflects three seasonally varying ocean processes: 1) thermal in‐ and outgassing, 2) mixed layer net community production (NCP) and 3) deep water ventilation. Previous studies have isolated the net biological seasonal signal (i.e., the sum of NCP and ventilation), after using air‐sea heat flux data to estimate the thermal signal. In this study, we resolve all three components of the APO seasonal cycle using a methodology in which the ventilation signal is estimated based on atmospheric N2O data, the thermal signal is estimated based on heat flux or atmospheric Ar/N2 data, and the production signal is inferred as a residual. The isolation of the NCP signal in APO allows for direct comparison to estimates of NCP based on satellite ocean color data, after translating the latter into an atmospheric signal using an atmospheric transport model. When applied to ocean color data using algorithms specially adapted to the Southern Ocean and APO data at three southern monitoring sites, these two independent methods converge on a similar phase and amplitude of the seasonal NCP signal in APO and yield an estimate of annual mean NCP south of 50°S of 0.8–1.2 Pg C/yr, with corresponding annual mean NPP of ∼3 Pg C/yr and a mean growing season fratio of ∼0.33. These results are supported by ocean biogeochemistry model simulations, in which air‐sea O2 and N2O fluxes are resolved into component thermal, ventilation and (for O2) NCP contributions.