Testing model estimates of recent ocean carbon uptake and deoxygenation using atmospheric and oceanic data

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Model studies suggest that global warming may reduce the ocean uptake of CO2 via changes in ocean circulation and carbon chemistry and lead to reductions of dissolved O2 in the ocean interior, causing an expansion of O2 minimum zones. These processes are possibly already underway in response to warming and wind changes over the past few decades. In this project, researchers at the Scripps Institution of Oceanography will test ocean model simulations of air-sea O2 and CO2 exchange using an approach emphasizing constraints from combined measurements of atmospheric O2/N2 ratio and CO2 concentrations and ocean measurements of chloro-fluorocarbons (CFCs). The approach is based on computing tracer "atmospheric potential oxygen" (APO), which is effectively the weighted sum O2 + 1.1CO2. By construction, APO is insensitive to land biospheric exchanges of CO2 and O2. Observations by the Scripps and Princeton O2 programs show that APO has decreased over the past two decades at a precisely measured rate. Two components of the APO decrease are well understood: (1) a fossil-fuel component and (2) a component related to uptake of anthropogenic CO2 by the oceans, which can be estimated based on ocean observations of CFCs. These two components can both be precisely quantified, and together appear to overestimate the observed APO decrease. The residual APO change constrains the air-sea exchanges of O2 and CO2 not counted as anthropogenic, i.e. due to climate impacts on the oceans. The APO trend is thus directly relevant to quantifying ongoing changes in ocean CO2 and O2. A core activity of this proposal will involve developing and applying data targets for model validation based on the observed APO changes. Working closely with four ocean modeling groups, the Scripps researchers will acquire output of model runs that simulate air-sea exchange of O2 and CO2 over the past few decades in response to climate changes and rising CO2. They will test not only the ability of the models to account for the long-term trends in APO but also for the seasonal cycles, which relate to the processes controlling O2 exchange at middle and high latitudes. Broader Impacts: This work will help to solidify the scientific understanding of two looming and related environmental threats: climate amplification of rising CO2 and ocean deoxygenation. It will also complement ongoing measurement activities of the Scripps O2 program by sustaining a small but viable modeling component. The project will enable dissemination of cutting edge research to the public at large through public presentations and will help foster young talent through support for a young scientist and a graduate student.