Meeting Documents

The Potential of the OCO-2 Observations to Constrain Air-Sea CO₂ Fluxes

Abstract

The measurements of column-averaged atmospheric CO₂ concentration (XCO₂) from the Orbiting Carbon Observatory-2 (OCO-2) have the potential to provide additional observational constraints on air-sea CO₂ fluxes by filling the gap in direct ocean measurements. However, the progress in understanding air-sea CO₂ fluxes with XCO₂ from OCO-2 has been limited. This presentation introduces the challenges in air-sea CO₂ flux estimation with XCO₂ observations and proposes an optimal strategy to estimate air-sea CO₂ fluxes. The performance of different inversion set-ups is evaluated through Observing System Simulation Experiments (OSSEs) by comparing the optimized fluxes with assumed true fluxes. The OSSE results indicate that the conventional inversion, simultaneously optimizing terrestrial biosphere and air-sea fluxes, reduces root mean square errors (RMSEs) in regional monthly air-sea fluxes by up to 22–24% and 6–10% in the low (<40°) and high (>40°) latitudes, respectively, with up to 22% error reduction in global annual air-sea fluxes. These limited adjustments are associated with an order of magnitude higher errors in terrestrial biosphere flux variability than in air-sea fluxes that drive atmospheric CO₂ changes. To isolate ocean signals within XCO₂ changes, we proposed a sequential approach that optimizes only air-sea fluxes with simulated ocean XCO₂ while prescribing optimized terrestrial biosphere fluxes optimized by simulated land XCO₂ as background fluxes. The sequential inversion achieves an 11% additional error reduction in global annual air-sea fluxes and a 33% further RMSE reduction in monthly air-sea fluxes in the southern high latitudes. However, we find that potential biases in ocean XCO₂ measurements may lead to a 24% RMSE increase in the Southern Ocean, despite employing sequential inversion. Our results highlight that sequential inversion is a promising technique for improving seasonal air-sea flux estimates in the Southern Ocean but mitigation of OCO-2 measurement biases is required for practical applications.