The Atmospheric Potential Oxygen forward Model Intercomparison Project (APO-MIP1): evaluating simulated atmospheric transport of air-sea gas exchange tracers and APO flux products

Atmospheric Potential Oxygen (APO, defined as O₂ + 1.1 × CO₂) is primarily a tracer of ocean biogeochemistry and fossil fuel burning. APO exhibits strong seasonal variability at mid-to-high latitudes, driven mainly by seasonal air-sea O₂ exchange. We present results from the first version of the Atmospheric Potential Oxygen forward Model Intercomparison Project (APO-MIP1), which forward transports three air-sea APO flux products in eight atmospheric transport models or model variants, aiming to evaluate atmospheric transport and flux representations by comparing simulations against surface station, airborne, and shipboard observations of APO. We find significant spread and bias in APO simulations at eastern Pacific surface stations, indicating inconsistencies in representing vertical and coastal atmospheric mixing. A framework using airborne APO observations demonstrates that most atmospheric transport models (ATMs) participating in APO-MIP1 overestimate tracer diffusive mixing across moist isentropes (i.e., diabatic mixing) in mid-latitudes. This framework also enables us to isolate ATM-related biases in simulated APO distributions using independent mixing constraints derived from moist static energy budgets from reanalysis, thereby allowing us to assess large-scale features in air-sea APO flux products. Furthermore, shipboard observations show that ATMs are unable to reproduce seasonal APO gradients over Drake Passage and near Palmer Station, Antarctica, which could arise from uncertainties in APO fluxes or model transport. The transport simulations and flux products from APO-MIP1 provide valuable resources for developing new APO flux inversions and evaluating ocean biogeochemical processes.