The goal of the project is to use the observed variability in the tracer �??atmospheric potential oxygen�?� APO �?? O2 + 1.1CO2 to improve estimates of the ocean �??priors�?�. APO is insensitive to land biospheric exchanges but shows strong variability of oceanic origin that depends on the rate of ocean uptake of CO2, ocean production/respiration, warming/cooling, and mixing – all processes which impact the air-sea CO2 flux. These links can be exploited to improve air-sea CO2 flux estimates. The main thrust will be to test and refine the �??prior�?� air-sea CO2 fluxes used in CarbonTracker by reconciling these fluxes also with annual-mean and seasonal air-sea fluxes of APO, as constrained by atmospheric data. This will involve separately developing constraints on the annual-mean component of the flux of APO and on its seasonal variation. In addition, this work will lead to additional constraints on the TM5 transport model used in CarbonTracker by comparing predictions of the vertical distribution of APO against observations. The project will take advantage of the rapid expansion of atmospheric O2 measurements over the past decades by programs at Scripps, at Princeton University, and at the National Institute for Environmental Studies (NIES) in Japan. It will also benefit from data recently collected on the NSF-funded �??HIPPO�?� (HIAPER Pole-to-Pole Observations) airborne campaign, which includes data from nearly pole to pole from the surface to the lower stratosphere on transects down the Pacific Ocean over five seasons. This work will help reduce uncertainty in ocean CO2 priors arising from uncertainty from modeled ocean transports, sparseness in pCO2 datasets, and it will help reduce uncertainty in the effects of atmospheric transport. The study will build on a method recently developed by Christian Roedenbeck of Max Planck Institute for Biogeochemistry in Jena (MPI Jena), Germany in collaboration with the PI and others that links CO2 and O2 fluxes in an ocean mixed-layer closure scheme for addressing the seasonal carbon-O2 linkages.