Summertime atmospheric boundary layer gradients of O2 and CO2 over the Southern Ocean

We present airborne observations of the vertical gradient of atmospheric oxygen (delta(O-2/N-2)) and carbon dioxide (CO2) through the atmospheric boundary layer (BL) over the Drake Passage region of the Southern Ocean, during the O-2/N-2 Ratio and CO2 Airborne Southern Ocean Study, from 15 January to 29 February 2016. Gradients were predominately anticorrelated, with excesses of delta(O-2/N-2) and depletions of CO2 found within the boundary layer, relative to a mean reference height of 1.7 km. Through analysis of the molar ratio of the gradients (GR), the behavior of other trace gases measured in situ, and modeling experiments with the Community Earth System Model, we found that the main driver of gradients was air-sea exchange of O-2 and CO2 driven by biological processes, more so than solubility effects. An exception to this was in the eastern Drake Passage, where positive GRs were occasionally observed, likely due to the dominance of thermal forcing on the air-sea flux of both species. GRs were more spatially consistent than the magnitudes of the gradients, suggesting that GRs can provide integrated process constraints over broad spatial scales. Based on the model simulation within a domain bounded by 45 degrees S, 75 degrees S, 100 degrees W, and 45 degrees W, we show that the sampling density of the campaign was such that the observed mean GR (+/- standard error), -4.0 +/- 0.8 mol O-2 per mol CO2, was a reasonable proxy for both the mean GR and the mean molar ratio of air-sea fluxes of O-2 and CO2 during the O-2/N-2 Ratio and CO2 Airborne Southern Ocean Study.