Air-sea gas exchange at extreme wind speeds measured by autonomous oceanographic floats

Measurements of the air-sea fluxes of N-2 and O-2 were made in winds of 15-57 m s(-1) beneath Hurricane Frances using two types of air-deployed neutrally buoyant and profiling underwater floats. Two "Lagrangian floats" measured O-2 and total gas tension (GT) in pre-storm and post-storm profiles and in the actively turbulent mixed layer during the storm. A single "EM-APEX float" profiled continuously from 30 to 200 in before, during and after the storm. All floats measured temperature and salinity. N-2 concentrations were computed from GT and O-2 after correcting for instrumental effects. Gas fluxes were computed by three methods. First, a one-dimensional mixed layer budget diagnosed the changes in mixed layer concentrations given the pre-storm profile and a time varying mixed layer depth. This model was calibrated using temperature and salinity data. The difference between the predicted mixed layer concentrations of O-2 and N-2 and those measured was attributed to air-sea gas fluxes F-BO and F-BN. Second, the covariance flux F-CO(z)=(wO(2)')(z) was computed, where w is the vertical motion of the water-following Lagrangian floats, O-2' is a high-pass filtered O-2 concentration and O(z) is an average over covariance pairs as a function of depth. The profile F-CO(z) was extrapolated to the surface to yield the surface O-2 flux F-CO(O). Third, a deficit of O-2 was found in the upper few meters of the ocean at the height of the ston-n. A flux F-SO, moving O-2 out of the ocean, was calculated by dividing this deficit by the residence time of the water in this layer, inferred from the Lagrangian floats. The three methods gave generally consistent results. At the highest winds, gas transfer is dominated by bubbles created by surface wave breaking, injected into the ocean by large-scale turbulent eddies and dissolving near 10-m depth. This conclusion is supported by observations of fluxes into the ocean despite its supersaturation; by the molar flux ratio F-BO/F-BN, which is closer to that of air rather than that appropriate for Schmidt number scaling; by O-2 increases at about 10-m depth along the water trajectories accompanied by a reduction in void fraction as measured by conductivity; and from the profile of F-CO(z), which peaks near 10 in instead of at the surface. At the highest winds O-2 and N-2 are injected into the ocean by bubbles dissolving at depth. This, plus entrainment of gas-rich water from below, supersaturates the mixed layer causing gas to flux out of the near-surface ocean. A net influx of gas results from the balance of these two competing processes. At lower speeds, the total gas fluxes, F-BO, F-BN and F-CO(O), are out of the ocean and downgradient. (c) 2006 Elsevier B.V. All rights reserved.