Influence of ocean surface processes on gas transfer during SO-GasEx

The exchange of carbon dioxide and other trace gases across the air-sea interface plays an important role in global and regional biogeochemical cycles. The gas transfer velocity (k) is thought to be controlled by near-surface turbulence at low to moderate wind speeds and by bubble-mediated processes at higher wind speeds. At low to moderate wind speeds, small-scale waves including microbreaking disrupt the diffusive boundary layer, contribute to mixing at the surface, and enhance exchange. Likewise, at higher wind speeds, large-scale wave breaking, or whitecapping, generates mixing and additionally enhances gas transfer via bubble-mediated exchange. One parameterization for k based on the direct covariance fluxes from previous Gas Exchange Experiment in the North Atlantic is shown to have a cubic dependence on wind speed [Wanninkhof and McGillis, 1999]. That result supports the hypothesis that, if bubble mediated exchange is important, the transfer velocity should increase proportionally with whitecap coverage (e.g., Keeling [1993]), since whitecap coverage been shown to increase with at least a cubic dependence on wind speed (e.g., Monahan and O-Muircheartaigh [1980]). However, the very large uncertainties under high wind speed conditions limit the universality of this result and the role that breaking waves and bubble modulated transfer play in air-sea gas exchange.

Here, we present results of the combination of turbulence, deep-ocean wave statistics, whitecapping, and CO2 gas exchange measured during the Southern Ocean Gas Exchange Experiment (SO GasEx) with sustained conditions between 10-20 m s-1. Directional ocean wave spectra, significant wave height, peak wave period, and peak wave direction were obtained with a Wave and Surface Current Monitoring System (WaMoS® II). WaMoS® II also has the capability to resolve two-dimensional maps of surface elevation snapshots with the significant advantage of continuous availability of wave data in rough seas. In addition, significant wave height was measured using a laser altimeter as well as a nadir-looking microwave system. Oceanic turbulent kinetic energy dissipation rates were measured using a pulse-coherent Doppler sonar mounted at 2-m depth from a drifting surface buoy. Lastly, wave-breaking statistics and whitecapping coverage are reported using two high-resolution digital cameras. We present results of process studies that investigate the various models for gas transfer (e.g., Asher and Wanninkhof [1998], Woolf [2005], Zhao et al. [2003]) that incorporate turbulence, sea state, and wave-breaking statistics with the goal of developing a focused parameterization.