Covering over 60 percent of the Earth's surface, the world's oceans are a major player in the Earth's carbon budget because they act as a significant sink of both natural and anthropogenic carbon dioxide (CO2). With a 36 % increase in atmospheric CO2 since the industrial revolution, a complete understanding of gas flux through the air-sea interface is a particularly pressing and time-sensitive issue.
Earlier studies developed gas exchange models based on parameterizations which
depend on the 10 meter wind speed, along with other easily measured in situ variables. Recent measurements suggest that fluxes may be significantly less than those predicted by the parameterizations, indicating that diffusive gas transport across the air-sea interface may depend on the sea-state rather than just the wind speed. This result is not surprising since previous parameterizations were derived from our knowledge of engineering flows over flat, rigid surfaces rather than flow over the marine boundary layer. The difference between the two is that individual stress and flux-scale components are not well resolved and further complicated by the presence of a free surface at the boundary. Thus, at the surface, the total stress is partitioned into viscous and (augmented by the existence of capillary waves), wave growth (momentum flux into the ocean which sustains the surface curvature), and air-flow separation component (occurring when the near surface air is unable to follow the local sea state). The gas scales are similarly partitioned into molecular and non-molecular diffusive components. The figure below shows some initial results from our theoretical study. The top shows scaled gas transfer velocity as a function of ten meter wind speed while the bottom shows the scaled gas transfer velocity as a function of friction velocity along with observations from a variety of tracer, gas flux experiments.
The study of air-sea gas exchange is multifaceted. Although the sea-state dependent interfacial model is a key element for understanding gas transport, breaking waves and bubbles remain the 'golden goose.' Breaking waves act as a source of vorticity and turbulence within the ocean surface layer, they dissipate surface-wave energy, and enhance gas transfer drawing turbulence closer to the boundary layer while entraining bubbles. Indeed the impact of bubbles is thought to increase the total transfer velocity (ktotal = kinterface + kbubbles) by as much as 50 percent. The cartoon below hopes to capture the complexity of this research. The dark blue line represents a spilling breaker, while the light blue line indicates the likely trajectory of a parcel of air just above the ocean surface. On the leeward side of the spilling breaker, air flow separation occurs and turbulence dominates within the separation bubble. A similar phenomena occurs on the oceanic side, indicated by the dotted green line. Mean CO2 concentration profiles (i.e. pCO2) have a logarithmic structure in the turbulent layer and a linear structure near the interface where molecular transport dominates. The vertical distribution of stress in the atmospheric layer is shown; the green profile is the turbulent stress, the red profile is the viscous stress, and the orange line represents the wave-induced stress. Within the bubble plume, bubbles will either completely dissolve (leading to a full exchange of gas) or they will expand and contract while ascending to the surface (giving a partial exchange of gas).
With an encompassing interfacial and bubble gas exchange model in place, interesting questions can begin to be answered. The obvious question pertains to high-wind events; to what extent does bubble mediated gas flux play a role and furthermore, under which sea states does bubble mediated flux dominate (if at all)? To what extent do isolated large wind and fetch event impact in overall gas exchange; is there a plateau in the total air-gas fluxes and under which sea-state conditions? Finally, how robust are these results under changing saturation and atmospheric gas concentrations