Molecular diffusion sustains the flux of soluble gases on the water side of the air-sea interface. The ''handover'' of this flux to more efficient eddy mixing begins with the smallest eddies, of size l, which interact with the surface diffusion boundary layer (DBL), of thickness Δ. Owing to the discrepancy of the scales, Δ≪l, the flow field on the Δ scale consists of horizontal motions of a velocity constant with depth and varying horizontally on the l scale. The vertical velocity is proportional to the divergence of the horizontal flow and increases linearly with depth. An exact solution of the advection-diffusion equation for the simple model of divergent stagnation point flow shows the mass transfer coefficient (velocity) k to be proportional to (aD)1/2 and DBL thickness Δ to be proportional to (D/a)1/2, where a is divergence, D diffusivity. Over a solid wall a similar model of Hiemenz flow yields a more complex relationship, also involving viscosity. These models reveal the mechanism by which the DBL is kept thin.

The most intense surface divergences on a wind-blown sea surface are associated with rollers on breaking wavelets. Vorticity and divergence in the rollers are both proportional to u*2/&ngr;, where u* is friction velocity and &ngr; is viscosity. The mass transfer coefficient resulting from divergences of this magnitude is then given by k=const u* Sc-1/2, where Sc is Schmidt number. Exact solutions of the advection-diffusion equation for model rollers reveal the details of the handover process. A thin DBL is maintained over divergences by the upward velocity. At convergences, narrow downward plumes convey DBL fluid into the turbulent interior. Flux lines (analogous to streamlines) are horizontal over divergences and dive down under convergences. Application to the sea surface requires a parameter quantifying the surface density of divergences. Laboratory data imply that a substantial fraction of the surface is covered by the divergences at higher wind speeds. However, in the open ocean straining by the large waves, and especially whitecapping, may significantly reduce the density of divergences and with it the area-average gas transfer rate. On the other hand, bubble and droplet production in whitecaps may diminish this effect or even reverse it. ¿ American Geophysical Union 1990