Shipboard Remote Sensing of Whitecap Lifetime

Under influence from the wind, waves grow until they become unstable and break. Breaking entrains air, creating plumes of bubbles below the surface. Smaller bubbles dissolve into the water, larger bubbles rise and form whitecaps. Whitecaps can be distinguished as either actively generated (stage A), or decaying (stage B). Stage A marks an acoustic period with turbulence, energy dissipation, ocean mixing, increased surface roughness, and bubble-driven gas diffusion. Buoyant bubbles drag water upward bringing surface active material and creating regions of divergence which enhance air-sea gas transfer. Stage A generation also enhances spray through the tearing of wave crests which significantly enhance sensible and latent heat fluxes. At stage B the bursting of bubbles produces small droplets which reside in the air long enough to reach moisture equilibrium and transform into sea salt aerosols. These have been found to increase planetary albedo directly and indirectly by acting as cloud condensation nuclei, they are also linked to the removal of atmospheric surface ozone and the activation of halogens, leading to ozone depletion. Whitecap coverage is used expansively to estimate these processes yet the routinely employed wind speed dependence has an order of magnitude uncertainty that, in large part, is due to significant variations in stage B lifetime. Here we present initial results from the first of two research cruises to understanding the factors at the air-sea interface which influence stage B lifetime. Whitecaps were observed using visible and infrared remote sensing which provides clear, unambiguous separation of stage A from stage B. Concurrent measurements were made of mean atmospheric and oceanic variables as well as momentum and enthalpy fluxes, surface tension, and subsurface bubble plume characteristics. These data were supplemented with wave information taken from existing buoys networks.