Variability of surface film distributions in a coastal ocean regime

Surface-active compounds produced by phytoplankton are preferentially adsorbed at the air-sea interface, resulting in a finite viscoelastic modulus. In low and moderate winds, ocean-atmosphere fluxes of mass, momentum and heat are strongly modulated by surface films via viscoelastic damping of small-scale waves and near-surface turbulence. Surfactant film distributions were surveyed during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) Low Wind experiments at the Martha’s Vineyard Coastal Observatory (MVCO). An automated microlayer sampling/fluorometry package was deployed to measure enrichment of surface-active colored dissolved organic matter (CDOM) in the microlayer. Surface tension was measured using calibrated spreading oils and elasticities were estimated from surface tension using an empirical relationship for the elasticity dependence on film surface pressure. These deployments focused on the response of the microlayer to changes in atmospheric forcing.

The MVCO site was significantly impacted by surface films for winds up to 6 m/s. Film distributions were patchy with features as small as a few tens of meters in scale, but with trends on kilometer scales. CDOM enrichments ranged from 2-200% of bulk CDOM concentration. Surface tension was typically 1-2 dyne/cm below that of pure seawater, but were as much as 25 dyne/cm below nominal in heavily filmed areas. Distributions were controlled by both water-side and atmospheric forcing. Banded features were frequently associated with near-surface temperature anomalies. Film distributions adjusted rapidly to increasing wind stress (erosion processes), but relatively slowly to decreasing wind stress (diffusion and readsorption). Observed CDOM and elasticity variations imply large variability in ripple damping and are consistent with reductions in the degree of saturation B(k) of 1-2 orders of magnitude for small-scale waves. These reductions in turn imply strong effects on momentum and mass transfer during low wind episodes.