Monitoring the Sea Surface Microlayer (SML) on Sentinel images

Slicks on the sea surface are usually related to oil spills, algal blooms or organic runoff around coastlines. An extensive network of slicks extending across the English Channel is seen on Sentinel 1 and Sentinel 2 images and are identified as comprising a film of natural surfactant material within the sea surface microlayer (SML). As the SML represents the interface between ocean and atmosphere, controlling the vital exchange of gases and aerosols, identification of the slicks on images can add a new dimension to climate modelling. Current models use primary productivity often combined with wind speed, but quantifying the global extent of surface films spatially and temporally is difficult due to their patchy nature. The slicks are shown to be visible on Sentinel 2 optical images affected by sun glint, due to the wave dampening effect of the surfactants. On a Sentinel 1 SAR image of the same day, they can be identified using the VV polarised band. The paper investigates the nature and spectral properties of the slicks in relation to sun glint, and evaluates the performance of chlorophyll-a, floating algae and floating debris indices on the slick-affected areas. No index was able to distinguish slicks from non-slick areas as successfully as the original sun glint image. This image was used to devise a tentative Surfactant Index (SI) which indicates over 40 % of the study area covered by slicks. As ocean sensors have lower spatial resolution and are generally designed to avoid sun glint, Sentinel 1 SAR may offer a useful alternative for monitoring the global spatial extent of surface films, until dedicated sensors and algorithms can be developed.