Pattern formation at the ocean surface: Sargassum distribution and the role of the eddy field

Positively buoyant organisms such as the macroalga Sargassum and the cyanobacterium Trichodesmium often form surface accumulations visible in satellite imagery that have lateral scale separation of tens of kilometers and cannot be explained by Langmuir circulation. Here we discuss the accumulation of floating materials in the ocean in presence of meso- and submesoscale activity. Using high-resolution simulations of the ocean mesoscale in both idealized (a 3-dimensional box where coherent eddies are forced by small-scale winds) and realistic domains (western Gulf of Mexico) where extensive concentrations of floating Sargassum have been recorded in satellite images, we show that the distribution of tracers at the ocean surface departs rapidly from that observed a few tens of meters below it. Such distribution does not resemble that observed for passive tracers in quasi-geostrophic turbulence. The strong divergence and convergence zones generated at the surface by ageostrophic processes in the submesoscale range are responsible for the creation of areas where the floating material accumulates. Floating particles are expelled from the core of mesoscale eddies and concentrate in convergence regions in patterns comparable to the ones observed through the satellite images. In light of those results, Sargassum and/or Trichodesmium may provide a useful proxy to track convergence/divergence processes resulting from ageostrophic processes at the ocean surface.