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

Lay Abstract Organisms that float at the ocean surface such as the macroalga Sargassum and the cyanobacterium Trichodesmium often accumulate in lines visible in satellite imagery. These lines have lateral scale separations of a few kilometers and cannot be explained by Langmuir circulation. Here we discuss the physical process responsible for the accumulation of floating materials in the ocean in the presence of mesoscale (tens of kilometers) and submesoscale (few kilometers) turbulence. Using high‐resolution simulations in both an idealized domain (a 3‐dimensional box where turbulence is excited by small‐scale winds and eddies form) and a realistic domain (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 meters below the surface. The strong divergence and convergence zones generated at the ocean surface by ageostrophic processes (i.e., processes for which an exact balance between the Coriolis force and the pressure gradient force is not attained) at scales between hundreds of meters and a few kilometers are responsible for the creation of lines where the floating material accumulates. Floating particles in the model simulations are expelled from the core of eddies and concentrate in small filaments. The resulting patterns are similar to the distribution of Sargassum seen in satellite images. Such organization may be common in many surface‐dwelling organisms and could play a role in enhancing biological interactions.