Analysis of surface drifter trajectories in the tropical Atlantic to track Sargassum.

Abstract

The Great Atlantic Sargassum Belt is growing in density and areal extent with an unexplained annual cycle. Large quantities of Sargassum have recently been reported in the central Atlantic Ocean and the Caribbean Sea, accompanied by frequent beaching events that have caused serious environmental, ecological, and economic problems. Sargassum consists of leafs that look like berries, these berries are filled with oxygen, allowing Sargassum to float on the surface. Using data collected in the summer of 2019 from undrogued Stokes and drogued SVP drifters, we investigate how the depth of the drifters impacts their trajectories and dispersion. We analyse for similarities and differences by comparing the difference in distance between drifters that are part of a pair, examining drifter trajectories, comparing their velocities and checking if there is a relation with the wind, waves or currents. We compare the trajectories of the drifters going eastward and we find that the undrogued Stokes drifters vary little in latitude while the drogued SVP drifters veer more south. By using multi linear regression, we find that this is possibly due to the wind. For the drifters going westward, we see that some of the drogued SVP drifters follow a different trajectory due to the presence of inertial oscillations. By comparing the velocities for the drifters, we find that the undrogued Stokes drifters drift faster than the drogued SVP drifters. This is expected because the wind has more effect on the surface than on the deeper layers in the ocean. We can conclude that if there will be any further research done into Sargassum, it matters if they use drogued or undrogued drifters. Sargassum floats at the surface thus the undrogued drifters are a better choice.