Weedlines: A Sargassum Research Blog

Physical Oceanography

Now that you have become familiar with what Sargassum is and why it is important, we would like to provide you with some information about the physical oceanographic processes that contribute to the distribution and aggregation of Sargassum features.

Free-floating species of Sargassum are distributed throughout the North Atlantic, Caribbean, and the Gulf of Mexico. The distribution of Sargassum is influenced by mesoscale features such as currents, gyres, eddies, and fronts. In the North Atlantic, a large aggregation of Sargassum is formed by transport via the Gulf Stream and other currents associated with the North Atlantic gyre. Gyres are wind-driven currents that result in the transport of water in a circular direction in an ocean basin. In the Northern Hemisphere where floating Sargassum is found, these gyres rotate in a clockwise direction as a result of Earth’s rotation and the Coriolis force. The pattern of transport in these gyres follows the contours of land surrounding the ocean basin. The North Atlantic gyre transports Sargassum to the Sargasso Sea via mesoscale eddies, which are rotating water masses that separate from the large basin-wide currents. In the Sargasso Sea, early research estimated Sargassum covered approximately two million square miles. Sargassum is also transported by the Loop Current in the Gulf of Mexico and the Florida Current, which meets with the Gulf Stream in the North Atlantic gyre.

Sargassum Distribution.jpg

Map of Sargassum distribution and associated ocean currents.
(Source: http://ocean71.com/chapters/sea-like-none-other/)

Sargassum is also commonly found along frontal regions, where a boundary forms between distinctly different water masses. Convergent fronts of water masses result in the formation of Langmuir cells, where Sargassum is commonly aggregated into windrows. Langmuir cells are formed as water masses converge and are forced down into the water column at the boundary. The movement of water away from the boundary at depth then forces the movement of the water back to the surface. This transport results in the formation of rotating cells referred to as “Langmuir cells.”

Langmuir cells.jpg

Diagram of Langmuir cells formed by convergent water masses.
(Source: Tajada-Martinez et al, 2011.  J. Appl. Mech. 79(1))

All of these mesoscale features mentioned result in the formation of Sargassum clumps, mats, and weedlines. By understanding the oceanographic processes that transport Sargassum, we can predict where our sampling efforts will be most successful on our research cruises this summer.

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