With the tide

Gigantic rings formed by the North Brazil Current, which transport nutrients from the Amazon River to the Northern Hemisphere, are more intense than expected

By Mônica Pileggi

Agência FAPESP – At certain times of the year, sailing along the far northern coastline of Brazil  may not be an easy task. The sensation of swimming against the tide is a sign that the sailor could be passing through a region in which formation of gigantic water rings is common.

This phenomenon is formed by the North Brazil Current, during which part of its flow revolves around itself and feeds the North Equatorial Counter Current, explains Guilherme Pimenta Castelão, researcher at the Ocean-Atmosphere Modeling Group at the National Space Research Institute (Inpe).

The rings were investigated by Castelão in his doctoral research. “The retro flow encloses, forming rings that span 400 kilometers in diameter. And these vortexes propagate to the Northeast, accompanying the coast,” he told Agência FAPESP. One of the researcher’s activities at Inpe is sharing knowledge about these rings with the group participating in FAPESP Research Program on Global Climate Change (RPGCC) for development of the Brazilian Global Climate System Model (MBSCG).

The North Brazil Current rings have been observed by researchers for years. Nevertheless, in an article published by the Journal of Geophysical Research, Castelão and his advisor, Bill Johns (of the University of Miami’s Rosenstiel School of Marine and Atmospheric Science), provide more details about the dynamic shielding of these marine vortexes thanks to Acoustic Doppler Current Profiler (ADCP) equipment.

The acoustic profile measures the velocity of currents at different depths using sonar pulses. Based on the data collected, the researcher discovered that the rings are more intense than originally thought, moving approximately 1 meter per second, and its maximum height of 38 centimeters is almost double that originally imagined.

“Due to the limited availability of quality data and the sparse understanding of these rings, it was believed that they were responsible for roughly 10% of this transportation. Today, we can affirm that the rings transport almost half of the warm current exchanged with the Northern Hemisphere,” says Castelão.   
 
The exchange of waters is a large scale system in which the two hemispheres exchange mass (salt) and heat. “There is a natural demand for this transportation. One of the ways that water is transported from one hemisphere to the other is through these rings,” he explains.

Maritime shields

With the high resolution data and speed in hand, the scientists observed a property known as vorticity, which is related to the tendency of a water flow to spin.

“With this, it is possible to characterize the structure of velocity and separate the ring in two regimes: internal – the equivalent of a solid body, spinning as if it was a disk – and the exterior, with the opposite vorticity characteristic of the internal regime. This second regime creates a dynamic shield, isolating the interior,” says Castelão.

Because of this kilometers-long shield where water velocity reaches 1 meter per second (considered high by specialists), many of the nutrients found in the Amazon River’s warm waters get stuck and are transported large distances to the Northeast.

Knowing more about the frequency – roughly six times per year – and the season – it is more common in the Southern Hemisphere’s autumn – when these rings occur is fundamental for shipping and offshore oil exploration.

The article Sea surface structure of North Brazil Current rings derived from shipboard and moored acoustic Doppler current profiler observations (doi:10.1029/2010JC006575) by G. P. Castelão and W. E. Johns can be read  subscribers of the Journal of Geophysical Research-Oceans at:  www.agu.org/pubs/crossref/2011/2010JC006575.shtml.