The effects of river discharge and seasonal winds on the shelf off southeastern South America

• Published in: Continental shelf research Vol. 28; no. 13; pp. 1607 - 1624
• Main Authors: Möller, Osmar O., Piola, Alberto R., Freitas, Ana Cristina, Campos, Edmo J.D.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 30-07-2008
• Subjects: Brackish; Freshwater; Marine; River discharge; Seasonal variations; Southwest Atlantic; Water masses; Wind effect; Brazil, Santa Catarina; Colombia, Magdalena, Santa Marta; PSW, Argentina; PSW, South America, Rio de la Plata; ASW, Brazil, Santa Catarina; Argentina, Buenos Aires, Mar Del Plata; South America, Rio de la Plata; PSW, Argentina, Buenos Aires, Mar del Plata; ASW, Colombia, Magdalena, Santa Marta; Seasonal variations; Wind effect; Water masses; River discharge; Southwest Atlantic
• ISSN: 0278-4343; 1873-6955
• DOI: 10.1016/j.csr.2008.03.012

The Río de la Plata waters form a low salinity tongue that affects the circulation, stratification and the distributions of nutrients and biological species over a wide extent of the adjacent continental shelf. The plume of coastal waters presents a seasonal meridional displacement reaching lower latitudes (28°S) during austral winter and 32°S during summer. Historical data suggests that the wind causes the alongshore shift, with southwesterly (SW) winds forcing the plume to lower latitudes in winter while summer dominant northeasterly (NE) winds force its southward retreat. To establish the connection between wind and outflow variations on the distribution of the coastal waters, we conducted two quasi-synoptic surveys in the region of Plata influence on the continental shelf and slope of southeastern South America, between Mar del Plata, Argentina and the northern coast of Santa Catarina, Brazil. We observed that: (A) SW winds dominating in winter force the northward spreading of the plume to low latitudes even during low river discharge periods; (B) NE winds displace the plume southward and spread the low salinity waters offshore over the entire width of the continental shelf east of the Plata estuary. The southward retreat of the plume in summer leads to a volume decrease of low salinity waters over the shelf. This volume is compensated by an increase of Tropical waters, which dominate the northern shelf. The subsurface transition between Subantarctic and Subtropical Shelf Waters, the Subtropical Shelf Front, and the subsurface water mass distribution, however, present minor seasonal variations. Along shore winds also influence the dynamics and water mass variations along the continental shelf area. In areas under the influence of river discharge, Subtropical Shelf Waters are kept away from the coastal region. When low salinity waters retreat southward, NE winds induce a coastal upwelling system near Santa Marta Cape. In summer, solar radiation promotes the establishment of a strong thermocline that increases buoyancy and further enhances the offshore displacement of low salinity waters under the action of NE winds.