Historical hydrographic data from the continental shelf off eastern South America are used to examine the thermohaline properties of the water masses in the region between 20°S to 40°S. The continental shelf water masses are originated by dilution of open ocean waters of the western boundary currents of the South Atlantic Ocean. Based on the temperature-salinity relation two distinct water masses are identified, namely the Subantarctic Shelf Water and the Subtropical Shelf Water. Subantarctic Shelf Water originates by dilution of Subantarctic Water, primarily in the southeast Pacific, due to excess precipitation and continental runoff enters the continental shelf near 55°S. The Subtropical Shelf Water is modified South Atlantic Central Water diluted by continental runoff from the coast of Brazil. In addition, substantial dilution of the upper shelf waters takes place at the mouth of Río de la Plata (approximately located at 36°S) and, in a lesser extent, at the Patos/Mirim Lagoon (at 32°S). The Río de la Plata, and the Patos outflows form a low salinity tongue that caps the shelf water leading to a salinity decrease to values well under 30 x 10-3. The low salinity tongue extends northward over the shelf and penetrates further north in winter than in summer.
There is little or no indication of mixing between subantarctic Shelf Water and Subtropical Shelf Water. These water masses are separated by an intense temperature, salinity and nutrient front. The front is oriented along the north-south direction and its location is variable. On average the front originates near the coast of southern Brazil at 32°S and extends southward towards the shelf break near 36°S. Based on the temperature and salinity distribution, beneath the low salinity surface layer the Shelf Front appears as an extension of the Brazil/Mlavinas Confluence over the continental shelf of South America. Thus, the distribution of the continental shelf water masses and the location and dynamics of the Shelf Front are probably linked to the dynamics and variability of the Brazil/Malvinas Confluence.