The Influence of Continental Freshwater Discharge upon Oceanographic Conditions on the Shelf of Southern Brazil and Uruguay

A number of field missions conducted in the area over the last two decades have revealed that the South Brazilian and Uruguaian shelves are dominated by a belt of low salinity water which extends for hundreds of miles along the coast. These relatively fresh waters originate from the admixture of river runoff. The two major sources of the freshwater inflow in the region are Plata river and Patos-Mirim lagoon estuaries. Some previous studies have attributed the low salinity observed over the shelf between 30 and 35 degrees of southern latitude to the discharge of Patos-Mirim lagoon. However, our recent combined ship/helicopter CTD survey of the inner shelf adjacent to the lagoon mouth indicated that despite of the inicial southward momentum of the discharged waters, the plume veers eastward, so that south of the estuary the thermohaline signature of the plume is observable only within a distance of 10-15 kilometers from the mouth. Thus the hypothesis that relates low salinity values observed over the Southern Brazilian shelf south of Rio Grande is not supported by our data. Rather, the low salinity belt can be related to Plata river discharge transported by a northward coastal current that we refer to as the Rio Grande Current (RGC). The existence of RGC, first predicted by our inverse model of regional circulation, was later confirmed by direct current measurements at a mooring station. Analysis of local historical data revealed an anomalously strong secular warming trend of SST (up to 1.6 degrees C per 100 years) in the areas controlled by freshwater input from Plata river and Patos-Mirim lagoon. The trend may be related to human impact upon the river runoff in the region.