O Rio da Prata, um dos maiores rios da Terra, descarrega no oceano águas de uma bacia de drenagem que cobre uma ampla área da América do Sul. Sua pluma extende-se ao longo do norte da Argentina, Uruguay e sul do Brasil influenciando amplamente os ecossistemas costeiros. A despeito disso, pouco se sabe a respeito dos mecanismos que a controlam. Relatamos aqui simulações conduzidas com o modelo POM na investigação do papel dos ventos e da descarga fluvial na dinâmica da pluma do Prata. Descargas com valores médios climatológicos e magnitudes representativas de El Niño e La Niña foram explorados. Somente sob descarga fluvial, a velocidade média de penetração da pluma ao longo da costa foi diretamente relacionada à vazão. Sua extensão variou entre 850 e 1550 km e no caso da vazão média uma banda de baixa salinidade formou-se desde o estuário até 30ºN na plataforma continental sul brasileira. O efeito de ventos de sudoeste, que causam subsidência, e de nordeste, que promovem a ressurgência costeira, foram investigados após 130 dias de descarga fluvial. Os resultados sugerem que a distribuição de baixa salinidade é muito mais sensível à direção dos ventos do que à descarga fluvial. Ventos de sudoeste são capazes de advectar o sinal de baixa salinidade ao longo da costa. Já os ventos de nordeste demonstram ser eficientes na erosão da pluma, que é destacada da costa por deriva de Ekman. Uma intrusão anormal da pluma de baixa salinidade em direção a baixas latitudes pode ser o resultado da posição original da pluma acoplada a eventos persistentes de fortes ventos de sudoeste.

The Rio de la Plata, one of the largest rivers on Earth, discharges into the ocean waters from basin that covers a large area of South America. Its plume extends along northern Argentina, Uruguay, and southern Brazil shelves strongly influencing the ecosystems. In spite of this, little is known about the mechanisms that control it. Here we report results of simulations with POM carried out to investigate the roles of wind and river discharge in Plata plume dynamics. Different outflows were explored, including an average climatological value and magnitudes representative of La Niña and El Niño. Forcing the model with river discharge the average plume speed was directly related to the outflow intensity. The Plata northward extension varied from 850 to 1550 km and for average discharge a band of low salinity waters formed from the estuary up to 30ºN of South Brazilian Shelf. Upwelling and downwelling winds were applied after 130 days. The distribution of low salinity waters over the shelf was more sensitive to the wind direction than to the river outflow variability. Downwelling winds were very capable of advecting the low salinity signal downshelf. Upwelling winds were efficient in eroding the plume, which was basically detached from the coast by Ekman drift. Abnormal plume intrusions toward low latitudes may be a result of the original plume position coupled with events of persistent strong downwelling favorable winds.

Satellite ocean colour data from the Sea-Viewing Wide Field-of-View Sensor are analyzed to monitor the space-time variability of the Plata river plume over the continental shelf and slope. SeaWiFS Chlorophyll-a (CSAT) product is used to understand the nature of changes in the colour of waters and how this relates to variations in wind direction and river outflow. We use a 5-year data set (1998-2002) of Global Area Coverage monthly mean composites. The analysis of CSAT data reveals strong seasonal variability over the continental shelf with maximum northeastward penetrations in the austral winter (June to August). The plume may reach ~27S, about 1000 km northeast of the Plata estuary. In summer the plume is confined to south of 32°S and the Plata waters are frequently exported to the open ocean through the Brazil/Malvinas Confluence. This seasonal shift is similar to the variation of surface salinity observed in historical hydrographic data and is presumably associated to the variable influence of the along-shore wind stress. Interannually, 2002 showed the maximum northeastward penetration and the strongest anomalies (6mg/m³). From late 1999 to early 2000, a strong negative CSAT anomaly (<6mg/m³) within the estuary could be linked to a period of very low Plata outflow (<12,000m³/seg). Results suggest that CSAT geophysical product is a good indicator of the presence of the Plata diluted waters. Other Case II ocean colour products are being explored (CDOM)