A numerical study of the barotropic circulation of the Río de la Plata estuary: sensitivity to bathymetry, the Earthʹs rotation and low frequency wind variability

The barotropic circulation of the Rı´o de la Plata is studied by means of process oriented high resolution 3-D numerical simulations, where its relation to the bathymetry, the Earth’s rotation, river runoff and low frequency wind fluctuations is explored. The model solutions indicate that the circulation at the Rı´o de la Plata is sensitive to bathymetry and the Earth’s rotation. Even though a bimodal circulation pattern has been reported in previous papers, our simulations suggest that in the absence of winds the normal path of the estuary flow should be a buoyant plume to the north. When the mean wind blows from directions between SSE and NNW this pattern is intensified. The southward path observed seems to be the result of the wind forcing when it acts from some directions, which explains why this path has only been sporadically registered. The wind driven circulation at the estuary can be explained in terms of two modes of circulation. The first, prevailing for winds with a cross-river component, is related to an infloweoutflow of water at the exterior part of the estuary and accounts for the seasonal signal observed in the salinity field. The second mode dominates when the wind blows along the estuary axis and has a very distinctive pattern of significant sea level increase or reduction at the upper part of the estuary. This mode accounts for two extreme situations often observed with important social implications: the Sudestada, which produces disastrous flooding, and the persistent northwest wind, which on occasions results in the collapse of the drinking water supply to the city of Buenos Aires. From the numerical experiments a dynamic classification of the estuary into three different regions with regard to bathymetry, the Earth’s rotation and winds is proposed. They are the uppercentral estuary, the exterior estuary and Samborombo´ n Bay. Our solutions indicate that during the summer this bay has a weak and retentive circulation pattern that favors the biota, allowing the region to become an area of spawning and nursery for several coastal species. Nevertheless, our results also indicate that the situation that favors the fisheries could change sensitively if the mean wind direction suffers even a small shift to the south as a result, for example, of climate change. Results from historical water level data analyzed by other authors suggest that such a shift could already be taking place.