Low-frequency currents and water mass spatial distribution on the southernBrazilian shelf

The Southern Brazilian Shelf (SBS) circulation is discussed in terms of the water mass distribution observed in seasonal hydrography and the subtidal frequency oscillations observed in shelf current and coastal wind time seri es. Low-salinity water that originated from river runoff is demonstrated to be an important inner-shelf feature, participating in water mass formation and distribution, while Tropical and Sub-Tropical waters (transported by B razil Current) are the main outer-shelf and slope waters. Cross-shelf trans port on the SBS shelf is maximum in austral spring when Patos Lagoon runoff peaks and monthly mean winds are upwelling-favorable, and along-shelf tran sport is maximum in the austral autumn and winter periods when La Plata Riv er runoff is driven toward the SBS by Argentina coastal winds and mean wind s over the SBS are downwelling-favorable, creating near shore bands of low- salinity water. The intrusion of water from Uruguay and Argentina shelves creates a cold, l ess-saline mid-shelf water mass which, together with local river runoff and the Brazil Current, are responsible for well defined cross-shelf gradients . Subtidal currents, recorded during a 3-month-long mooring in the austral au tumn of 1997, suggest an Ekman response to along-shelf wind forcing with a time lag of 14 h. Power spectra and coherence functions characterize wind i nfluence as occurring primarily in the synoptic period band of 2-10 days, w ith most energetic peak at 4 x 10(-3) cph (10.4 days). The residual current , computed according to Mardia's directional data statistics (Mardia, 1972. Statistics of Directional Data. Academic Press, New York), flows parallel to the coastline and equatorward in agreement with a bouyancy-driven curren t. (C) 2001 Elsevier Science Ltd. All rights reserved.