PHYSICAL OCEANOGRAPHY OF THE AMAZON SHELF

The Amazon shelf is subject to energetic forcing from a number of diff erent sources, including near-resonant semi-diurnal tides, large buoya ncy flux from the Amazon River discharge, wind stress from the northea sterly tradewinds and strong along-shelf flow associated with the Nort h Brazil Current. Although the large volume of river discharge produce s a pronounced salinity anomaly, the water motions on the shelf are do minated by the other forcing factors. Tidal velocities of up to 200 cm s(-1) are generally oriented in the cross-shelf direction. Tide-induc ed mixing influences the position and structure of the bottom salinity front that separates the well-mixed nearshore region from the stratif ied plume. High concentrations of suspended sediment trapped along the frontal zone increase the stability of the tidal boundary layer and t hus reduce the bottom stress. At subtidal frequencies, motion is prima rily along-shelf toward the northwest, both in the plume and in the am bient, high-salinity water of the outer-shelf. The plume is generally 5-10 m thick, with a salinity of 20-30 psu. The along-shelf velocity w ithin the plume varies as a function of the along-shelf wind stress. T his variability results in large temporal variations in plume structur e and freshwater content on the shelf. The net northwestward motion of the Amazon plume and of the ambient shelf water appears to be the res ult of a large-scale pressure gradient associated with the North Brazi l Current system.