Observations of currents and water properties in the Amazon Frontal Zone

Measurements of currents and water properties were obtained at various locations in the frontal zone of the Amazon River outflow plume during four cruises in 1989–1991 as part of A Multidisciplinary Amazon Shelf Sediment Study. A salinity front resembling a salt wedge is found approximately 150 km seaward of the river mouth. It continues northwestward more than 400 km along the shelf between the 10‐ and 20‐m isobaths. Tidal currents, which are oriented principally in the cross‐shelf direction, reach 200 cm s −1 during spring tides and drop to less than 80 cm s −1 during neaps in the vicinity of the river mouth. The nontidal flow is strongly sheared, with near‐surface speeds of up to 100 cm s −1 and weak near‐bottom velocities. The direction of the near‐surface flow is generally northwestward except near the river mouth, where river discharge leads to a significant offshore component of flow. The strong velocity shears are stabilized by strong stratification by salinity and by highly concentrated suspended sediment (or fluid mud) near the seabed. Variations in stratification and frontal position are due principally to spring‐neap variations in tidal mixing and seasonal variations in runoff.