The Amazon River Plume during AMASSEDS: Subtidal current variability and the importance of wind forcing

Current variability over the north Brazilian shelf, particularly within the Amazon Plume, is characterized using observations from a moored array deployed about 300 km north of the Amazon River mouth from February to June 1990 as part of A Multidisciplinary Amazon Shelf SEDiment Study (AMASSEDS). The moored array consisted of a transect perpendicular to the coast with inner shelf (18 m depth) and midshelf (65 m) moorings and a third mooring near the shelf break, in 100 m of water. To our knowledge, the AMASSEDS moored array includes the first long-term (2 months) moored observations made within the Amazon Plume. The current variability is dominated by two components, semidiurnal cross-shelf currents with peak velocities of 50 to 200 cm/s and vertically sheared subtidal (timescales days to weeks) along-shelf currents. This study focuses on the subtidal flow within the Amazon Plume which is strong and variable with along-shelf currents ranging from −50 cm/s to over 150 cm/s. The variability in the along-shelf current within the plume is wind driven. The relatively weak (<0.5 dyn/cm2) along-shelf wind stresses in this region drive strong along-shelf current variability because the Amazon Plume is thin and the interfacial drag is weak. However, in the absence of an along-shelf wind stress the plume flow is northwestward at 40 to 80 cm/s, indicating that direct wind forcing does not account for the mean northwestward flow of the Amazon Plume. In the cross-plume direction there is a tendency for the onshore wind stress due to the persistent trade winds to balance the typically offshore buoyancy force and the Coriolis force associated with the wind-driven along-shelf flow. Thus the wind stress is a key factor influencing variability in both the flow and cross-shelf structure of the Amazon Plume in the vicinity of the moored array.