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/cm 2 ) 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.