Current meter observations from an array of three subsurface moorings located on the Brazil continental slope near 4°N are used to describe the annual cycle and low-frequency variability of the North Brazil Current (NBC). The moored array was deployed from September 1989 to January 1991, with further extension of the shallowest mooring, located over the 500-m isobath near the axis of the NBC, through September 1991. Moored current measurements were also obtained over the adjacent shelf for a limited time between February and May 1990. The NBC has a large annual cycle at this latitude, ranging from a maximum transport of 36 Sv (Sv ≡ 106 m3 s−1) in July–August to a minimum of 13 Sv in April–May, with an annual mean transport of approximately 26 Sv. The mean transport is dominated by flow in the upper 150 m, and the seasonal cycle is contained almost entirely in the top 300 m. Transport over the continental shelf is 3–5 Sv and appears to be fairly constant throughout the year, based on the available... Abstract Current meter observations from an array of three subsurface moorings located on the Brazil continental slope near 4°N are used to describe the annual cycle and low-frequency variability of the North Brazil Current (NBC). The moored array was deployed from September 1989 to January 1991, with further extension of the shallowest mooring, located over the 500-m isobath near the axis of the NBC, through September 1991. Moored current measurements were also obtained over the adjacent shelf for a limited time between February and May 1990. The NBC has a large annual cycle at this latitude, ranging from a maximum transport of 36 Sv (Sv ≡ 106 m3 s−1) in July–August to a minimum of 13 Sv in April–May, with an annual mean transport of approximately 26 Sv. The mean transport is dominated by flow in the upper 150 m, and the seasonal cycle is contained almost entirely in the top 300 m. Transport over the continental shelf is 3–5 Sv and appears to be fairly constant throughout the year, based on the available...

The Amazon shelf is subject to energetic forcing from a number of different sources, including near-resonant semi-diurnal tides, large buoyancy flux from the Amazon River discharge, wind stress from the northeasterly tradewinds and strong along-shelf flow associated with the North Brazil Current. Although the large volume of river discharge produces a pronounced salinity anomaly, the water motions on the shelf are dominated by the other forcing factors. Tidal velocities of up to 200 cm s −1 are generally oriented in the cross-shelf direction. Tide-induced mixing influences the position and structure of the bottom salinity front that separates the well-mixed nearshore region from the stratified plume. High concentrations of suspended sediment trapped along the frontal zone increase the stability of the tidal boundary layer and thus reduce the bottom stress. At subtidal frequencies, motion is primarily along-shelf toward the northwest, both in the plume and in the ambient, 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 within the plume varies as a function of the along-shelf wind stress. This variability results in large temporal variations in plume structure and freshwater content on the shelf. The net northwestward motion of the Amazon plume and of the ambient shelf water appears to be the result of a large-scale pressure gradient associated with the North Brazil Current system.