Physical oceanography of the Amazon shelf

TLDR

The Amazon shelf is driven by energetic forcing from semi-diurnal tides, large river discharge, northeasterly trade winds, and along-shelf flow associated with the North Brazil Current. Despite a pronounced salinity anomaly from river discharge, water motions are dominated by tidal and wind-driven processes, with cross-shelf tidal velocities up to 200 cm s⁻¹, tide-induced mixing shaping the bottom salinity front, suspended sediment stabilizing the boundary layer, subtidal along-shelf currents flowing northwest, a 5–10 m thick plume of 20–30 psu salinity whose velocity varies with wind stress, and overall northwestward plume transport driven by a large-scale pressure gradient of the North Brazil Current.

Abstract

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.