Lagrangian flow observations of the Amazon River discharge into the North Atlantic

Abstract

Nine satellite‐tracked drifters with near‐surface drogues were launched in or near the mouth of the Amazon River on the north Brazil shelf during 1989–1991 at four different stages of river discharge. The drifters initially moved along shelf toward the northwest over the north Brazil shelf with mean speeds varying from 41 to 128 cm/s and a maximum along‐shelf velocity of 197 cm/s. Many drifters experienced strong acceleration near 2°N between the 5‐m and 20‐m isobaths, caused in part by the sharp reduction in cross‐shelf cross‐sectional area near the shore‐perpendicular Cabo Norte shoal. The drifter trajectories showed strong cross‐shelf tidal variability in the flow field near the Amazon River mouth, and the largest low‐frequency, along‐shelf variability was located over the open shelf just north of Cabo Norte. Good visual correlation between drifter along‐shelf velocity and along‐shelf wind stress supports the dynamical model of Lentz (this issue) that subtidal fluctuations in near‐surface along‐shelf currents in the Amazon River Plume are strongly wind driven. Drifters left the north Brazil shelf after about 5 to 30 days. Three drifters moved northwestward parallel to the northeast coast of South America, crossed the shelf break near 8°N, and moved into deeper North Atlantic water where they began looping in anticyclonic eddies (often called retroflection eddies or loopers) with periods of approximately 11 days as they moved northwestward to the Caribbean Sea. Four drifters initially moved along shelf toward the northwest and then offshore between 3°N and 7°N, where they followed the retroflection of the North Brazil Current. Only one drifter entered the zonal current system that characterizes the equatorial Atlantic circulation.