NORTH BRAZIL CURRENT RETROFLECTION EDDIES

During 1989-1992, six different anticyclonic eddies were observed to translate up the coast of South America between 7-degrees-N and 12-degrees-N. These eddies, which are similar to those recently observed with coastal zone color scanner images, current meters, and altimetry, are inferred to have formed from pieces of the North Brazil Current, which retroflects or veers offshore near 7-degrees-N to flow eastward. The maximum near-surface diameter of the eddies when newly formed is estimated to be 400 km. The retroflection eddies were identified in trajectories of four surface drifters and three SOFAR floats at depths from 900-1200 m that looped as they translated northwestward with a mean velocity of 9 cm/s. The two longest trajectories were (1) by a 900-m float beginning near 7-degrees-N in August 1989 that looped 14 times over 151 days with a maximum diameter of 140 km and (2) by a surface drifter beginning near 7-degrees-N in October 1990 that looped 9 times over 116 days with a maximum diameter of 250 km and maximum swirl speed of 80 cm/s at that diameter. The mean rotation periods of these two eddies were 11 and 13 days, respectively, with the rotation period of small loops at 900 m around 7 days. Three of the eddies were observed during the period August 1989 to April 1990. Retroflection eddies are considered to provide significant northward volume transport, approximately 3 Sv, along the western boundary even during the months when the North Brazil Current has been observed to retroflect into the countercurrent.