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The North Brazil Current retroflection: Seasonal structure and eddy variability


  • William E. Johns,

  • Thomas N. Lee,

  • Friedrich A. Schott,

  • Rainer J. Zantopp,

  • Robert H. Evans


We report results from a 1-year (September 1987 to September 1988) moored current meter array spanning the continental margin off French Guiana near 8° N in the western tropical Atlantic. Current profiles were recorded at three sites: at the shelf break, over the mid-continental slope, and at the base of the continental rise. Upper level mean currents showed a northwestward flowing North Brazil Current (NBC) and offshore retroflection of this flow into the North Equatorial Countercurrent from late summer through about January. Generally weak upper level mean flows were observed during the spring (February–June). Persistent northwestward mean flow was observed at 900 m depth over the continental slope, indicating northward transport of Antarctic Intermediate Waters in a subsurface boundary flow at speeds of 10–15 cm s−1. Deep currents over the continental rise showed a strong southeastward Deep Western Boundary Current (DWBC) extending from 2500 m to the bottom, with mean core speeds of nearly 30 cm s−1 at 4300 m depth. Transport estimates based on these data and a few geostrophic sections suggest a DWBC transport of 20–40 × 106 m3 s−1 at this location. Low-frequency current fluctuations were dominated by a well-defined 40- to 60-day oscillation with peak-to-peak meridional velocity amplitudes of > 1 m s−1 during the fall. Analysis of historical coastal zone color scanner imagery suggests that these oscillations are related to quasi-periodic generation and subsequent westward movement of ≈ 400 km diameter eddies from the NBC retroflection. These results contrast sharply with earlier indications of a quasi-permanent “Demerara Eddy” in this region, and suggest that this commonly observed feature is in fact a transient phenomenon associated with the time-dependent behavior of the NBC retroflection.

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