The North Brazil Current retroflection: Seasonal structure and eddy variability

Abstract

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 ø 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 retrofiection 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 x 106 m z 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 retrofiection.

The warm water inflow into the western tropical Atlantic boundary regime, spring 1994

“…How this partition works and what kind of seasonality it has are still to be deter mined. The NECC is heavily supplied via cross-equatorial in flow during July to December or January, expressed by a strong surface current retroflection near 3°-5°N [Johns et al, 1990;Didden and Schott, 1992], while in spring to early summer the NECC is weak or even flowing westward [ Richardson and Reverdin, 1987]. The NECC can also be supplied from the north, as was determined by Wilson et al [1994] for August 1989, when significant transports out of the NEC turned south ward near the western boundary and joined the NECC.…”

“…Water mass transfer out of the retroflection zone partially occurs through eddy shedding, and detached anticyclonic rings have been observed to propagate along the South American coast toward the entrances of the Caribbean by surface color measurements [Muller-Karger et al, 1988;Johns et al, 1990], by trapped drifters [Richardson et al, 1994], and in Geosat altim etry [Didden and Schott, 1993]. The weakness and even reversal of the NECC in spring, combined with only weak seasonality of the SEC inflow, was presumed to result in increased north westward transport out of the NBC toward the Caribbean in spring [Richardson and Philander, 1987], but so far, evidence for this transport cycle could not be produced [e.g., Didden and Schott, 1992].…”

The influence of salinity on tropical Atlantic instability waves

“…Variability in these regions is not associated with TIWs in the traditional sense, but it does involve the instability of the flow field such as the variability associated with the retroflected NBC and the NECC. The variability of the currents in this region is often discussed in the context of eddies and rings [e.g., Johns et al, 1990;Johns et al, 1998;Castelão and Johns, 2011]. Whether the variability in the northeast and southwest Atlantic are better described as eddies or Rossby waves [e.g., Polito and Sato, 2003] are beyond the scope of this study.…”

Decadal- to interannual-scale source water variations in the Caribbean Sea recorded by Puerto Rican coral radiocarbon

“…Current theories of how water is transported northward across the equator include: 1) eddy shedding from the North Brazil Current Retroflection (Didden and Schott 1993;Johns et al 1990;Richardson et al 1994), 2) direct flow into the southern Caribbean by the seasonal northwestward flowing Guyana Current via the North Brazil Current (Lumpkin and Garzoli 2005), and 3) northward transport of surface waters by Ekman drift in the ocean interior (Halliwell et al 2003;Lumpkin and Garzoli 2005;Mayer and Weisberg 1993). In order to get equatorial 14 C-depleted waters around Puerto Rico, it has to be transported northward to the latitude of Puerto Rico, either before or after entering the Caribbean.…”