Upper layer currents in the western tropical North Atlantic (original) (raw)
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Upper layer currents in the western tropical North Atlantic (1989–1991)
Journal of Geophysical Research, 1999
Shipboard acoustic Doppler current profiler (ADCP) measurements and hydrographic observations of temperature, salinity, and dissolved oxygen are used to examine the upper water column flow field in the North Brazil Current (NBC) retroflection region of the western tropical Atlantic Ocean. Observations are presented from six cruises, one conducted in August 1989 and the other five conducted during the Western Tropical
Upper layer circulation in the western tropical North Atlantic Ocean during August 1989
Journal of Geophysical Research, 1994
Shipboard acoustic Doppler current profiler velocity measurements and profiles of temperature, salinity, and dissolved oxygen measurements are used to map the flow field above the cr 0 = 26.8 isopycnal (approximately the upper 300 m of the water column) in the North Brazil Current (NBC) retroflection region (0 ø to 14øN, 60 ø to 40øW) during August of 1989. The water column is divided into a near-surface, upper thermocline layer (above cr 0 = 24.5) and a main to subthermocline layer (or 0 = 24.5 to cr 0 = 26.8). In the upper layer the eastward flowing North Equatorial Countercurrent
The North Brazil Current retroflection: Seasonal structure and eddy variability
Journal of Geophysical Research, 1990
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. 1. INTRODUCTION Advances in tropical oceanography over the past few decades have led to the realization that low-latitude western boundary currents play an important role in cross-equatorial transport and recirculation of water within the zonal equatorial current systems. In the Atlantic, observations have shown that an intense western boundary current exists along the coast of South America that carries water northward from the South Equatorial Current where it impinges on the east coast of Brazil (Figure l a). This boundary current has been referred to by various names in the literature (e.g., the North Brazil Current, Brazilian Coastal Current, North Brazilian Coastal Current); however, we will adopt the convention used by the majority of FOCAL/SEQUAL (Programme Fran•ais Ocean et Climat dans l'Atlantique Equatorial/Seasonal Response of the Equatorial Atlantic) investigators and refer to it here as the North Brazil Current (NBC). There are few direct measurements of this current system at present, although observations by Flagg et al. [1986] in December 1980 showed the NBC to be 100-200 km wide, with
The Shallow and Deep Western Boundary Circulation of the South Atlantic at 5°–11°S
Journal of Physical Oceanography, 2005
Repeated shipboard observation sections across the boundary flow off northeastern Brazil as well as acoustic Doppler current profiler (ADCP) and current-meter records from a moored boundary array deployed during 2000–04 near 11°S are analyzed here for both the northward warm water flow by the North Brazil Undercurrent (NBUC) above approximately 1100 m and the southward flow of North Atlantic Deep Water (NADW) underneath. At 5°S, the mean from nine sections yields an NBUC transport of 26.5 ± 3.7 Sv (Sv ≡ 106 m3 s−1) along the boundary; at 11°S the mean NBUC transport from five sections is 25.4 ± 7.4 Sv, confirming that the NBUC is already well developed at 11°S. At both latitudes a persistent offshore southward recirculation between 200- and 1100-m depth reduces the net northward warm water flow through the 5°S section (west of 31.5°W) to 22.1 ± 5.3 Sv and through the 11°S section to 21.7 ± 4.1 Sv (west of 32.0°W). The 4-yr-long NBUC transport time series from 11°S yields a seasonal ...
Retroflections of the North Brazil Current during February 2002
Deep Sea Research Part I: Oceanographic Research Papers, 2005
Retroflections of the North Brazil Current (NBC) are examined using observational data collected in the western tropical Atlantic between 351W-441W and 111S-71N. Acoustic Doppler Current profiler, conductivity temperature depth oxygen and expendable Bathythermograph data were obtained during a cruise conducted in February 2002. The water column was separated into two layers, the upper layer from the sea-surface to the 24.5 isopycnal and lower layer from the 24.5 to the 26.8 isopycnal. The upper layer includes the NBC, the South Equatorial Current, the North Equatorial Countercurrent (NECC) and a portion of the Equatorial Undercurrent (EUC). The lower layer includes the North Brazil Undercurrent (NBUC), the South Equatorial Undercurrent (SEUC), the North Equatorial Undercurrent (NEUC) and another portion of the EUC. With respect to earlier work, the data show that (1) the SEUC is primarily fed by waters from a recirculation gyre on its northern boundary with some minor transport retroflecting from the NBUC; (2) the EUC is fed primarily by southern hemisphere waters that retroflect from the NBC, but there is some northern hemisphere water in the near surface flow that connects the EUC and NEUC during this cruise; (3) the NECC during this cruise has no connection to the NBC and is fed by northern hemisphere waters originating in the North Equatorial Current; and (4) the majority of the upper layer NBC that crosses 441W does not appear to retroflect from the boundary farther north to return equatorward. New results include:
Ocean Science Discussions, 2010
The properties and circulation of water masses are examined using data collected from a hydrographic and Acoustic Doppler Current profiler in the Western Tropical Atlantic during two cruises of the GEOTRACES process study "AMANDES" (AMazon-ANDEans): AMANDES I (October-November 2007) and AMANDES II (January 2008). In the upper layer (from the sea surface to 150 m) means of vertical sections of velocity are showing the structure of the Current (NBC) and North Equatorial Countercurrent. In the lower layer (below 150 m) the subsurface velocity core of the North Brazil UnderCurrent, Western Boundary Undercurrent (WBUC) and northern branch of the South Equatorial Current (nSEC) could be observed. In October the WBUC flows southeastward with a velocity of about 0.3 m s −1. In the studied area during October 2007, the NBUC and nSEC are transporting South Atlantic Central Water (SACW) from the Southern Hemisphere whereas the WBUC transports North Atlantic Central Water (NACW) southeastward. In the deep layers, the North Atlantic Deep Water (NADW) is composed of three components: the Upper North Atlantic Deep Water-UNADW (between 1310 and 1650 m), the Middle North Atlantic Deep Water (between 1930 and 2400 m), the Lower North Atlantic Deep Water (centered around 3430 m). Off Guyana, the Antartic Intermediate Water (AAIW) changes of composition be
Direct measurements of western boundary currents off Brazil between 20°S and 28°S
Journal of Geophysical Research: Oceans, 1998
Current measurements from three moored arrays on the Brazilian continental slope between 20øS and 28øS are investigated for the existence and strength of western boundary currents from near the surface down to the North Atlantic Deep Water. The Brazil Current is found to deepen southward from 100 rn to more than 670 rn and to strengthen its volume transport to 16.2 x !06 m3/s. Antarctic Intermediate Water is transported in a well-developed boundary current southward at 28øS and northward north of Cabo Frio (24øS). This result supports earlier suggestions derived from the analysis of hydrographic data that Antarctic Intermediate Water enters the Brazil Basin from the east and bifurcates as it meets the continental break off Brazil. North Atlantic Deep Water is transported southward in a weakly developed boundary current that leads to lower estimates of volume transport than expected from earlier hydrographic data analysis. 1. Introduction The South Atlantic is known to be the highway on which the major interoceanic exchange of water masses and, consequently, of heat and salt occurs. As a result of the Meteor expedition 1925-1927 into the South Atlantic, Wast [!935] and Defant [1936a, b] illustrated the South Atlantic's deep and near-surface circulation, which in its general structure still holds for the subtropical western South Atlantic off Brazil. Much of the associated volume transports is carried in western boundary currents that in early theories are required to balance the interior ocean circulation [Stommel, 1948; Stommel and Aarons, 1960]. Principally, this concept could apply to all layers of deep and intermediate waters. For the Brazil Basin, one therefore would expect not only the near-surface Brazil Current balancing the wind-driven circulation, but also bound~ ary currents in the layers of the Antarctic Intermediate Water (AAIW), the Upper Circumpolar Deep Water (UCDW), the three layers of the North Atlantic Deep Water (NADW), and the Antarctic Bottom Water (AABW). While deep boundary currents carrying NADW southward and AABW northward have been described in the literature, a northward spreading of AAIW and UCDW within boundary currents has been less studied, and analysis was restricted to hydrographic data. For broad reviews of the present knowledge of the South Atlantic's water masses and general circulation as derived from hydrographic measurements, the reader is referred to Reid [!989], Peterson and Stramma [199!], DeMadron and Weatherly [1994], and Tsuchiya et al. [!994]. Few direct current observations were available from the boundary current system along the South American coast when the World Ocean Circulation Experiment (WOCE) began with its Deep Basin Experiment (DBE) in 1990. The then available records from moored current meters show the existence of a
Journal of Geophysical Research, 2000
Hydrographic and velocity observations of the North Brazil Current (NBC) retrofiection region during the 1990-1991 Western Tropical Atlantic Experiment (WESTRAX) are examined with the intent of extracting dynamical information about the NBC eddy shedding. A comparison is performed between the depth structure of empirical orthogonal functions and the dynamical normal modes of the NBC retrofiection region on a beta plane centered on 5øN. The barotropic and first two baroclinic modes account for about 75% of the vertical structure of the NBC flow. Thus, a three-layer model of the NBC region is suitable for dynamical studies of the NBC retrofiection. In terms of flow structure the upper layer represents the retrofiection of the surface layers of the NBC to feed the North Equatorial Countercurrent. The middle layer represents the separating subthermocline waters of the NBC which feed the North Equatorial Undercurrent. The third layer represents a weak meandering flow that may be thought of as the Deep Western Boundary Current signature in the three-layer ocean. In terms of PV, a well-defined front separates the NBC waters from the North Equatorial Current in the upper layer. Both upper and middle layers present closed PV contours associated with the eddy in the process of pinching off from the retrofiecting NBC. The NBC separation region, although equatorial, complies reasonably well with the basic quasigeostrophic (QG) assumptions. Therefore QG methods are applied to investigate the NBC meander growth. By isolating the effect of PV anomalies in each of the layers on each of the layers, baroclinic growth is verified to occur during the NBC eddy shedding. 28,559 28,560 SILVEIRA ET AL.: NORTH BRAZIL CURRENT RETROFLECTION DYNAMICS also revealed five retroflection eddies. They found that the eddies had mean widths of 250 km at the surface and 140 km at 900 m. Average northwest translation speeds were 9 cm s-•.. Modeling efforts have also examined the NBC eddyshedding phenomenon. Fratantoni et al. [1995] successfully simulated the generation of the NBC retroflection eddies with the U.S. Navy nonlinear, primitive equation, layered ocean model forced by climatological winds and a prescribed thermohaline meridional overturning circulation (MOC). These authors verified that at least 2-3 rings per year separated from the modeled NBC. Because the model failed to shed eddies in the absence of an imposed MOC, Fratantoni et al. [1995] suggested that the increased boundary current transport due to the MOC is an important factor in the NBC ring formation. Ma [1996] used analytical and numerical methods in an equivalent-barotropic flamework to further investigate the formation and translation of the NBC eddies. He found that the formation of the eddies was due to the short Rossby waves, nonlinearities, and the western boundary. The translation mechanisms seemed to be caused by interaction of the eddies with the boundary and the fi effect.
Upper ocean circulation in the western tropical Atlantic in boreal fall 2000
Deep Sea Research Part I: Oceanographic Research Papers, 2005
The upper ocean large-scale circulation of the western tropical Atlantic from 11.51S to the Caribbean in November and December 2000 is investigated from a new type of shipboard ADCP able to measure accurate velocities to 600 m depth, combined with lowered ADCP measurements. Satellite data and numerical model output complement the shipboard measurements to better describe the large-scale circulation. In November 2000 the North Brazil Undercurrent (NBUC) was strongly intensified between 11 and 51S by inflow from the east, hence the NBUC was formed further to the north than in the mean. The NBUC was transporting 23.