North Brazil Current rings and transport of southern waters in a high resolution numerical simulation of the North Atlantic (original) (raw)
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North Brazil Current retroflection and transports
Journal of Geophysical Research, 2004
1] A subset of data collected as a part of a larger program, the North Brazil Current Rings (NBCR) Experiment, is analyzed to study the variability of the transport of the North Brazil Current (NBC) and its relation with the shedding of rings. It is concluded that there is a direct relation between the latitude of penetration, the number of rings shed, and the intensity of the NBC. The data set consists of dynamic height time series derived from three inverted echo sounders and a shallow pressure gauge deployed along a section perpendicular to the South American coast between the continent and 7°N, and between 48°and 45°W. Velocity and hydrographic data collected during the NBCR cruises are also analyzed and used to validate the results. The 15-month mean transport of the NBC is 16 ± 2 Sv. The 18-month mean of the retroflected southeastward flow is 22 ± 2 Sv. Both flows display considerable variability. The retroflected southeast flow reaches its maximum value during September 1999, near the time when the climatological North Equatorial Countercurrent (NECC) reaches its maximum strength and it is minimum when the climatological NECC reverses or is not present in the basin. The mean difference between the NBC flow and the retroflected flow during August-December 1999 when the NECC is fully established is À7 Sv. The excess in the retroflected flow is due to North Atlantic water joining the retroflected flow from the South Atlantic. The combination of both flows constitutes the NECC.
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:
Low-Latitude Circulation and Mass Transport Pathways in a Model of the Tropical Atlantic Ocean*
Journal of Physical Oceanography, 2000
An eddy-resolving numerical ocean circulation model is used to investigate the pathways of low-latitude intergyre mass transport associated with the upper limb of the Atlantic meridional overturning cell (MOC). Numerical experiments with and without applied wind stress and an imposed MOC exhibit significant differences in intergyre transport, western boundary current intensity, and mesoscale ring production. The character of interaction between low-latitude wind-and overturning-driven circulation systems is found to be predominantly a linear superposition in the annual mean, even though nonlinearity in the form of diapycnal transport is essential to some segments of the mean pathway. Within a mesoscale band of 10-100 day period, significant nonlinear enhancement of near-surface variability is observed. In a realistically forced model experiment, a 14 Sv upperocean MOC return flow is partitioned among three pathways connecting the equatorial and tropical wind-driven gyres. A frictional western boundary current with both surface and intermediate depth components is the dominant pathway and accounts for 6.8 Sv of intergyre transport. A diapycnal pathway involving wind-forced equatorial upwelling and interior Ekman transport is responsible for 4.2 Sv. Translating North Brazil Current rings contribute approximately 3.0 Sv of intergyre transport.
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
The Evolution and Demise of North Brazil Current Rings
Journal of Physical Oceanography, 2006
Subsurface float and surface drifter observations illustrate the structure, evolution, and eventual demise of 10 North Brazil Current (NBC) rings as they approached and collided with the Lesser Antilles in the western tropical Atlantic Ocean. Upon encountering the shoaling topography east of the Lesser Antilles, most of the rings were deflected abruptly northward and several were observed to completely engulf the island of Barbados. The near-surface and subthermocline layers of two rings were observed to cleave or separate upon encountering shoaling bathymetry between Tobago and Barbados, with the resulting portions each retaining an independent and coherent ringlike vortical circulation. Surface drifters and shallow (250 m) subsurface floats that looped within NBC rings were more likely to enter the Caribbean through the passages of the Lesser Antilles than were deeper (500 or 900 m) floats, indicating that the regional bathymetry preferentially inhibits transport of intermediate-depth ring components. No evidence was found for the wholesale passage of rings through the island chain.
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
Journal of Marine Research, 2000
Data from a hydrographic section carried out in January-March 1994 offshore from the eastern coast of South America from 50S to 10N, are used to quantify the full-depth exchanges of water between the western boundary currents and the ocean interior. In the upper and intermediate layers, the westward transport associated with the southern branch of the South Equatorial Current was 49 Sv at the time of the cruise. The transports of the central and northern branches in the upper 200 m were 17 Sv and 12 Sv, respectively.After subtraction of the parts that recirculate in the subtropical, subequatorial, and equatorial domains, the fraction of the South Equatorial Current that effectively contributes to the warm water export to the North Atlantic is estimated at 18 Sv. The poleward boundary of the current southern branch is at 31S through the whole thickness of the subtropicalgyre, but the latitude of the northern boundary varies from 7°308S at the surface to 27S at 1400 m depth. The estimated latitude of its bifurcation into the Brazil Current and North Brazil Undercurrent also varies downward from about 14S at the surface to 28S at a depth of 600 m.
Water Mass Transports and Pathways in the North Brazil‐Equatorial Undercurrent Retroflection
Journal of Geophysical Research: Oceans
The cross-equatorial northward flow in the western tropical Atlantic Ocean is carried mainly by western boundary currents flowing at surface and intermediate levels: the North Brazil Current (NBC) and the North Brazil Undercurrent (NBUC), transporting from salty thermocline South Atlantic Central Waters to low-salinity Antarctic Intermediate Waters (AAIW; e.g.,
The warm water inflow into the western tropical Atlantic boundary regime, spring 1994
Journal of Geophysical Research, 1995
During March 1994 a survey of the western boundary of the tropical Atlantic, between 10øN and 10øS, was carried out by conductivity-temperature-depth and current profiling using shipboard and lowered acoustic Doppler current profilers. In the nearsurface layer, above cr o = 24.5, the inflow into the boundary regime came dominantly from low latitudes; out of the 14 Sv that crossed the equator in the upper part of the North Brazil Current (NBC), only 2 Sv originated from south of 5øS, while 12 Sv came in from the east at 1ø-5øS with the South Equatorial Current (SEC). After crossing the equator near 44øW, only a minor fraction of the near-surface NBC retroflected eastward, while a net through flow of about 12 Sv above cro -24.5 continued northwestward along the boundary. By contrast, in the isopycnal range cro --24.5-26.8 encompassing the Equatorial Undercurrent (EUC), the source waters of the equatorial circulation were dominantly of higher-latitude South Atlantic origin. While only 3 Sv of eastern equatorial water entered the region through the SEC at 3ø-5øS, there was an inflow of 10 Sv of South Atlantic water in the North Brazil Undercurrent (NBUC) along the South American coast that originated south of 10øS. The transport of 14 Sv arriving at the equator along the boundary in the undercurrent layer was almost entirely retroflected into the EUC with only marginal northern water additions along its path to 35øW. The offequatorial undercurrents in the upper thermocline, the South and North Equatorial Undercurrents carried only small transports across 35øW, of 5 Sv and 3 Sv, respectively, dominantly supplied out of SEC recirculation rather than out of the boundary current. Still deeper, three zonal undercurrents were observed: the westward-flowing Equatorial Intermediate Current (EIC) in the depth range 200-900 m below the EUC, and two offequatorial eastward undercurrents, the Northern and Southern Intermediate Countercurrents (NICC, SICC) at 400-1000 m and 1ø-3 ø latitude. In the lower part of the NBUC there was an Antarctic Intermediate Water (AAIW) inflow along the coast of 6 Sv, and there was a clear connection at the AAIW level to the SICC by low salinities and high oxygens and a weaker suggestion also that some supply of the NICC might be through AAIW out of the deep NBUC. As regards the seasonal signal south of the equator, ship drift currents [Richardson and Walsh, 1986; Arnault, 1987], evaluation of the historical expendable bathythermograph (XBT) data file [Molinari and Johns, 1994], and satellite altimetry [e.g., Romaneessen, 1993] suggest that it is small compared with the region of the North Equatorial Countercurrent (NECC) regime. After an evaluation of the high-resolution World Ocean Circulation Experiment Community Modeling Effort (WOCE CME) model of the North Atlantic revealing that the northward flowing boundary current off North Brazil, between 5øS and 10øS, was in fact an equatorward undercurrent [Schott and BOning, 1991], recent observational programs confirmed that undercurrent structure from direct velocity profiles [Stramma et al., 1995]; it also can be seen in geostrophic results of older Nansen bottle sections [Silveira et al., 1994]. The North Brazil Undercurrent (NBUC) was found to be quasi-continuous along the coast, between 10øS and 5øS, with very little inflow out of the SEC in that latitude range. However, after passing Cape San Roque at 5øS, the SEC overrides the NBUC, causing its structure to change from an undercurrent to a surface-intensified current, the North Brazil Current. The NBC, after crossing the equator, partially continues northwestward, partially retroflects into the Equatorial Under-24,745 24,746 SCHOTT ET AL.: INFLOW INTO WESTERN TROPICAL ATLANTIC BOUNDARY REGIME