South Atlantic mass transports obtained from subsurface float and hydrographic data (original) (raw)
Related papers
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.
The fate of the Deep Western Boundary Current in the South Atlantic
Deep Sea Research Part I: Oceanographic Research Papers, 2015
The pathways of recently ventilated North Atlantic Deep Water (NADW) are part of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). In the South Atlantic these pathways have been the subject of discussion for years, mostly due to the lack of observations. Knowledge of the pathways of the AMOC in the South Atlantic is a first order prerequisite for understanding the fluxes of climatically important properties. In this paper, historical and new observations, including hydrographic and oxygen sections, Argo data, and chlorofluorocarbons (CFCs), are examined together with two different analyzes of the Ocean general circulation model For the Earth Simulator (OFES) to trace the pathway of the recently ventilated NADW through the South Atlantic. CLIVAR-era CFCs, oxygen and salinity clearly show that the strongest NADW pathway in the South Atlantic is along the western boundary (similar to the North Atlantic). In addition to the western boundary pathway, tracers show an eastward spreading of NADW between $ 17 and 25°S. Analyzed together with the results of earlier studies, the observations and model output presented here indicate that after crossing the equator, the Deep Western Boundary Current (DWBC) transports water with the characteristics of NADW and a total volume transport of approximately 14 Sv (1 Sv ¼10 6 m 3 s À 1 ). It crosses 5°S as a narrow western boundary current and becomes dominated by eddies further south. When this very energetic eddying flow reaches the Vitória-Trindade Ridge ( $ 20°S), the flow follows two different pathways. The main portion of the NADW flow continues along the continental shelf of South America in the form of a strong reformed DWBC, while a smaller portion, about 22% of the initial transport, flows towards the interior of the basin.
Variability and sources of the southeastern Atlantic circulation
Journal of Marine Research, 1996
The 1992-1993 Benguela Sources and Transport (BEST) time series provide a quantitative view of the Benguela Current transport and the eddy field crossing 3OS, as well as an estimate of the relation between its barotropic and baroclinic components. This is done by a simultaneous analysis of the BEST data derived from inverted echo sounders, pressure sensors, current meter moorings, CTD, and ADCP stations. The analysis of the time series indicates that the annual mean baroclinic transport of the Benguela Current is 13 Sv with a total transport of 16 Sv. Through the combination of instruments the total baroclinic plus barotropic transport of the upper 2600 m was obtained without making any assumption about the level of no motion. Results from this calculation corroborated the assumption that 1000 m as a level of no motion could be used as a fairly good approximation.
South Atlantic meridional fluxes
Deep Sea Research Part I: Oceanographic Research Papers, 2013
The properties of the meridional overturning circulation (MOC) and associated meridional heat transport (MHT) and salt fluxes are analyzed in the South Atlantic. The oceanographic data used for the study consist of Expendable bathythermograph (XBT) data collected along 27 sections at nominally 351S for the period of time 2002-2011, and Argo profile data collected in the region. Previous estimates obtained with a shorter record are improved and extended, using new oceanographic sections and wind fields. Different wind products are analyzed to determine the uncertainty in the Ekman component of the MHT derived from their use. Results of the analysis provide a 9-year time series of MHT, and volume transport in the upper layer of the MOC. Salt fluxes at 351S are estimated using a parameter introduced by numerical studies, the M ov that represents the salt flux and helps determine the basin scale salt feedback associated with the MOC. Volume and heat transport by the western and eastern boundary currents are estimated, and their covariablity is examined. Analysis of the data shows that the South Atlantic is responsible for a northward MHT with a mean value of 0.54 7 0.14 PW. The MHT exhibits no significant trend from 2002 to 2011. The MOC varies from 14.4 to 22.7 Sv with a mean value of 18.1 7 2.3 Sv and the maximum overturning transport is found at a mean depth of 1250 m. Statistical analysis suggests that an increase of 1 Sv in the MOC leads to an increase of the MHT of 0.04 7 0.02 PW. Estimates of the M ov from data collected from three different kinds of observations, contrary to those obtained from models, feature a positive salt advection feedback (M ov o 0) suggesting that freshwater perturbations will be amplified and that the MOC is bistable. In other words, the MOC might collapse with a large enough freshwater perturbation. Observations indicate that the mean value of the Brazil Current is À 8.6 7 4.1 Sv at 241S and À 19.4 7 4.3 Sv at 351S, increasing towards the south. East of 31E, the northward flowing Benguela Current and Agulhas rings have a net northward transport of 22.5 7 4.7 Sv. No significant correlation is observed between the MOC and the Brazil Current transport, and most of the compensation derives from the eastern boundary and interior transports. Products from the Ocean general circulation model For the Earth Simulator (OFES) are used to validate methodology used to extend the XBT record, and to aid in the interpretation of the observed findings.
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:
CFC-113 shows Brazil eddy crossing the South Atlantic to the Agulhas retroflection region
Journal of Geophysical Research, 1996
Using the new ocean tracer CFC-113 and other hydrographic and chemical data, we have identified and "aged" a Brazil eddy transported across the South Atlantic to the Cape Basin. This is the first observation of such an eddy so far east. It was observed during two RRS Discovery cruises in January and May 1993, and using TOPEX-POSEIDON altimeter data, we have been able to track it between the two locations. TS and nutrient characteristics show that the eddy could not have an eastern Atlantic origin. Comparison with eddies found in the Brazil /Falklands Confluence suggests that this is the most likely source. From the CFC-113:CFC-11 ratio it appears that the eddy is in excess of 4 years old, and from the two sightings and the altimeter data we have estimated a transport rate for the eddy and given credence to the CFC-113:CFC-11 and CFC-113:CFC-12 ageing technique in surface waters.
Journal of Geophysical Research: Oceans
The first in situ continuous full-water-column observations of the eastern boundary currents (EBCs) at 34.5°S in the South Atlantic are obtained using 23 months of data from a line of Current and Pressure recording Inverted Echo Sounders (CPIES) spanning the Cape Basin. The CPIES are used to evaluate the mean structure of the EBC, the associated water masses, and the volume transport variability. The estimated northward time-mean Benguela Current absolute geostrophic transport is 24 Sv, with a temporal standard deviation of 17 Sv. Beneath this current the time-mean transport is southward, indicating the presence of a deep-EBC (DEBC), with a time-mean transport of 12 Sv, and a standard deviation of 17 Sv. Offshore of these currents, the shallow and deep flows are more variable with weak time means, likely influenced by Agulhas Rings transiting through the region. Hydrographic data collected along the CPIES line demonstrate that the DEBC is carrying recently ventilated North Atlantic Deep Water, as it flows along the continental slope. This is consistent with a previously hypothesized interior pathway bringing recently ventilated North Atlantic Deep Water from the Deep Western Boundary Current across the Atlantic to the Cape Basin. The observations further indicate that much of the DEBC must recirculate within the basin. Spectral analyses of the shallow and deep EBC transport time series demonstrate that the strongest variability occurs on timescales ranging from 30 to 90 days, associated with the propagation of Agulhas Rings. Plain Language Summary Heat and freshwater transported by the overturning circulation in the Atlantic have been shown to have significant influences on global climate. Boundary current variations in the Southeast Atlantic have furthermore been shown to play a major role in the overturning circulation. This study presents, for the first time, full-depth, daily observations of the eastern boundary currents at 34.5°S based on moored instruments. The 23 months of observations (velocity and water mass properties) reveal the presence of two distinct eastern boundary currents, with equatorward flow in the upper layer and poleward flow in the deep layer. The variability of both currents is characterized, in unprecedented detail, and is shown to be dominated by large anticyclonic eddies transiting across the moorings. The observations further indicate that much of the deep current must recirculate within the Cape Basin, with a smaller portion of the flow participating in the global overturning circulation.
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.,
On mean and seasonal currents and transports at the western boundary of the equatorial Atlantic
Journal of Geophysical Research, 1993
Current measurements from two consecutive yearlong deployments of three moored stations at the western end of the equator in the Atlantic, along 44øW, are used to determine the northwestward flow of warm water in the upper several 100 m and of the southeastward counterflow of North Atlantic Deep Water (NADW). Measurements from three acoustic Doppler current profilers (ADCPs) looking upward from 300 m toward the surface allowed calculation of a time series of upper layer transports over 1 year. Mean transport through the array for the upper 300 m is 23.8 Sv with an annual cycle of only +3 Sv that has its maximum in June-August and minimum in northern spring. Estimated additional mean northwestward transport in the range 300-600 m is 6.7 Sv, based on moored data and shipboard Pegasus and lowered ADCP profiling. In the de•th range 1400-3100 m a current core with maximum annual mean southeastward speed of 30 cm s--is found along the continental slope that carries an estimated upper NADW transport of 14.2-17.3 Sv, depending on the extrapolation used between the mooring in the core and the continental slope. This transport is higher than off-equatorial estimates and suggests near-equatorial recirculation at the upper NADW level, in agreement with northwestward mean flow found about 140 km offshore. Below 3100 m and above the 1.8øC isotherm, only a small core of lower NADW flow with speeds of 10-15 cm s -1 is found over the flat part of the basin near 1.5øN, clearly separated from the continental slope by a zone of near-zero mean speeds. Estimated transport of that small current core is about 4.5 Sv, which is significantly below other estimates of near-equatorial transport of lower NADW and suggests that a major fraction of lower NADW may cross the 44øW meridian north of the Ceara Rise. Intraseasonal variability is large, although smaller than observed at 8øN near the western boundary.
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