Upper layer currents in the western tropical North Atlantic (original) (raw)
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 ...
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.
The mid-depth circulation of the northwestern tropical Atlantic observed by floats
Deep-sea Research Part I-oceanographic Research Papers, 2009
A comprehensive analysis of velocity data from subsurface floats in the northwestern tropical Atlantic at two depth layers is presented: one representing the Antarctic Intermediate Water (AAIW, pressure range 600–1050 dbar), the other the upper North Atlantic Deep Water (uNADW, pressure range 1200–2050 dbar). New data from three independent research programs are combined with previously available data to achieve blanket coverage in space for the AAIW layer, while coverage in the uNADW remains more intermittent. Results from the AAIW mainly confirm previous studies on the mean flow, namely the equatorial zonal and the boundary currents, but clarify details on pathways, mostly by virtue of the spatial data coverage that sets float observations apart from e.g. shipborne or mooring observations. Mean transports in each of five zonal equatorial current bands is found to be between 2.7 and 4.5 Sv. Pathways carrying AAIW northward beyond the North Brazil Undercurrent are clearly visible in the mean velocity field, in particular a northward transport of 3.7 Sv across 16°N between the Antilles islands and the Mid-Atlantic Ridge. New maps of Lagrangian eddy kinetic energy and integral time scales are presented to quantify mesoscale activity. For the uNADW, mean flow and mesoscale properties are discussed as data availability allows. Trajectories in the uNADW east of the Lesser Antilles reveal interactions between the Deep Western Boundary Current (DWBC) and the basin interior, which can explain recent hydrographic observations of changes in composition of DWBC water along its southward flow.
Deep western boundary current variability off northeastern Brazil
Deep Sea Research Part I: Oceanographic Research Papers, 1993
Observations from a year-long deep current meter mooting at 80N, 52"W are used to describe the structure and variability of the Deep Western Boundary Current (DWBC) in the tropical Atlantic. The DWBC has a deep core near 4300 m depth, is extremely swift and narrow (35 cm s-~ core speed, 60 km width), and shows a total transport of 22 Sv between 2500 m and the bottom. Approximately 3 Sv of the DWBC transport are carded in near-bottom layers below 0 = 1.8"C, which appears to be recirculating in the western tropical Atlantic north of about 4*N.
Ocean Science, 2012
The first direct estimates of the temporal variability of the absolute transport of the Deep Western Boundary Current (DWBC) at 34.5 • S in the South Atlantic Ocean are obtained using just under one year of data from a line of four pressure-equipped inverted echo sounders. Hydrographic sections collected in 2009 and 2010 confirm the 5 presence of the DWBC, one of the main deep pathways of the Meridional Overturning Circulation, based on neutral density, temperature, salinity, and oxygen values. Both observations confirm that the DWBC reconstitutes itself after breaking into eddies in the western sub-tropical Atlantic near 8 • S. The amplitude and spectral character of the DWBC transport variability are comparable with those observed at 26.5 • N, where 10 longer records exist, with the DWBC at 34.5 • S exhibiting a transport standard deviation of 25 Sv and variations of ∼ 40 Sv occurring within periods as short as a few days. There is little indication of an annual cycle in the DWBC transports, although the observation record is too short to be definitive, and the dominant time scale during the first year of the experiment was about 9-10 days. A "Monte Carlo-style" analysis us-15 ing 27 yr of model output from the same location as the observations indicates that another 48-60 months of data will be required to encompass a fairly complete span of deep transport variability. The model suggests the presence of an annual cycle in DWBC transport, however the statistical significance of the annual cycle with even 27 yr of model output is low, suggesting that annual period variations in the model are weak 20 as well. Johns et al., 2008; Meinen et al., 2012), however the pathways and variability of the 978 equipped inverted echo sounders (PIES) deployed across the DWBC at 34.5 • S on the western boundary of the South Atlantic Ocean just north of the Brazil-Malvinas Confluence, and also to demonstrate that the pilot array of PIES is successfully observing the DWBC. The variability observed during the first year of deployment will be compared to 27 yr of output from a high-resolution model to evaluate the statistical information 25 contained in a one-year record. 979 Abstract 980 OSD 9, 977-
The zonal currents and transports at 35°W in the tropical Atlantic
Geophysical Research Letters, 2003
The total of 13 existing cross-equatorial shipboard current profiling sections taken during the WOCE period between 1990 and 2002 along 35°W are used to determine the mean meridional structure of the zonal top-to-bottom circulation between the Brazilian coast, near 5°S, and 5°N and to estimate mean transports of the individual identified shallow, intermediate and deep current branches. One of the