Fate of North Brazil Current Rings (original) (raw)
Related papers
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.
North Brazil Current Rings experiment : RAFOS float data report : November 1998 - June 2000
2002
Twenty-one RAFOS floats were tracked at depths of 200-1000 meters in and around several North Brazil Current Rings between November 1998 and June 2000. This was part of an experiment to study the role of these current rings in transporting upper level South Atlantic water across the equatorial-tropical gyre boundary into the North Atlantic subtropical gyre. The float trajectories in combination with surface drifters and satellite imagery reveal the sometimes complex life histories of several rings and their fate as they collide with the Lesser Antilles
Velocity structure of North Brazil Current rings
2002
1] High-resolution shipboard surveys of four North Brazil Current rings are presented, which are the first such dedicated surveys to be made of these features. Of the four rings surveyed, three fundamentally different types of ring structures are found: (1) a shallow, surface-trapped structure with velocities confined to the top 200 m (two rings), (2) a deep-reaching structure with significant swirl velocities ($0.2 m/s) extending to 2000 m (one ring), and (3) a thermocline-intensified structure with almost no detectable surface signature (one ring). The results of this study indicate that North Brazil Current rings can have highly variable vertical structures, and that assessing their overall role in cross gyre exchange in the tropical Atlantic will require a careful combination of remote sensing and in-situ observations.
2003
Output from a very high resolution (1/12 deg.) North Atlantic simulation with the Miami Isopycnic Coordinate Ocean Model (MICOM) is analyzed in a region of the Tropical Atlantic characterized by the presence of the North Brazil Current (NBC) retroflection and North Brazil Current rings. The model mean and seasonal circulations present a good qualitative agreement with observations. Quantitatively, the modeled NBC in summer and fall does not completely retroflect into the North Equatorial Counter Current, and the model upper 100 m NBC is more intense than the observed values by 3-4 Sv. The modeled NBC generates a variety of rings, which we classify as 'shallow', 'intermediate', 'deep', and 'subsurface'. An average of 8.3 rings of all types are generated per year, of which 6 are surface intensified, in good agreement with altimetry (5.7 rings per year, . The transport of southern origin water by the the rings was estimated using two methods. First, the transport was computed kinematically from the rings' volume, resulting in an average transport of 6.6 Sv. Second, an estimation of southern water transport based on an explicit calculation of water mass content was done, resulting in an average transport of 7.5 Sv. The rings' contribution represents 40% of the total meridional transport from the surface to the intermediate water layers. Possible mechanisms operating in the model ring generation are briefly discussed.
2009
The complex nature of the North Brazil Current (NBC) has lead to various studies regarding its seasonal variations and eddy-shedding regime. Horizon Marine, Inc. has conducted operational analysis of the NBC for the past 8 years, making use of drifting buoys, satellite imagery, and ADCP data. We have validated observations made by previous studies such as references [1-5] during this time period. Energy fluctuations within the NBC Retroflection can create a closed circulation at the point of inception of the North Equatorial Counter Current (NECC). This closed circulation can spin off as it gains momentum to create a separate anticyclonic eddy, known as an NBC ring. Our observations indicate that the NBC Retroflection sheds between 5 and 8 NBC rings over the period of 1 year. These rings vary from 100 to 500 km in diameter and 70 to 200 cms-1 in surface intensity. The depth of the coherent velocity structure can vary from 200 m to 1000 m as described in. The structure and intensity of each ring varies seasonally as does its migration rate and trajectory. The energy sector offshore Trinidad and Tobago is most strongly impacted by the NBC ring influence on local oce¿anographie conditions, most critically the vigorous near-surface currents associated with the rings' strong vertically coherent velocity fields. The extent of this influence depends on the NBC ring proximity to the shelf break, the ring swirl velocity, and vertical variation in the ring velocity field. In this study, we quantify characteristic patterns of oce¿anographie variation imposed by NBC rings observed in our operational dataseis. Of central importance is determining the variations in migration patterns of NBC rings with changing seasonality.
Investigation of Brazil Current rings in the confluence region
Journal of Geophysical Research, 2006
TOPEX/Poseidon-derived along track SHA, climatological temperature, and salinity fields were used within a two-layer scheme to estimate the depth of the 8°C isotherm in the southwestern Atlantic. These fields were used to monitor the formation and characteristics of the Brazil Current warm-core anticyclonic rings shed by the first meander trough after poleward excursions of the Brazil Current (BC). Results reveal that 40 warm-core rings were shed by the BC between January 1993 and October 1998. The observed lifetime ranges between 1 and 4 months, with a mean value of approximately 2 months. At any given time, two to three anticyclonic rings coexisted in the Brazil-Malvinas confluence region. Most of the rings drifted southward without coalescing with their parent current. Only four rings were identified as being reabsorbed by the BC front after they were shed. No evidence of propagation or absorption of these anticyclones into the eastern limb of the subtropical gyre was observed. These rings have a mean horizontal length scale of 55 km, mean upper-layer thickness of 260 m, and mean translation speed of 10 km d À1. Volume anomaly and available potential energy computations showed a mean value of 3.6 Â 10 12 m 3 and 2.5 Â 10 15 J, respectively. The upper layer transport of the BC was also computed, and a relationship between variations in the southward transport and ring shedding activity was examined. Computation of the heat flux anomaly of the BC rings is estimated to be approximately 0.045 PW per annum. Compilation of these results indicates that warm-core rings created by meandering boundary current extensions in different regions are generally similar.
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.
Journal of Geophysical Research: Oceans, 2020
In order to investigate intra‐ and inter‐annual variability of North Brazil Current (NBC) rings, angular momentum eddy detection and tracking algorithm (AMEDA) was used for identification of their occurrence, trajectories, and parameters. Based on 24 years (1993–2016) of geopotential height and geostrophic current fields reanalysis data from ARMOR 3D (¼°), we identified an average rate of five NBC rings shed by year. The rings present an average lifetime of 15.3 (±5.4) weeks, average speed‐based radius (Rmax) of 139.8 (±23.6) km, and mean sea surface height anomaly (SSHa) of 9.4 (±4.0) cm. The mean observed maximum azimuthal velocity (Vmax) was 0.27 (±0.08) m/s, while the averaged Rossby number (Ro) value was 0.08 (±0.04) and averaged kinetic energy (KE) was of 255.3 (±154.8) cm2/s2. NBC rings have larger dimensions, rotate faster, live less, and transfer more energy in boreal winter months. In contrast, those shed during boreal summer and early fall last longer, have smaller diamet...
Brazil Current surface circulation and energetics observed from drifting buoys
Journal of Geophysical …, 2009
1] The southwestern Atlantic mean surface circulation, its associated variability and energetics are studied through the analysis of 13 years of surface drifter data binned onto a 0.5°Â 0.5°grid. Special attention is given to the following three main regional features of the domain: the Brazil Current, the Malvinas Current, and the Brazil-Malvinas Confluence. For the western boundary currents, the mean field reveals velocities larger than 45 cm s À1 , associated with high levels of variability (standard deviation of more than 15 cm s À1 ), while the highest values of standard deviation are found in the Brazil-Malvinas Confluence (>25 cm s À1 ). Conversely, over most of the domain the mean velocities found (20-30 cm s À1 ) and associated standard deviations (<15 cm s À1 ) are generally lower. High values of kinetic energy of the mean flow per unit mass (265-1000 cm 2 s À2 ) are estimated in the western boundary currents and their extensions. Eddy kinetic energy along the main jet of the western boundary currents is generally lower than the kinetic energy of the mean flow, except for some areas along the Brazil Current path. In contrast, the Brazil-Malvinas Confluence reveals eddy kinetic energy levels comparable or larger ($2500 cm 2 s À2 ) than the mean kinetic energy. Away from the boundary, most of the kinetic energy of the surface circulation is in the eddy field. Furthermore, the analysis of the kinetic energy conversion term suggests the presence of barotropic instabilities along the Brazil Current. Over most of the Brazil Current core, the kinetic energy conversion term points from the mean to eddy kinetic energy.
On the origins of the North Brazil Current
Journal of Geophysical Research, 1994
The analysis of the hydrographic data obtained during the "Nordeste III" (NEIII) survey by the R/V Altairante Saldanha (Brazilian Navy) between May 4 and July 12, 1986, is reported in this work. Distributions of geostrophic speeds and transports (relative to 1000 m) reveal the North Brazil Current (NBC) and other major features of the tropical Atlantic circulation west of 30ø30'W between 4øS and 10ø30'S. The well-developed NBC appears in all analyzed crossisobath sections bordering the Brazilian continental slope between 10øS and 5øS, transporting .an average of 21 Sv. The velocity core of the NBC is found to be centered at about 100-200 m, instead of at the surface, as previously thought. The central branch of the South Equatorial Current (CSEC) enters the study region at about 5øS-6øS and separates into two portions. The northern portion of the CSEC transports about 15 Sv northwestward, and north of 5øS, it coalesces with the NBC to form a 300-km-wide current that transports more than 36 Sv equatorward. The southern branch of the CSEC transports 14 Sv southwestward until about 9ø30'S, where it makes a cyclonic turn to the north and merges with northward flow from the southernmost branch of South Equatorial Current (SSEC). The high-salinity signature of waters from the SSEC appears in the NBC, indicating that it is formed to the south of the study area. By contrast, historical ship drift and Lagrangian drifter observations suggest that the NBC and the Brazil Current are formed at the bifurcation of the CSEC. This apparent contradiction is explained by the surface Ekman drift which obscures the geostrophic flow patterns.