North Brazil Current Rings Research Articles

An objective reference system for studying rings in the ocean

Rings are marine vortices with a scale of hundreds of kilometers that can last for months, whose associated transport and mixing play an important role in the ocean dynamics. Such features are traditionally treated as a geostrophic flow, but since the centrifugal acceleration is not negligible in the inner core, the cyclo-geostrophic balance is a better approximation for the rings. In the present work, we describe a novel objective technique to identify the ring center, which is used as the origin of a convenient framework to handle rings under the cyclo-geostrophic balance. Furthermore, we correct the velocity field by the translation to isolate the swirl movement, a procedure ignored by other methodologies. We show that the lack of such correction would lead to an error of 30km on the center definition of a ship surveyed North Brazil Current Ring with 160km of radius. Another distinct characteristic of our approach is the flexibility in the spatio-temporal structure of the data, because it allows for ungridded data, an important ability for in situ observations. That also enables the use of a hybrid dataset composed from different instruments. The error on the Monte Carlo experiments to identify the center of the propagating ring is less than 10km, and depends on the level of noise, sampling strategy, and strength of the ring, among other factors. This technique was fully implemented in PyRings, an open Python library with a collection of procedures to handle oceanic rings and mesoscale eddies in general.

Analysis of horizontal and vertical ring structure based on analytical model and satellite data: Application to the North Brazil Current Rings

A theoretical analysis of the principal oceanic ring parameters (tangential and angular velocities, shape function, horizontal and vertical scale of the ring) is described. The theoretical model consists of a reduced gravity model of the lens-like vortices with solid body rotation. The application of this approach is tested by comparison with data from the North Brazil Current Rings Experiment and remote sensing data. Specifically, we used the data corresponding to the surfaceintensified North Brazil Current ring (R-3) surveyed in February–March 1999, using direct velocity and hydrographical measurements. The theoretical model was used for evaluating the geometrical structure of the surface-intensified rings that produce remarkable signals in satellite data. The principal ring parameters from the model were compared with those from satellite data (altimetry and drifter information), which were estimated by using the method of minimization of the multivariable objective functions. Although the proposed model is linear in its conception, a good agreement was observed between the model and the primary characteristics of the observed rings. The model, however, allows for improvement in its assumptions, since its application is rather limited to intense ocean rings.

Seasonal and Interannual Modulation of the Eddy Kinetic Energy in the Caribbean Sea

Abstract Variability of the mesoscale eddy field in the Caribbean Sea is analyzed over the period 1993–2009 using geostrophic anomalies derived from altimeter data and a high-resolution regional model. The Colombia Basin presents the largest values of eddy kinetic energy (EKE) and its semiannual cycle, with a main peak in August–October and a secondary peak in February–March, is the dominant feature in the whole Caribbean EKE cycle. The analysis of energy conversion terms between low-frequency currents and eddies explains these peaks by enhanced baroclinic and barotropic instabilities, in response to seasonally varying currents in the region of the Guajira Peninsula. The semiannual acceleration of the atmospheric Caribbean low-level jet intensifies the southern Caribbean Current (sCC) twice a year in this region, together with its vertical and horizontal velocity shears. The asymmetry of the EKE seasonal cycle in the Colombia Basin is explained by a summer peak in the annual cycle of the whole sCC. Numerical results suggest that the arrival of more energetic North Brazil Current rings during part of the year have almost no impact on the seasonal cycle of EKE in the Colombia Basin. Instead, they are shown to contribute, together with the annual cycle of the Caribbean inflow through the southern passages of the Lesser Antilles, to an annual peak of EKE in the Venezuela Basin between May and August. At the interannual scale the mechanism is similar: interannual variability of the alongshore wind stress controls the speed of the southern Caribbean Current and the energy of the eddies in the Colombia Basin through instability.

Obstacles and benefits of the implementation of a reduced-rank smoother with a high resolution model of the tropical Atlantic Ocean

Abstract. Most of oceanographic operational centers use three-dimensional data assimilation schemes to produce reanalyses. We investigate here the benefits of a smoother, i.e. a four-dimensional formulation of statistical assimilation. A square-root sequential smoother is implemented with a tropical Atlantic Ocean circulation model. A simple twin experiment is performed to investigate its benefits, compared to its corresponding filter. Despite model's non-linearities and the various approximations used for its implementation, the smoother leads to a better estimation of the ocean state, both on statistical (i.e. mean error level) and dynamical points of view, as expected from linear theory. Smoothed states are more in phase with the dynamics of the reference state, an aspect that is nicely illustrated with the chaotic dynamics of the North Brazil Current rings. We also show that the smoother efficiency is strongly related to the filter configuration. One of the main obstacles to implement the smoother is then to accurately estimate the error covariances of the filter. Considering this, benefits of the smoother are also investigated with a configuration close to situations that can be managed by operational center systems, where covariances matrices are fixed (optimal interpolation). We define here a simplified smoother scheme, called half-fixed basis smoother, that could be implemented with current reanalysis schemes. Its main assumption is to neglect the propagation of the error covariances matrix, what leads to strongly reduce the cost of assimilation. Results illustrate the ability of this smoother to provide a solution more consistent with the dynamics, compared to the filter. The smoother is also able to produce analyses independently of the observation frequency, so the smoothed solution appears more continuous in time, especially in case of a low frenquency observation network.

Sea surface structure of North Brazil Current rings derived from shipboard and moored acoustic Doppler current profiler observations

[1] In the western tropical Atlantic, the North Brazil Current retroflection periodically sheds large anticyclonic rings, which then propagate northwestward. Between 1998 and 2000, the North Brazil Current Rings Experiment sampled a large number of these rings by shipboard and moored acoustic Doppler current profiler. Ten of the sampled rings are analyzed in this study, focusing on their sea surface dynamic properties. The rings exhibit a radial structure consisting of two regimes, an “inner” core region in near solid body rotation and an “outer” ring regime with an approximately exponentially decaying structure. The observations show a sharp change in vorticity at the regime transition and the presence of a strong opposite vorticity shield bounding the inner solid body core. We show that Gaussian models, commonly used to represent the surface expression of these and other rings, are adequate for determining the sea surface height anomaly but tend to poorly estimate other properties such as the maximum swirl velocity. Therefore, we propose a new two-part model as a better approximation of the rings' radial structure. According to the cyclogeostrophic balance approximation, the sea surface height distribution across the inner ring has a parabolic shape, while the outer ring has an exponential structure similar to the velocity field. Interestingly, many of the observed rings have an intensity very close to the theoretical limit for anticyclones at these latitudes, which is believed to be due to inertial instability.

On the propagation and decay of North Brazil Current rings

Near the western boundary of the tropical North Atlantic, where the North Brazil Current (NBC) retroflects into the North Equatorial Countercurrent, large anticyclonic rings are shed. After separating from the retroflection region, the so‐called NBC rings travel northwestward along the Brazilian coast, until they reach the island chain of the Lesser Antilles and disintegrate. These rings contribute substantially to the upper limb return flow of the Atlantic Meridional Overturning Circulation by carrying South Atlantic Water into the northern subtropical gyre. Their relevance for the northward transport of South Atlantic Water depends on the frequency of their generation as well as on their horizontal and vertical structure. The ring shedding and propagation and the complex interaction of the rings with the Lesser Antilles are investigated in the Family of Linked Atlantic Model Experiments (FLAME) model. The ring properties simulated in FLAME reach the upper limit of the observed rings in diameter and agree with recent observations on seasonal variability, which indicates a maximum shedding during the first half of the year. When the rings reach the shallow topography of the Lesser Antilles, they are trapped by the island triangle of St. Lucia, Barbados and Tobago and interact with the island chain. The model provides a resolution that is capable of resolving the complex topographic conditions at the islands and illuminates various possible fates for the water contained in the rings. It also reproduces laboratory experiments that indicate that both cyclones and anticyclones are formed after a ring passes through a topographic gap. Trajectories of artificial floats, which were inserted into the modeled velocity field, are used to investigate the pathways of the ring cores and their fate after they encounter the Lesser Antilles. The majority of the floats entered the Caribbean, while the northward Atlantic pathway was found to be of minor importance. No prominent pathway was found east of Barbados, where a ring could avoid the interaction with the islands and migrate toward the northern Lesser Antilles undisturbed.

Modulation of the inflow into the Caribbean Sea by North Brazil Current rings

A two-year long record from a triangular mooring array between the Lesser Antilles islands Tobago, Barbados, and St. Lucia is used to investigate the inflow into the Caribbean Sea, the amount of South Atlantic Water (SAW) carried with the inflow, and the role of North Brazil Current (NBC) rings in the observed variability. The data set consists of time series from temperature/conductivity recorders and current meters in the moorings, bottom-mounted inverted echo sounders at the Tobago and St. Lucia mooring positions, and supplementary shipboard measurements. The acoustic travel time measurements of the inverted echo sounders and the conductivity/temperature time series are used for continuous estimation of dynamic height profiles and geostrophic currents between the surface and 1000 dbar as well as the amount of SAW found at the mooring positions. The observations show a domination of intraseasonal variability between 0 and 15 Sv, superimposed on the long-term fluctuations. With time scales of one to three months, these represent the signature of the NBC rings. The baroclinic transport time series shows nine periods of increased variability, indicative of the rings interacting with the Lesser Antilles island arc; with the exception of one, these periods were associated with corresponding sea surface height anomalies. No marked seasonality was observed in the transport variability or the ring frequency. The arrival of individual rings leads to a weakening of the inflow into the Caribbean. Nevertheless, the rings carry large amounts of SAW into the area, and the immediate increase of the transport towards the end of a ring event suggests a subsequent flow of this SAW-rich water into the Caribbean. At St. Lucia, rings sometimes cause a short-term decrease of SAW content, indicative of an influx of northern hemispheric water and a blocking situation. The average transport of SAW into the Caribbean south of St. Lucia during the observations amounted to 5.5 Sv, with no significant seasonal cycle, but a small positive trend in SAW fraction as well as in transport of about 15% and 1 Sv, respectively; a corresponding trend in the baroclinic volume transport was not observed.

The mesoscale variability in the Caribbean Sea. Part II: Energy sources

The processes which drive the production and the growth of the strong mesoscale eddy field in the Caribbean Sea are examined using a general circulation model. Diagnostics of the simulations suggest that: (1) The mean currents in the Caribbean Sea are intrinsically unstable. The nature of the instability and its strength vary spatially due to strong differences of current structure among basins. (2) The greatest and most energetic eddies of the Caribbean Sea originate in the Venezuela Basin by mixed barotropic-baroclinic instability of an intense jet, formed with waters mostly from the surface return flow of the Meridional Overturning Circulation and the North Equatorial Current which converge and accelerate through the Grenada Passage. The vertical shear of this inflow is enhanced by an eastward undercurrent, which flows along the south American Coast between 100 and 250 m depth. The shallow eddies (less than 200 m depth) formed in the vicinity of the Grenada Passage get rapidly deeper (down to 1000 m depth) and stronger by their interaction with the deep interior flow of the Subtropical Gyre, which enters through passages north of St. Lucia. These main eastern Caribbean inflows merge and form the southern Caribbean Current, whose baroclinic instability is responsible for the westward growth and strengthening of these eddies from the Venezuela to the Colombia Basin. (3) Eddies of lesser strength are produced in other regions of the Caribbean Sea. Their generation and growth is also linked with instability of the local currents. First, cyclones are formed in the cyclonic shear of the northern Caribbean Current, but appear to be rapidly dissipated or absorbed by the large anticyclones coming from the southern Caribbean. Second, eddies in the Cayman Sea, which impact the Yucatan region, are locally produced and enhanced by barotropic instability of the deep Cayman Current. (4) The role of the North Brazil Current (NBC) rings is mostly to act as a finite perturbation for the instability of the mean flow. Their presence near the Lesser Antilles is ubiquitous and they appear to be linked with most of the Caribbean eddies. There are some evidences that the frequency at which they form near the Grenada Passage is influenced by the frequency at which the NBC rings impinge the Lesser Antilles. But large Caribbean eddies also form without a close influence of any ring, and comparison between simulations shows that mean eddy kinetic energy and eddy population in the Caribbean Sea are not substantially different in absence or presence of NBC rings: their presence is not a necessary condition for the generation and growth of the Caribbean eddies.

Laboratory experiments on mesoscale vortices colliding with an island chain

The present laboratory study investigates the behavior of a self‐propagating barotropic cyclonic vortex colliding perpendicularly with aligned circular cylinders and determines the condition for a vortex to bifurcate and split into multiple vortices and/or to generate dipoles downstream of the cylinders. During the experiments, four parameters were varied: G, the gap width between the cylinders; d, the diameter of the incident vortex; Ydis, a parameter expressing the initial vortex positions; and Disl, the total length of the “middle” island. It has been observed that as long as 0.1 < G/d ≤ 0.4, the vortex fluid was funneled between two cylinders at one of the gaps and a dipole generally formed, much like water ejected from a circular nozzle generates a dipole ring. After the dipole formed, the cyclonic part of the dipole became dominant and self propagated away from the cylinders. Furthermore, in some experiments having 0.2 < Disl/d ≤ 0.5, after a weak dipole formed, the remnant of the original vortex moved zonally “south.” When the remnant of the vortex came in contact with a new cylinder, fluid peeled off the outer edge of the vortex and a so‐called “streamer” went around the cylinder in a counterclockwise direction. Under the right conditions, this fluid formed a new cyclonic vortex in the wake of the cylinder, causing bifurcation of the original vortex into two vortices, as observed in previous studies. In general, independently of the configurations and Ydis, the number of cyclonic vortices downstream of the cylinders was one, either originating from the dipole or generated by the bifurcation of the original vortex. The vortex center position, radius, and circulation, before and after the interaction, were computed from its velocity field. It was found that for 0.1 < G/d ≤ 0.4, intense vortices experienced greater amplitude loss than weak vortices. The formation of both a dominant cyclone and an anticyclone (i.e., a dipole) downstream of the aligned cylinders, representing an island chain, is in agreement with recent oceanic observations of North Brazil Current (NBC) rings interacting with the Lesser Antilles in the Eastern Caribbean Sea. Since the passages of the Lesser Antilles have values of 0.07 ≤ G/d ≤ 0.3, the oceanic observations might be explained by the experimental results reported in this paper.

Caribbean current variability and the influence of the Amazon and Orinoco freshwater plumes

The variability of the Caribbean Current is studied in terms of the influence on its dynamics of the freshwater inflow from the Orinoco and Amazon rivers. Sea-surface salinity maps of the eastern Caribbean and SeaWiFS color images show that a freshwater plume from the Orinoco and Amazon Rivers extends seasonally northwestward across the Caribbean basin, from August to November, 3–4 months after the peak of the seasonal rains in northeastern South America. The plume is sustained by two main inflows from the North Brazil Current and its current rings. The southern inflow enters the Caribbean Sea south of Grenada Island and becomes the main branch of the Caribbean Current in the southern Caribbean. The northern inflow (14°N) passes northward around the Grenadine Islands and St. Vincent. As North Brazil Current rings stall and decay east of the Lesser Antilles, between 14°N and 18°N, they release freshwater into the northern part of the eastern Caribbean Sea merging with inflow from the North Equatorial Current. Velocity vectors derived from surface drifters in the eastern Caribbean indicate three westward flowing jets: (1) the southern and fastest at 11°N; (2) the center and second fastest at 14°N; (3) the northern and slowest at 17°N. The center jet (14°N) flows faster between the months of August and December and is located near the southern part of the freshwater plume. Using the MICOM North Atlantic simulation, it is shown that the Caribbean Current is seasonally intensified near 14°N, partly by the inflow of river plumes. Three to four times more anticyclonic eddies are formed during August–December, which agrees with a pronounced rise in the number of anticyclonic looper days in the drifter data then. A climatology-forced regional simulation embedding only the northern (14°N) Caribbean Current (without the influence of the vorticity of the NBC rings), using the ROMS model, shows that the low salinity plume coincides with a negative potential vorticity anomaly that intensifies the center jet located at the salinity front. The jet forms cyclones south of the plume, which are moved northwestward as the anticyclonic circulation intensifies in the eastern Caribbean Sea, north of 14°N. Friction on the shelves of the Greater Antilles also generates cyclones, which propagate westward and eastward from 67°W.

Remote sensing observations of the coherent and non-coherent ring structures in the vicinity of Lesser Antilles

Abstract. The North Brazil Current Rings (NBCR) penetration into the Caribbean Sea is being investigated by employing a merged altimeter-derived sea height anomaly (TOPEX/Poseidon, Jason-1 and ERS-1, 2), the ocean surface color data (SeaWiFS) and Global Drifter Program information. Four strategies are being applied to process the data: (1) calculations of the Okubo-Weiss parameter for NBCR identification, (2) longitude-time plots (also known as Hovmöller diagrams), (3) two-dimensional Radon transforms and (4) two-dimensional Fourier transforms. A twofold NBCR structure has been detected in the region under investigation. The results have shown that NBC rings mainly propagate into the Caribbean Sea along two principal pathways (near 12° N and 17° N) in the ring translation corridor. Thus, rings following the southern pathway in the fall-winter period can enter through very shallow southern straits as non-coherent structures. A different behavior is observed near the northern pathway (~17° N), where NBC rings are thought to have a coherent structure during their squeezing into the eastern Caribbean, i.e. conserving the principal characteristics of the incident rings. We attribute this difference in the rings' behavior to the vertical scales of the rings and to the bottom topography features in the vicinity of the Lesser Antilles.

Amazon and Orinoco River Plumes and NBC Rings: Bystanders or Participants in Hurricane Events?

AbstractThe Amazon and Orinoco River plumes and North Brazil Current (NBC) rings are investigated during the 1 June through 30 November Atlantic hurricane season to identify their impact on upper-ocean temperatures in the region and to draw attention to their potential role in hurricane maintenance and intensification. The analysis uses ocean temperature and salinity stratification data, infrared and microwave satellite-derived sea surface temperature (SST) data, and Atlantic tropical storm and hurricane tracks data. The Amazon–Orinoco River plume spreads into the western equatorial Atlantic Ocean forming an extensive (0°–20°N, 78°–33°W) 10–60-m-thick buoyant surface layer associated with the warmest surface temperatures (up to +3°C) in the region due to the freshwater barrier layer effect. At times the warm Amazon–Orinoco River plume is bisected by cool-surface NBC rings. For the 1960 to 2000 time period, 68% of all category 5 hurricanes passed directly over the historical region of the plume, revealing that most of the most destructive hurricanes may be influenced by ocean–atmosphere interaction with the warm plume just prior to reaching the Caribbean. Statistical analyses of tropical Atlantic SSTs and tropical cyclone wind speeds reveal a significant and unique relationship between warm (cool) SSTs in the Amazon–Orinoco River plume and stronger (weaker) tropical cyclone wind speeds between 35° and 55°W. This implies that warmer (cooler) plume SSTs due to increased (decreased) river discharge may directly contribute to a more (less) vigorous hurricane season.

Laboratory experiments on vortices colliding with multiple islands

AbstractLaboratory experiments have been performed to investigate the physical processes that govern the interaction of a self‐propagating barotropic cyclonic vortex with aligned circular cylinders. The motivation was to understand the dynamics which controls the interaction of North Brazil Current (NBC) rings with the Lesser Antilles (LA) in the Eastern Caribbean Sea. The geometrical parameter regulating the flow in the experiments was the ratio of the gap size to the diameter of the incident vortex, G /d. The range 0.03 ≤ G /d ≤ 0.4 was studied. Since the passages of the LA have values of 0.07 ≤ G /d ≤ 0.3 [2], the experimental results might explain recent oceanic observations [3]. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Globorotalia puncticulata: Population divergence, dispersal and extinction related to Pliocene–Quaternary water masses

The isolating effect of water mass partitioning of populations on the morphology, stratigraphic distribution and extinction of planktonic foraminifera is assessed from the Pliocene–Quaternary record of Globorotalia puncticulata. Southern Hemisphere, Mediterranean and North Atlantic data on these aspects of its history are examined and appear consistent with a limited dispersal biogeographic model wherein populations are largely confined by hydrographic barriers. Earliest populations appeared during the latest Miocene in Southern Hemisphere middle latitude water masses. However, morphometric analysis shows that significant differentiation in the axial shape of shells had developed by 4 Ma between Southwest Pacific populations from ODP Site 1123 (temperate water) and ODP Site 1119 (subantarctic water). These sites are in close proximity but separated by the Subtropical Front. At Site 1123 inflation of late-formed chambers and reduction in the number included in the outer whorl created shell profiles that anticipated the globose form of Globorotalia inflata. The latter's gradual evolution from G. puncticulata s.s. took place in this temperate water mass, with the earliest morphotypes with three chambers in the outer whorl present by 4.1 Ma. In contrast, subantarctic populations at Site 1119 retained four chambers but their axial shape was modified. The development of a large, highly arched aperture and increase in the number of chambers in the outer whorl in Mediterranean–North Atlantic Globorotalia puncticulata bononiensis is an example of population differentiation later in the Pliocene. Chronostratigraphy shows that the northward expansion of central temperate water populations commenced with their occupation of Southwest Pacific subtropical water about 4.8 Ma. The rather abrupt entry of substantial populations into Mediterranean and North Atlantic water at 4.5 Ma marked a major biogeographic expansion and established G. puncticulata as a bipolar species. It was widely distributed about 3 Ma, with major populations in several water masses during a period of middle Pliocene warmth. After this acme North Atlantic and Mediterranean G. puncticulata bononiensis populations collapsed as late Pliocene Northern Hemisphere glacials intensified. They were extinguished in MIS 96 (2.4 Ma). Concurrently, G. puncticulata s.s became extinct in the warm subtropical Southwest Pacific. Subantarctic populations persisted but in turn were decimated in severe glacials during the Middle Pleistocene Transition. Most had disappeared by MIS 16 (0.66 Ma). However, at Sites 594 and 1119 there was a small Lazarus-like revival in MIS 11 (0.41 Ma). The highest known occurrence is in MIS 9 (0.33 Ma) at Site 1119. Confinement of the species to subantarctic water in the Pleistocene may have raised its vulnerability to extinction. While stable isotope data indicate that the lineage's evolution is related to depth habitat selection about the thermocline, its biogeography suggests that hydrographic barriers significantly isolated populations and probably facilitated speciation. Eddies such as the North Brazil Current rings provide conduits for inter-water mass transfer of populations but the history of G. puncticulata suggests that such mechanisms seldom operated successfully. The morphology of the lectotype of G. puncticulata s.s., from beach sand at Rimini, Italy, is consistent with a lower Pliocene source. Reports of living occurrences are poorly documented and the species is considered to be extinct.

The Evolution and Demise of North Brazil Current Rings*

Abstract 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.

Laboratory experiments on mesoscale vortices interacting with two islands

The present study investigates the interaction between a self‐propagating cyclonic vortex with two right vertical cylinders and determines the conditions for a vortex to bifurcate into two or more vortices. As in previous studies, after the cyclonic vortex came in contact with a cylinder, fluid peeled off the outer edge of the vortex and a so‐called “streamer” went around the cylinder in a counterclockwise direction. Under the right conditions, this fluid formed a new cyclonic vortex in the wake of the cylinder, causing bifurcation of the original vortex into two vortices. In some cases, two streamers formed and went around the two cylinders, each forming a new cyclonic vortex. During the experiments, three parameters were varied: G, the separation between the cylinders; d, the diameter of the incident vortex; and y, the distance of the center of the vortex from an axis passing through the center of the gap between the cylinders. The number of vortices generated by the interaction depends on the ratio G/d and on the geometry of the encounter, which is given by the ratio y/g, where g = G/2. An unexpected and revealing result was the formation of a dipole vortex downstream of the two islands for values of −2 < y/g < 0, 0.25 ≤ G/d ≤ 0.4, and ReG > 200, where ReG = UGG/ν is the Reynolds number and UG is the maximum velocity of the vortex fluid in the gap. A possible mechanism is that the flow within the vortex was funneled between the two islands, and provided it had a sufficiently high velocity, a dipole formed, much like water ejected from a circular nozzle generates a dipole ring. The formation of a vortex of opposite sign to the incident vortex (i.e., anticyclonic) is in agreement with recent observations of North Brazil Current (NBC) rings interacting with the islands of Saint Vincent and Barbados in the eastern Caribbean. The passage between the islands of Saint Vincent and Barbados has values of G/d of approximately 0.5; hence the laboratory result suggests that both cyclonic and anticyclonic vortices could form downstream of them.

Caribbean Current and eddies as observed by surface drifters

Abstract Recent satellite-tracked surface drifter trajectories were analyzed to describe the mean currents and eddies in the Caribbean Sea. The structure of the Caribbean Current and its variability were determined from high-resolution 1 2 -degree maps of the mean velocity and eddy kinetic energy. Looping drifter trajectories were used to identify discrete cyclones and anticyclones, and their characteristics were described and related to the structure of the mean flow. The translation rate of eddies in different areas was found to be similar to the mean velocity of the local background flow fields, suggesting that the eddies were largely advected by the background flow. Ten energetic anticyclones translated westward at 13 cm/s in the Venezuela and Colombia Basins. These anticyclones tended to lie in two bands, centered near 15°N and 17°N, coinciding with two jets of the Caribbean Current. The northern weaker jet contains water primarily from the North Atlantic; the southern stronger jet contains water from the tropical and South Atlantic. The anticyclones are thought to have formed in the eastern Caribbean from the anticyclonic vorticity derived from North Brazil Current rings. The ring vorticity enters the eastern Caribbean through island passages and is probably amplified by the anticyclonic shear on the northern side of the jets. Southwest of Cuba a cyclone–anticyclone pair was observed to translate slowly (∼2 cm/s) westward into the Yucatan Current. The cyclone was tracked for 10.5 months with four drifters, making it the longest tracked of the Caribbean eddies.

North Brazil current rings viewed by TRMM Microwave Imager SST and the influence of the Amazon Plume

In the western equatorial Atlantic, 5 years of satellite Sea Surface Temperature (SST) measurements (1998–2002) from the cloud penetrating Tropical Rainfall Measuring Mission (TRMM) Microwave Imager reveal SST signatures of rings shedding from the North Brazil Current (NBC) as it separates from the South American coastline north of the Amazon River Delta and retroflects eastward between 5 and 10°N. By removing the spatial-mean SST from a 7° by 7° square of the nearly instantaneous measurements of each satellite pass, the 46.7 day aliasing period of the diurnal solar cycle is reduced, and seven to eight rings are observed per year with relatively warm (cold) SST anomalies of up to 1°C in the first (second) half of the year. The sense of the SST anomalies carried by the NBC rings are determined by the contrast between the NBC SST and the regional SSTs that are influenced by the far-reaching seasonally varying Amazon River freshwater plume. Within a 1.6-year period, 12 of the SST anomalies are validated by in situ mooring array data confirming the predicted sense of the SST anomalies for each season. According to historical hydrographic data, during the first half of the year, the Amazon Plume is generally contained northwest along the coast, whereas during the second half of the year, the Amazon Plume surrounds the NBC retroflection on the west and the north, and from the surface down to 50m, imposing a dramatic surface salinity contrast up to −4 and a surface temperature contrast up to +2°C across the front. The surface layer characteristics of the rings shed from the NBC retroflection reveal varying influence of the Amazon Plume. Of the four rings surveyed in the NBCR experiment, Amazon Plume water is found only on the edges of three surface-intensified rings, whereas it completely covers the surface layer of the one thermocline-intensified ring. The maximum current cores of the NBC and retroflection are observed within tens-of-meters of the edges of the Amazon Plume. As the fresher and typically warmer surface waters associated with the Amazon Plume are buoyant relative to the saltier and typically colder surface waters carried by the NBC, the varying position of the Amazon Plume may seasonally influence the surface dynamics in the region.

Impact of variable pelagic environments on natural larval growth and recruitment of the reef fish Thalassoma bifasciatum

New recruits of the bluehead wrasse Thalassoma bifasciatum were censused and collected from nearshore reefs of Barbados, West Indies, every 2 weeks for 20 months. Their temporal coincidence with low salinity (<34·5) water during their pelagic larval stage was determined by comparing the otolith records of new recruits with conductivity and temperature records from a current meter moored 2 km off the west coast of the island. Larval residence in a low salinity North Brazil Current (NBC) ring appeared to have a negative impact on growth. Larvae that encountered a NBC ring for at least 7 days during either the first half of the larval period exhibited slower larval growth than those that did not encounter a ring for 7 days during any part of their larval period. As a result of this slower growth, larvae that encountered low salinity waters had a longer pelagic larval duration and were larger at the time of settlement. The magnitude of settlement was not distinctly related to the presence or absence of a NBC ring, but the largest settlement event occurred at the end of the longest ring event. Early juvenile growth did not vary between fish that had encountered a ring and those that did not, so size differences at settlement were propagated through the first week of life on the reef. The potentially opposing attributes of fast and slow‐growing larvae (e.g. fast growing larvae with shorter larval stage duration but smaller size at settlement and higher susceptibility to reef predation), and the resulting differential mortality on the reef may promote the persistence of individuals in the population with contrasting life history traits, and contribute to the lack of a relationship between larval growth and recruitment success. Positive transport related effects of rings (i.e. enhanced retention during some ring events) may further complicate matters by outweighing the negative impact of rings on larval growth.

North Brazil Current retroflection and transports

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 −7 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.