1. Introduction
The North Brazil Current (NBC) is an intense western boundary current and the dominant surface circulation feature in the western tropical Atlantic Ocean. The NBC separates from the South American coastline at 6°–8°N and curves back on itself (retroflects) to feed the eastward North Equatorial Countercurrent (NECC) and close the anticyclonic (clockwise) wind-driven equatorial gyre (Fig. 1). The retroflection of the NBC is dynamically similar to the Agulhas Current south of Africa (e.g., Lutjeharms 1996) and, like the Agulhas, the NBC occasionally curves back upon itself so far as to pinch off large warm-core vortices (Johns et al. 1990). These NBC rings, which can exceed 450 km in overall diameter and 2000 m in vertical extent, swirl anticyclonically at speeds approaching 100 cm s−1 while translating northwestward toward the Caribbean on a course parallel to the South American coastline (Didden and Schott 1993; Richardson et al. 1994; Fratantoni et al. 1995, 1999). After translating for 3–4 months the rings are destroyed through interaction with abrupt topography in the vicinity of the Lesser Antilles. Locally, NBC rings and their filamentary remains episodically disrupt surface circulation patterns in the eastern Caribbean, impact the distributions of salinity and icthyoplankton (e.g., Kelly et al. 2000; Cowen and Castro 1994; Borstad 1982), and pose a physical threat to expanding offshore oil and gas exploration on the South American continental slope. Globally, the five to six rings generated annually by the equator-crossing NBC are responsible for up to one-third of the equatorial-to-subtropical mass transport associated with the upper limb of the Atlantic meridional overturning circulation (MOC), a fundamental component of the earth climate system (Fratantoni et al. 1995, 1999, 2000; Goni and Johns 2001).
Due to their geographic location NBC rings are particularly difficult to investigate using satellite remote sensing techniques. Rings shed from subtropical boundary currents (e.g., the Gulf Stream, Kuroshio, Agulhas Current, East Australian Current) are often discernable in satellite infrared imagery due to the contrast between their (warm or cold) core temperature and that of the surrounding environment (e.g., Brown et al. 1983). Relatively weak surface temperature gradients in the western tropical Atlantic warm pool (e.g., Servain and Lukas 1990) make sea surface temperature (SST) imagery ineffective for NBC ring identification. Similarly, the relatively small sea surface height (SSH) signature associated with the azimuthal velocities of the low-latitude NBC rings are difficult (but not impossible: see Didden and Schott 1993) to distinguish from an energetic background eddy field using satellite altimetry. Recently, Goni and Johns (2001) assembled a census of NBC rings using TOPEX/Poseidon SSH anomaly data merged with a mean field derived from historical hydrography. While this approach appears promising, the relatively low temporal (10 days) and spatial (2°–3° longitude) measurement resolution inhibits detailed altimetric study of NBC ring structure and evolution from altimetry alone. Pauluhn and Chao (1999) combined TOPEX altimetry with an eddy-resolving numerical simulation and were able to identify a number of translating NBC rings.
Because of the difficulty of remote observation, NBC rings were not “discovered” until the late 1980s. Prior to this time it was generally understood that the western tropical Atlantic is an energetic and eddy-rich environment (e.g., Bruce et al. 1985). However, the essential evidence that discrete, westward-translating rings are shed by the retroflecting NBC resulted from the analysis of Coastal Zone Color Scanner imagery (Johns et al. 1990). As reported earlier by Muller-Karger et al. (1988), ocean color imagery of the western tropical Atlantic reveals a unique tracer of ocean circulation. The Amazon River discharges onto the continental shelf of equatorial Brazil resulting in elevated nutrient concentrations and enhanced biological productivity. A plume of phytoplankton-rich, high-chlorophyll water is advected northwestward along the nearshore edge of the NBC and into the interior as the NBC retroflects into the North Equatorial Counter Current (NECC).1 Remotely sensed surface chlorophyll measurements derived from ocean color observations reveal filaments of highly productive Amazon-influenced water adjacent to and surrounding relatively lifeless midocean water. This contrast permits visualization of the cyclic advance and retreat of the NBC retroflection and the accompanying formation of pinched-off NBC rings (Fig. 2).
In this article we present new observations detailing the generation and evolution of NBC rings using three years of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color measurements. We also present and make ancillary use of surface drifter trajectories and hydrographic observations resulting from a recent National Science Foundation–sponsored regional field program. The acquisition, processing, and analysis of the SeaWiFS data is described in section 2. Our results, including a summary of ring characteristics and case studies illustrating the genesis and demise of three NBC rings, are presented in section 3. A brief discussion of these results is provided in section 4, and our conclusions are summarized in section 5.
2. Data and methods
SeaWiFS measures chlorophyll a and water leaving radiances at six wavelengths (Hooker et al. 1992; McClain et al. 1998). We obtained daily global fields of SeaWiFS level-3 chlorophyll a data from the Goddard Distributed Active Archive Center at the NASA/Goddard Space Flight Center for the period September 1997–September 2000. The level-3 data product consists of calibrated, atmospherically corrected data corresponding to a period of 1 day and stored in a global, equal-area grid with cells approximately 81 km2. From each daily global field we extracted a regional subfield encompassing the western tropical Atlantic and the eastern Caribbean Sea. The sole use of SeaWiFS imagery in the present study is for identification of near-surface property gradients. Further details regarding the SeaWiFS instrument, calibration methodologies, and data processing can be found in Hooker et al. (1992) and McClain et al. (1998).
Due to the intense convection associated with the migrating intertropical convergence zone (ITCZ) the western tropical Atlantic is often obscured by clouds. Individual daily images in the study region ranged from cloud-free to 95% obscured with significant daily and seasonal variation. To minimize the impact of occasional cloudiness on our ability to discern large-scale spatial structures we generated a sequence of 265 overlapping 7-day composite images centered every fourth day. The compositing algorithm retained the lowest value from each grid cell of the component images.2 The 7-day composite period is short compared to the 50–60-day period of NBC ring generation, but is approximately one-half the rotation period of a translating NBC ring. Based on examination of several representative composites and their component images we do not believe the smoothing and smearing of spatial features inherent in the compositing process significantly impacts the results presented herein.
Each of the 265 composite images was manually analyzed to determine the position of the tracer fronts delineating the onshore edge of the NBC, the southern edge of the NECC, and the circumference of any translating NBC rings. The position of the northwest corner of the NBC retroflection was recorded for each image, as were the center and major and minor diameters of any NBC rings present. Time series were constructed of the ring center position and along- and across-shore diameters for each individual ring. The time difference between the observed separation of a ring from the retroflection and the passage of the ring center through the 55°W meridian was used to compute northward and westward translation speeds. East of 58°W the rings are generally easy to visualize with well-defined cores of midocean (presumably South Atlantic&sol=uatorial gyre) water surrounded by highly productive Amazon-influenced water. West of this longitude ring tracking is more difficult due to additional sources of elevated chlorophyll (the Orinoco River discharge and enhanced productivity near the Lesser Antilles) and smearing of surface gradients by wind and lateral mixing. Surface drifter trajectories (discussed below) confirm that the rings continue to translate at least as far west as Barbados (61°W) while their surface color expression gradually erodes or is masked by competing processes.
The subjective manual interpretation of images is informative and relatively precise, but not particularly efficient. As we hope to extend this analysis in the future, we developed a semiobjective method for inferring NBC ring formation and translation statistics from ocean color imagery. Two indices were defined:
Retroflection position anomaly (RPA): For each 7-day composite image we determined the distance from an arbitrary upstream reference point to the northwest corner of the NBC retroflection. This location was extracted manually from each image, although in hindsight a relatively simple automated method could be devised. The record-length (3-yr) mean position was subtracted from each observation to form an anomaly describing the alongshore extension/retraction of the NBC retroflection relative to its mean position.
Chlorophyll concentration anomaly (CCA): For each 7-day composite image we computed the area-averaged chlorophyll a concentration in a 3° latitude by 1° longitude subregion centered at 9.5°N, 55°W. Rings that pass this meridian are completely separated from the NBC retroflection but have not begun to experience significant deformation due to topographic interaction. A centered, 90-day moving average was subtracted from each observation to minimize the contribution of seasonal concentration variations. The resulting anomaly time series describes the mesoscale variability in ocean color at a site well removed from the NBC retroflection and near the center of the NBC ring translation “corridor.”
3. Results
a. Ring generation
A total of 14 individual ring formation events were visually identified during the 3-yr study period (Table 1; Fig. 3). Displacement vectors depicting the overall translation of each ring during the observed time period are shown in Fig. 4. Ring formation dates were determined subjectively by examination of individual 7-day composite images. Formation was inferred when the low-chlorophyll ring core was visibly separated from the low-chlorophyll interior of the NBC retroflection by a band or ridge of relatively high-chlorophyll water. On several occasions a ring appeared to separate but was subsequently observed to reattach to the retroflection. These abortive separation events were not included in the statistics enumerated in Table 1. Rather, we cataloged only those formation events that could be linked to a separated, freely translating ring observed at or downstream of 55°W.
A majority of the 14 observed ring shedding events are correlated with sudden, large (>250 km) southeastward retractions of the NBC retroflection and commensurate reduction in RPA (Fig. 5). Similarly, nearly all of the identified rings are evident as 30–50-day periods of negative CCA. These anomalous low periods correspond to passage of individual NBC ring cores (composed of relatively “dead” midocean water) through the 55°W monitoring domain. The retroflection retraction signaling ring generation precedes the arrival of a low-chlorophyll event at 55°W by approximately 1 month. We were unable to identify a consistent relationship between the amplitude of the retroflection retraction and the duration or the intensity of the subsequent low-CCA event at 55°W.
Four additional low-CCA events, two each in the fall of 1998 and 1999, could not be correlated with subjective observations of NBC ring translation past 55°W, nor could they be related to a retraction of the NBC retroflection. Yet, these low-chlorophyll events are almost identical in duration and amplitude to the 14 visually verified ring passage events. In Figs. 3 and 5 these questionable CCA events have been labeled Q1–Q4. One of these features (Q2) was carefully surveyed during a research cruise in December 1998 and found to have the expected hydrographic and velocity characteristics of an NBC ring. Based on this in situ validation and the general similarity of the four events, we surmise that Q1–Q4 are in fact NBC rings that were visually unrecognizable in the composite imagery. Given the seasonality of the Amazon discharge, the NBC transport, and the local wind field it is not altogether surprising that our ability to visually detect NBC rings via their ocean color signature should exhibit a seasonal dependence. This interesting and unexpected complication is discussed further in section 4.
Although NBC transport varies significantly during the year (Fig. 3a) we detect no particular seasonality in ring generation (Fig. 6). This finding is generally consistent with a recent studies of NBC ring generation using TOPEX altimetry3 (Pauluhn and Chao 1999; Goni and Johns 2001) and with moored observations (Fratantoni et al. 1995), but is markedly different from previous numerical results and inferences from sparse Lagrangian observations (e.g., Richardson et al. 1994). Molinari and Johns (1994) demonstrated that the NBC retroflection is a consistent, year-round feature in the western tropical Atlantic thermocline, thus dispelling previous notions that NBC ring generation must be a seasonal phenomenon. The few existing in situ observations of NBC rings indicate significant ring-to-ring structural variability. At present we are unable to determine if this variability is purely random or a consequence of the underlying seasonality of the NBC and its environs.
Questions remain as to the degree of interannual variability in ring generation. Goni and Johns (2001) report significant (factor of 2) differences in the number of rings formed from one year to the next. While the present study found no year-to-year variation in ring formation, the 3-yr study period is hardly adequate to address interannual variability. It should also be noted that the Goni and Johns (2001) observations appear to stabilize in the late 1990s with approximately six rings formed per year from 1996 to the present (G. Goni 2000, personal communication).
b. Ring translation
Once free of the retroflection the NBC rings were observed to move northwestward parallel to the continental slope at a mean speed of 14.5 ± 4.3 cm s−1. This speed is consistent with previous estimates from surface drifters (Richardson et al. 1994), satellite altimetry (Didden and Schott 1993; Pauluhn and Chao 1999; Goni and Johns 2001), and numerical simulations (Fratantoni et al. 1995). The translation speed of individual rings varies from a minimum of 7 cm s−1 to a maximum of 22 cm s−1 in the vicinity of 55°W. As will be illustrated below in the form of case studies, the translation speed of a particular ring can vary considerably during its brief lifetime with speeds as high as 30 cm s−1 occasionally noted. A portion of the translation speed variability results from simultaneous deformation of the overall ring geometry (see below). The mean trajectory traced by NBC ring centers is generally parallel to the continental shelfbreak in water 3000–4000 m deep (Fig. 4; Didden and Schott 1993; Fratantoni et al. 1995).
The relative importance of NBC ring self-propagation versus advection by a background flow was investigated briefly by Fratantoni et al. (1995) and in greater detail by Jacob (1997). The latter study concluded that NBC rings self-propagate via the β effect at a speed consistent with analytical models (e.g., Cushman-Roisin et al. 1990). There is therefore no requirement for a background northwestward flow to explain their movement. This provides support for the notion first advanced by Richardson et al. (1994) that the northwestward “Guyana Current” historically depicted in regional circulation schematics and pilot charts may be an artifact of non-eddy-resolving measurements of NBC rings.4 The amplitude and seasonality of northwestward flow over the wide and shallow (<100 m) continental shelf remains uncertain, although moored measurements upstream of the retroflection suggest an annual-mean flow of up to 5 Sv (Sv ≡ 106 m3 s−1) (Johns et al. 1998). Lagrangian observations of a coastal current (Limeburner et al. 1995; Glickson et al. 2000) are generally inconclusive as most surface drifters launched on the shelf upstream of the retroflection are quickly pulled offshore into the NBC.
c. Ring geometry
A summary of geometric parameters corresponding to ring observations near 55°W is shown in Table 2. To simplify the characterization of ring geometry we approximated each ring as an ellipse. Mean major (minor) axis lengths of 304 (213) km yield a mean ring-core surface area of approximately 52 000 km2 at this longitude. Typical aspect ratios (ratio of minor to major axis length) near 0.7 suggest that the standard kinematic approximation of an oceanic ring as a circular, axisymmetric vortex may not apply in this region. There is a tendency for the major axis of the elliptical ring to align itself parallel to the sloping western boundary, particularly as the ring translates to the west of 55°W and encounters topography that is increasingly perpendicular to the ring's preferred westward translation direction. Several rings were observed to dramatically change their orientation as they evolved (see case studies below). When cast in terms of cross- and alongshore dimensions these observations suggest mean radii of 100 and 150 km, respectively. The circulation associated with the NBC retroflection and separated NBC rings extends beyond the low-chlorophyll core. In situ velocity measurements (e.g., Fratantoni et al. 1999; Wilson et al. 1999) indicate the overall scale of circulation associated with an NBC ring may exceed 450 km in diameter.
The observed ocean color gradients provide a means to visualize the generation and evolution of NBC rings. But do the ocean color patterns we observe correspond in a meaningful way to the expected horizontal velocity structure of an NBC ring? For example, may we assume that the color front that visibly defines the ring boundary is dynamically related to the radius of maximum velocity, typically regarded as the boundary between the ring core and the external environment? As part of a recent in situ measurement program, synoptic surveys of hydrographic properties and absolute vector velocity were obtained in and around the NBC retroflection and several NBC rings (Fratantoni et al. 1999). On several occasions these in situ surveys coincided with relatively cloud-free atmospheric conditions permitting simultaneous representation of the surface circulation with SeaWiFS. In Fig. 7 we show velocity vectors at a depth of 15 m superimposed on a nearly simultaneous SeaWiFS image of the NBC retroflection. Note particularly the line of stations near the northwestward extent of the retroflection (
d. Ring volume transport
The effective transport of South Atlantic water into the subtropical North Atlantic by NBC rings has been estimated at approximately 1 Sv per ring by several authors (Johns et al. 1990; Didden and Schott 1993; Richardson et al. 1994; Fratantoni et al. 1995). Based on such estimates, the annual mass flux carried by 5–6 rings amounts to roughly one-third of the interhemispheric transport required to close the upper limb of the Atlantic MOC (e.g., Schmitz and McCartney 1993; Schmitz 1995). The relative consistency of the published mass flux estimates tends to obscure the fact that they are based on very limited data and use different assumptions about horizontal scale, vertical penetration, and number of rings that are formed in a typical year.
The present dataset provides a new opportunity to estimate the transport potential of NBC rings using methods independent from those employed by previous investigators. To compute ring volume we assume each ring may be approximated as an elliptical cylinder with a known surface area (Table 2) and a vertical penetration depth estimated from recent in situ ring observations. Three different NBC rings were surveyed near 57°W during regional research cruises conducted between November 1998 and February 2000 (Fratantoni et al. 1999). In Fig. 8 we show vertical profiles of temperature, salinity, and dissolved oxygen anomalies computed at the core of each ring relative to the ring's immediate background environment. As fundamental constituents of the density field, temperature and salinity are intimately related to the velocity field, and hence the profiles of temperature and salinity anomaly reflect the disparate azimuthal velocity structure of these three rings (Fratantoni et al. 1999). Dissolved oxygen, however, is a passive tracer that, at depth, can only be advected by the ring's velocity field or diffused. The three oxygen anomaly profiles shown in Fig. 8 are similar in form, each tending toward zero near a depth of 600 m. This suggests that the vertical segment of the swirling ring core that is hydrographically distinct from its surrounding environment may be approximated5 by the depth interval 0–600 m. This vertical interval was used in combination with the SeaWiFS surface area measurements to compute the ring volume and effective annualized per ring transports shown in Table 2.
The resulting 1.0 ± 0.4 Sv per ring transport value deduced from our SeaWiFS observations is strikingly similar to, yet completely independent of, earlier NBC ring transport estimates. Necessary simplifications in the present calculation suggest that the ubiquitous 1 Sv per ring value we achieved is probably an upper bound on the effective annual transport. In particular, Richardson et al. (1994) and Fratantoni et al. (1995) noted that NBC rings observed with Lagrangian devices and a current meter mooring, respectively, indicated reduced core diameter at depth relative to at the surface. This suggests a ring core geometry more similar to a truncated cone than to a cylinder, and therefore smaller in volume. A significant contribution of the present study to the assessment of ring transport is our independent estimate of the number of rings formed per year (6), a value that can only be obtained through remote observation and/or extended in situ monitoring. Additional detailed study of NBC ring water mass and velocity structure is required to refine this coarse estimate of volume transport.
e. Ring evolution: Case studies
1) Rings F and G
A time series of SeaWiFS images depicting the generation and evolution of Rings F and G is shown in Fig. 9. Just prior to the separation of Ring F in early January 1999, the NBC retroflection was in an extended state reaching almost to 9°N (Fig. 10a). An indentation or “neck” had formed where the first meander of the southeastward NECC approached the northwestward NBC. On approximately 1 January the retroflection retracted sharply (see Fig. 5) leaving behind a pinched-off ring centered over the northeast corner of the Demerara Rise. Ring F moved hesitantly toward the northwest for about two weeks before its translation speed stabilized near 12 cm s−1 (Fig. 10d). This ring was one of the smallest identified during the study, with a surface area approximately one-half the average value observed at 55°W. While approximately round shortly after formation (aspect ratio near 1.0 on 10 January) Ring F gradually became more eccentric with time and rotated such that its major axis was oriented parallel to the continental slope. This alongshore stretching is particularly evident in the trajectories of surface drifters launched in the ring in February 1999 (Fig. 9). The drifters in the core of Ring F abruptly stopped their looping behavior in early March as the center of circulation neared Trinidad and the ring became extremely elongated. Just prior to the apparent collapse of its vortical circulation, Ring F extended as far north as Martinique (13°N) with an overall length of almost 600 km. Though we find no evidence that Ring F penetrated the Lesser Antilles as a coherent vortex, the distribution of its fractured remnants (identified by surface drifters) indicate that some fraction of the ring's core volume entered the southeastern Caribbean through a series of shallow (50 m) passages in the Grenadines.
Ring G was generated immediately following ring F on approximately 10 February 1999. The retroflection did not extend as far north prior to the formation of this ring (Fig. 5) nor was any significant “necking” of the retroflection noted (Fig. 11). Nevertheless, in contrast to Ring F, Ring G is one of the largest rings observed during this study with a surface area almost 50% larger than average. This size discrepancy suggests the possibility that the extreme elongation of Ring F near the end of its life cycle may have been due in part to interaction with its neighbor. One common feature of vortex–vortex interactions, studied extensively in the laboratory and in numerical simulations, is that of a large, strong vortex entraining filaments of fluid from a neighboring, smaller, weaker vortex (e.g., Provenzale 1999). The close proximity of Rings F and G (not atypical of other observed NBC rings) suggests that such interaction may be a common and important component of downstream ring evolution. Such ring–ring interaction could result in variability in the location at which ring core water is released to the surrounding environment. As seen in Fig. 9, the trajectories of drifters intentionally launched near the ring center indicate a closed circulation considerably smaller than the radius of maximum velocity. Over time the radius at which the drifters loop becomes larger but remains within the assumed radius of maximum velocity, which we associate with the low-chlorophyll ring core. By late February Rings F and G are separated by only a narrow (150 km) band of high-chlorophyll water that, we assume, is experiencing considerable horizontal shear resulting from the opposing azimuthal velocities of the two rings. The demise of Ring G is somewhat different from its predecessor with very few of the core drifters entering the Caribbean. Instead, the ring maintains its vortical circulation while moving northward parallel to and east of the Lesser Antilles. Most of the drifters in the ring core accelerate as they pass through a deep but narrow channel between the islands of Barbados and St. Lucia.
While no drifter in Ring G demonstrated a complete loop around Barbados (but see Ring K, below) the movement of the low-chlorophyll ring core strongly suggests that it passed directly over the island, completely engulfing it (see Fig. 9, 11–19 April). How is this possible? At a depth of 1000 m (i.e., somewhat below the vertical center of the ring's azimuthal circulation) the island of Barbados appears to the ring as an obstacle approximately 40 km wide and 100 km long. With a core diameter at least five times the width of this obstacle (and a total extent perhaps twice that size) the ring is able to maintain a (possibly distorted) vortical circulation. This does not appear to be the case when an NBC ring meets the considerably larger ridge encompassing the Lesser Antilles, as in the case of Ring F. A similar problem, that of Mediterranean Water eddies (meddies) colliding with an isolated cylindrical seamount, has been studied in the laboratory by Cenedese (2002) for both advected and self-propagating vortices. Cenedese found that for ratios of seamount diameter to vortex diameter of less than 0.2 (the regime corresponding to NBC Ring–Barbados interaction) the vortex moved past the topographic obstacle with minimal disturbance.
2) Ring K
The evolution of Ring K is depicted in a time series of SeaWiFS images shown in Fig. 12. This ring was formed in January 2000 in a manner quite similar to that of Ring F, accompanied by a clear extension, “necking,” and subsequent retraction of the retroflection (Figs. 5 and 13a). Ring K accelerated rapidly toward the west. The mean translation speed over its first month of existence was 21 cm s−1, making it one of the fastest observed during this study. This rapid translation was accompanied by significant deformation of its already elliptical shape, with an aspect ratio gradually decreasing from 0.8 to near 0.5. By early March the major axis of the ring had assumed a position parallel to the continental slope east of Tobago.
Near the middle of March the center of ring K passed directly over Barbados. Unlike Ring G, we have detailed verification of this event as a surface drifter in the ring core completes a closed loop around the island and then continues looping as the ring moves northward (see Fig. 10: 20 March). This anticyclonic circulation persisted until the ring center reached about 17°N. From this point the ring gradually decayed without further translation (not shown; see Glickson et al. 2000). None of the drifters launched in Ring K entered the Caribbean, although three drifters grounded while heading in that direction, one each on St. Lucia, Martinique, and Guadeloupe.
4. Discussion
We have demonstrated that it is possible, albeit difficult, to extract useful information about NBC ring generation and evolution from remotely sensed ocean color data. One particular source of difficulty stems from a recurrent July–October minimum in surface chlorophyll contrast and the resulting pattern of successive “missing rings” (Q1–Q4) apparent in Fig. 3. In addition to horizontal advection by wind and the NBC, surface chlorophyll concentration on the equatorial continental shelf is assumed to be related to the Amazon discharge through an unknown and probably nonlinear biological response to changing nutrient input as well as inherent seasonality in phytoplankton productivity. It is reasonable to hypothesize that the annually varying Amazon discharge (Oltman 1968; Figueiredo et al. 1991), the NBC transport (Johns et al. 1998), and the local wind stress (Lentz 1995a,b) conspire to reduce the amount of “tagged” chlorophyll-rich water on the outer continental shelf that is entrained into the NBC. This in turn leads to a reduced ability to visually distinguish the NBC retroflection and any pinched-off rings in ocean color imagery.
As shown in Fig. 3, events Q1–Q4 occur within a few months of the climatological annual minimum in Amazon River discharge, shortly after the NBC transport begins to decline from its annual maximum, and during a period of relatively light southwestward winds. Lentz (1995b) examined seasonal variations in the configuration of the Amazon plume using climatological hydrographic data and found that, on average, the Amazon discharge was contained much closer to shore (200–300 km) during July–December than during January–June (400–500 km). This difference in offshore extent was attributed to a combination of high Amazon discharge and the more-frequent occurrence of southwestward winds during the first half of the year. This is consistent with synoptic observations of the Amazon plume by Lentz and Limeburner (1995) that clearly reveal a minimum in the offshore extent of low-salinity water during November 1991 coincident with minimum Amazon discharge. We surmise that the large (50%) reduction in local wind stress and reduced Amazon discharge during boreal summer and fall (Fig. 3) could significantly reduce the amount of Amazon-influenced water on the outer continental shelf that is available to become entrained into the NBC. Further observation, analysis, and/or numerical simulation is required to further explore the relationship between the various forcing mechanisms described above and their relative impact on surface chlorophyll distributions in the western equatorial Atlantic.
5. Summary and conclusions
We have presented unique observations of North Brazil Current ring generation and evolution using SeaWiFS ocean color imagery and satellite-tracked surface drifters. These observations provide insight into the complete life cycle of an NBC ring from its genesis in the low-latitude Atlantic to its demise near the Lesser Antilles. In combination with recent in situ observations our relatively small collection of remotely observed rings enabled a new and independent estimate of the contribution of NBC rings to intergyre volume transport in the western tropical Atlantic. The main conclusions of this investigation can be summarized as follows:
Ocean color imagery of the western tropical Atlantic reveals the dynamically relevant spatial and temporal scales associated with NBC ring generation and evolution. Strong horizontal gradients in surface chlorophyll can be used to infer the position of the NBC retroflection and the radius of maximum velocity of NBC rings.
The effectiveness of SeaWiFS imagery for visual NBC ring identification appears to have a seasonal limitation associated with the annual cycle of Amazon River discharge and/or wind-forced cross-shelf transport. However, detectable mesoscale variability in chlorophyll concentration persists along the NBC ring translation corridor during all seasons of the year.
Based on SeaWiFS observations and in situ tracer measurements, a reasonable upper bound on the effective annualized transport of a single NBC ring is approximately 1.0 ± 0.4 Sv. This value is both consistent with and independent from previous estimates.
Approximately Six NBC rings are formed each year. Successive rings evolve in close proximity to one other. Our observations suggest that neighboring rings may exert substantial influence on each other. Interaction between adjacent rings may control the time and location at which trapped ring core water is released to the tropical/subtropical gyre circulation.
Progressive deformation of the elliptical ring geometry occurs rapidly relative to the rate of northwestward ring translation. This implies that precise prediction of high-velocity events at a particular location (e.g., an island, oil rig, etc.) depends on knowledge of both the ring's translation and deformation histories.
The societal importance of a thorough understanding of NBC ring generation and evolution extends beyond their role as a link in the Atlantic MOC. For example, expanding deep water oil and gas exploration efforts offshore of Venezuela and Trinidad depend on accurate characterization and prediction of regional circulation variability in order to assure the safety and profitability of these endeavors. In the absence of an extensive in situ monitoring program, the ability to predict the movement and intensity of NBC rings on an operational basis depends largely on our ability to collect and interpret remote observations. Individually, satellite altimetry and ocean color measurements provide important information regarding the generation and evolution of NBC rings, and each approach has its benefits and limitations. Altimetric measurements are immune to cloud cover and allow direct estimation of circulation intensity, but at marginally useful spatial and temporal resolution. Ocean color observations provide high-resolution, near-synoptic views of surface circulation patterns, but provide no intensity information and are susceptible to cloud interference and seasonal changes in discharge-related productivity. A logical next step in the development of an operational ring tracking system might involve combination of the best features of these two observational methods, perhaps using an assimilating numerical model to blend real-time surface observations into a dynamically consistent three-dimensional picture of the tropical Atlantic circulation.
Acknowledgments
This work was supported by the National Science Foundation through Grant OCE 97-29765. The ocean color data were provided by the SeaWiFS Project at Goddard Space Flight Center. The data were obtained from the Goddard Distributed Active Archive Center under the auspices of the National Aeronautics and Space Administration. Use of this data is in accord with the SeaWiFS Research Data Use Terms and Conditions Agreement. Supplemental support for the surface drifter measurements was provided by the National Oceanic and Atmospheric Administration. The in situ observations reported here result from the collaborative data collection and analysis efforts of the North Brazil Current Rings Experiment coinvestigators (P. L. Richardson, D. M. Fratantoni, W. E. Johns, S. Garzoli, W. D. Wilson, and G. J. Goni). We acknowledge the thoughtful comments made by P. L. Richardson and W. E. Johns on an early version of this manuscript. We thank Semyon Grodsky and an anonymous reviewer for their insightful comments and suggestions for improvement.
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Cartoon depicting the major upper-ocean circulation features in the western tropical Atlantic. The South Equatorial Current (SEC), North Brazil Current (NBC), and North Equatorial Countercurrent (NECC) close the wind-driven equatorial gyre. NBC rings provide a mechanism for mass and tracer transport from this gyre through the tropical gyre and into the southern extent of the North Atlantic subtropical gyre
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
A SeaWiFS chlorophyll a image depicting the NBC retroflection and a recently separated NBC ring. Relatively lifeless midocean water (dark blue; within the retroflection and the ring) contrasts sharply with the highly productive waters influenced by the nutrient-rich Amazon outflow. Note also the plume of high-chlorophyll water spreading northwestward from the mouth of the Orinoco River. The ability to track NBC rings with SeaWiFS degrades significantly in this region
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
(a) Repeating annual cycles of Amazon River discharge (solid) from Figueiredo et al. (1991) and NBC volume transport (dashed) as measured near 4°N by Johns et al. (1998). (b) Repeating annual cycles of wind direction (solid) and stress (dashed) at 49°W, 3°N from the global climatology of Hellerman and Rosenstein (1983). Wind direction follows the oceanographic convention (i.e., the direction in which the wind is blowing). (c) A graphical depiction of the time period during which each identified ring formed, passed the 55°W meridian, and was lost to remote observation. Several of these rings were observed in situ during a series of research cruises conducted between Nov 1998 and Feb 2000. The formation date of rings Q1–Q4 was approximated based on observed mean translation speeds
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
Displacement of the 14 NBC rings enumerated in Table 1 during the period of observation with SeaWiFS imagery. The mean position of the NBC retroflection is illustrated schematically for reference
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
Retroflection position anomaly (RPA) and chlorophyll concentration anomaly (CCA) time series (see text for explanation). Where possible, shaded lines indicate visual correlation between retraction of the NBC retroflection (indicated by sharp decline in RPA) and subsequent passage of a ring across the 55°W meridian (indicated by a local minimum in CCA). Individual measurements (circles) taken from SeaWiFS imagery are connected by a cubic spline. CCA events Q1–Q4 do not visually correspond to ring generation events. See text for details
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
Histograms depicting the quarterly frequency of observed NBC ring generation. Note that Goni and Johns (2001) do not specifically report the date of ring generation, but rather the date on which a ring is first identified using TOPEX altimetry (G. Goni 2000, personal communication). This difference in methodology is most likely responsible for the differences shown
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
A composite SeaWiFS image (centered on 8 Dec 1998) of the NBC retroflection augmented by in situ absolute velocity measurements at depth 15 m. Note the correspondence between high gradients in ocean color and local maxima in the NBC jet
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
Vertical profiles of temperature, salinity, and dissolved oxygen anomaly computed within the core relative to a profile on the ring's periphery. Three profiles are shown, one each for rings observed near 57°W in 1998 (solid), 1999 (dotted), and 2000 (dashed). The oxygen anomaly profiles suggest that the strongest water mass contrast between the core of an NBC ring and its surrounding environment occurs above a depth of 600 m
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
Time series of composite SeaWiFS images depicting the evolution of Rings F and G. Color palette is identical to that used in Fig. 2. Seven-day segments of satellite-tracked surface drifter trajectories corresponding to the SeaWiFS image are also shown. The length of each line segment reflects the speed of the drifter. The most recent drifter position (i.e., the head of each 7-day “worm”) is depicted by a small circle
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
Summary diagram depicting the generation and evolution of Ring F. (a) Simplified representation of the color front bounding the NBC retroflection and Ring F during various stages of its life cycle. Dashed contour corresponds to the last observed configuration of the retroflection prior to identification of a separated ring. (b) Movement of the ring center during the period of observation. Circles appear every 4 days. Approximated positions (required due to cloud cover) are denoted by open circles. (c–e) Time series of latitude (solid), longitude (dashed), translation speed, and aspect ratio (ratio of the major and minor axes of an elliptically shaped ring)
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
As in Fig. 10 but for Ring G
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
As in Fig. 9 but for Ring K
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
As in Fig. 10 but for Ring K
Citation: Journal of Physical Oceanography 32, 3; 10.1175/1520-0485(2002)032<1058:NBCRGA>2.0.CO;2
Rings identified and tracked using SeaWiFS
Geometric parameters of NBC rings observed near 55°W.a
Contribution Number 10462 of the Woods Hole Oceanographic Institution.
See Geyer and Beardsley (1995) and accompanying reports for a recent summary of Amazon River outflow plume observations.
Unlike sea surface temperature imagery in which the coldest pixels generally signify clouds, the SeaWiFS level 3 imagery is distributed with cloud values set to an arbitrarily high flag value. Hence a multiday composite that selects the lowest chlorophyll value eliminates cloud-flagged pixels in much the same way a warmest-pixel SST compositing algorithm would function.
Goni and Johns (2001) segregate their observations into “Rings” and “Eddies” depending on the altimetric observability of the NBC retroflection at the time an anticyclonic feature was identified. For comparison with our SeaWiFS observations, a portion of which overlap the TOPEX measurements of Goni and Johns, we have regrouped their census results neglecting this distinction.
Recent observations in the western Indian Ocean have similarly demonstrated that the Mozambique Current is best described as a succession of eddies rather than as a feature of the mean circulation (DeRuijter et al. 2002).
For this calculation we ignore the apparent reversal in oxygen anomaly in the 100-m-thick surface mixed layer. This feature is most likely due to nonconservative upper-ocean sources and sinks of oxygen rather than lateral mixing between the ring core and its environment.
