Malvinas Current Research Articles

Why Does the North Brazil Current Regularly Shed Rings but the Brazil Current Does Not?

Abstract Both the North Brazil Current (NBC) and the Brazil Current (BC) are western boundary currents (WBCs) that separate from the western Atlantic coast. The NBC retroflects and sheds several rings per year (at the retroflection region), whereas the BC rarely sheds rings near its separation point. Traditionally, the difference between these two WBCs has been attributed to the Malvinas Current (MC), whose momentum flux opposes the poleward momentum flux of the BC, thus preventing rings shedding at the point where the current leaves the coast. Even in the absence of the MC, rings from the separating BC would have never been regularly generated because of the relatively large slant of the coastline relative to the zonal direction. Using the recently proposed theory of Zharkov and Nof, it is demonstrated that the large inclination of the coastline between 20° and 45°S (approximately 50°) lies within the regime that does not allow the BC a continuous shedding of rings. In contrast, the inclination of the coastline between 5° and 8°N is sufficiently small to allow the NBC a continuous and smooth shedding of rings. The importance of the coastline inclination comes about through a ring β-induced westward propagation rate. In the small inclination case, the alongshore migration is fast, allowing the newly formed rings to quickly escape from their generation zone (i.e., before they are recaptured by the newly born rings generated behind). In contrast, in the high inclination case, the alongshore speed is so small that the rings spend a long time in the generation area and, consequently, are usually recaptured by the new rings generated just behind them. The authors argue, paradoxically, that the rings occasionally shed by the BC are probably due to the MC that advects the rings away from the generation area, preventing their recapture by the current behind them. Although no new analytical solutions are presented, the authors elaborate on the application of the recapturing condition to the NBC and BC and show new numerical simulations for both the NBC and the BC.

Identification, characteristics and seasonal evolution of surface thermal fronts in the Argentinean Continental Shelf

The location and seasonal variability of surface thermal fronts along the Argentinean Continental Shelf (38–55°S) were studied using 18 years (1985–2002) of sea surface temperature (SST) satellite data. Monthly SST gradients were calculated and a threshold was used to identify frontal pixels. Frontal areas were classified into 4 zones according to their seasonal evolution and the main forcings leading to the front's formation were identified for each group. The shelf break front was easily detected due to the large number of frontal pixels in the region and its high mean gradient values. This front showed a marked annual cycle and relatively constant position associated to the bottom slope; it tended to be located where the core of the Malvinas current is closest to the shelf. Tidal fronts also showed a strong annual cycle, being detected in three well-defined regions during spring and summer. Along the coasts of Tierra del Fuego and Santa Cruz, the combination of strong tidal mixing and low-salinity coastal plumes led to semi-annual seasonal cycles of frontal intensity and persistence that showed a relative maximum in winter. A similar behavior (semi-annual) was found at the coast off the Buenos Aires Province. There, the coastal dilution and the bathymetric gradient generated near-coastal fronts that changed direction seasonally. In the northern mid-shelf, a front linked to the intrusion of warm waters formed in the San Matías Gulf was identified during the winter.

Variations in the Malvinas Current volume transport since October 1992

We present a set of new current meter measurements collected to monitor the Malvinas Current (MC) near its merger with the Brazil Current at 40–41°S from December 2001 to February 2003 below a Jason‐1 altimeter track. These measurements are compared to former measurements obtained 8 years earlier at the same location; they also provide new information on the core of the MC on the continental slope above the 1000‐m isobath where a mooring had been previously lost. There, most of the velocity variation is along‐isobath (80% of the variance) and shows a significant annual cycle. The two data sets provide coherent means and statistical parameters on the vertical structure of the flow. A 14‐year‐long time series of MC volume transport is derived using satellite altimetry. The good correlation between the altimetry‐derived transport and the transport estimated from the current meter data persists in time (over 0.7 for each measurement period). A spectacular shift in the spectral composition of transport variations was observed: from 1992 until the end of 1997 transport variations occurred at rather short periods (50–90 days and to some degree around 180 days) whereas, after year 2000, longer periods including a seasonal cycle predominated. Altimetry‐derived anomalies of surface geostrophic velocities along the core of the MC show a similar shift in spectral composition suggesting a remote‐forcing origin.

Size variations of the vestibula of Krithe gnoma DO CARMO & SANGUINETTI, 1999 (Ostracoda): a new procedure for their analysis

The species Krithe gnoma was described from Holocene sediments on the continental margin of Brazil, where it occurs on both shelf and slope. The shelf distribution of this species is restricted to those areas influenced by the Malvinas current between 32°11' and 22°31'S. K. gnoma was selected due to the excellent resolution of images of the anterior vestibule. So the range in size of the vestibula of this species was used to determine whether or not the dimensions of the anterior vestibule were to some degree related to amount of dissolved oxygen in the water at the sampling site. Female valves of the 7th and 8th instars were measured and their sizes and those of the anterior vestibule were compared. These valves and the reference carapace were collected at six localities in the southern shelf area. The anterior vestibule of right and left valves of the 7th instar and one of the 8th instar from discrete localities were measured. In the reference carapace, the larger left valve also has the larger vestibule. A gerontic 8th instar has valves larger than those of the previous instars, but its vestibula are smaller than those of the 7th instar. The vestibula of the left valves of the 7th instar collected at discrete localities showed consistent differences in size related to the oxygen content of the sampling site. The relationship between vestibule size and oxygen content is inversely proportional. However, the poor correlation factor (R²<<1) of these results are not precise enough to support Peypouquet's hypothesis. But the size of the vestibula of K. gnoma does show some degree of relationship to oxygen content. So it is important to measure the vestibula of species of Krithe using the procedure described here. Rigorous application of this procedure will eliminate the possibility of discrimination based on variations in size due to ontogeny and dimorphism leaving only those related to actual valve size. In any case, variations in the size of vestibula may have a phenotypic origin.

On the origins of the variability of the Malvinas Current in a global, eddy‐permitting numerical simulation

This article investigates the origins of the variability of the Malvinas Current (MC) transport using the results of an eddy‐permitting ocean general circulation model. First, the dynamical links between the variabilities of the MC and the Antarctic Circumpolar Current (ACC) transports at the Drake Passage are established. Time series analyses indicate that the connection between the transport variations of the ACC and the MC is masked by high‐frequency oscillations. Although a substantial portion of this high‐frequency variability is produced and dissipated locally, there are also anomalies that propagate between those regions. The most conspicuous of these signals has an average speed of ∼6–7 cm s−1 and a period of ∼150 days. Second, the contribution of the wind forcing to the variability of the ACC and the MC is determined from Principal Estimator Patterns analysis. The analysis indicates that the variance of the transport of both currents is more directly related to the variability of the wind stress than to the wind stress curl. The transport variability is particularly sensitive to zonal wind stress anomalies in the latitudinal range of the Drake Passage that extends farther north over the South Indian Ocean. A rather counterintuitive result of our analysis is that the wind stress forcing over the South Atlantic Ocean is unimportant to the variability of the MC transport.

Environmental factors controlling the phytoplankton blooms at the Patagonia shelf-break in spring

The shelf-break front formed between Argentinean shelf waters and the Malvinas Current (MC) flow shows a conspicuous band of high phytoplankton biomass throughout spring and summer, detected by ocean color sensors. That area is the feeding and spawning ground of several commercial species of fish and squid and is thought to play an important role in CO 2 sequestration by the ocean. Phytoplankton blooms in this area have been attributed mainly to coccolithophorids, a group of calcite-producing phytoplankton. Here we present the environmental factors associated with the spring bloom at the Patagonian shelf-break (40°–48°S) in the austral spring 2004. A remarkable bloom of diatoms and dinoflagellates (approximately 1200 km long) was observed along the front, where integrated chlorophyll values ranged from 90.3 to 1074 mg m −2. It is suggested that supply of macro-nutrients by upwelling and probably iron by both upwelling and shelf transport contribute to maintaining the spring bloom. Strong water column stability along the front allowed the accumulation of algal cells mainly in the top 50 m and their maintenance in the euphotic layer. East of the shelf-break front, macronutrient levels were high (surface nitrate=16.6 μM, phosphate=0.35 μM, silicate=4.0 μM), associated with low phytoplankton biomass (<2 mg m −3). This was due to mixing and advection associated with the MC flow and to grazing pressure at a transitional site between the MC and the high chlorophyll patch. Primary production rates (determined by the 14C technique) ranged between 1.9 and 7.8 g C m −2 d −1. Primary production was highest near 42°S partly because of the elevated phytoplankton biomass, which consumed most of the nitrate and phosphate in surface waters in this region. These high primary production rates are comparable with maximal seasonal productivity at eastern boundary currents. The large bloom extent at the Patagonian shelf-break (approximately 55,000 km 2 patch of >2 mg m −3 chlorophyll), the associated primary production rates and diatom dominance indicate a potentially significant biological control of gases such as O 2 and CO 2 in surface layers. The main factors favoring the development and maintenance of these blooms are nutrient supply from MC upwelling and water column stability. Other processes such as mixing or grazing play an important role in biomass modulation in the region.

Shallow water Asteroidea and Ophiuroidea of Uruguay: composition and biogeography

Asteroidea (five species) and Ophiuroidea (18 species) recorded along the Uruguayan littoral (up to 50 m depth) are summarized and their distributional pattern are discussed. Species are gathered into four groups: 1) A. taxa distributed from North/South Carolina, or from the Caribbean Sea, to Southern Buenos Aires Province (Argentina), B. species ranging from North/South Carolina, Florida, Sao Paulo, to the mouth of Rio de la Plata, 2) species ranging from Rio de Janeiro south to the San Matias Gulf, 3) taxa recorded from Chile or from Antarctic/Subantarctic waters north to Uruguay, 4) a single cosmopolitan species. While ophiuroids are represented in each one of the mentioned groups, asteroids are restricted to the first one. As it happens with other elements of the Uruguayan fauna, this complex pattern is a consequence of the confluence of the warm Brazilian and the cold Malvinas (Falkland) currents, and of the discharge of fresh, muddy waters of the Rio de la Plata. Rev. Biol. Trop. 56 (Suppl. 3): 205-214. Epub 2009 January 05.

Population genetic structuring of the king weakfish, Macrodon ancylodon (Sciaenidae), in Atlantic coastal waters of South America: deep genetic divergence without morphological change

Phylogeographic patterns in Macrodon ancylodon sampled from 12 locations across all its range were investigated using mitochondrial DNA cytochrome b sequences, and analysed together with patterns of morphometric differentiation. Populations of the North Brazil and the Brazil currents, with warmer waters, form a clade (tropical clade) separated by 23 fixed mutations from the populations that inhabit regions of colder waters influenced by the Brazil and Malvinas currents (subtropical clade). No gene flow exists between the tropical and subtropical clades, and most likely also between the two groups of the tropical clade. Distribution of these clades and groups is correlated with flow of currents and their temperatures, and is facilitated by larval retention and low adult migration. Despite differentiation at the molecular level, fishes analysed from all these current-influenced regions are morphometrically homogeneous. Throughout its range M. ancylodon inhabits the same, or very similar niche; thus, stabilizing selection probably promotes the retention of highly conserved morphology despite deep genetic divergence at the mitochondrial DNA cytochrome b.

Circumpolar Deep Water Circulation and Variability in a Coupled Climate Model

Abstract The natural variability of Circumpolar Deep Water (CDW) is analyzed using a long-term integration of a coupled climate model. The variability is decomposed using a standard EOF analysis into three separate modes accounting for 68% and 82% of the total variance in the upper and lower CDW layers, respectively. The first mode exhibits an interbasin-scale variability on multicentennial time scales, originating in the North Atlantic and flowing southward into the Southern Ocean via North Atlantic Deep Water (NADW). Salinity dipole anomalies appear to propagate around the Atlantic meridional overturning circulation on these time scales with the strengthening and weakening of NADW formation. The anomaly propagates northward from the midlatitude subsurface of the South Atlantic and sinks in the North Atlantic before flowing southward along the CDW isopycnal layers. This suggests an interhemispheric connection in the generation of the first CDW variability mode. The second mode shows a localized θ−S variability in the Brazil–Malvinas confluence zone on multidecadal to centennial time scales. Heat and salt budget analyses reveal that this variability is controlled by meridional advection driven by fluctuations in the strength of the Deep Western Boundary and the Malvinas Currents. The third mode suggests an Antarctic Intermediate Water source in the South Pacific contributing to variability in upper CDW. It is further found that NADW formation is mainly buoyancy driven on the time scales resolved, with only a weak connection with Southern Hemisphere winds. On the other hand, Southern Hemisphere winds have a more direct influence on the rate of NADW outflow into the Southern Ocean. The model’s spatial pattern of θ−S variability is consistent with the limited observational record in the Southern Hemisphere. However, some observations of decadal CDW θ−S changes are beyond that seen in the model in its unperturbed state.

Ocean‐atmosphere in situ observations at the Brazil‐Malvinas Confluence region

This paper presents a description of marine atmospheric boundary layer (MABL) and oceanic boundary layer (OBL) interactions at the Brazil‐Malvinas Confluence. Although this region is known as one of the most energetic zones of the World Ocean, very few studies have addressed the mechanisms of OA interaction there. Based upon novel, direct in situ simultaneous OA observations, our results show that the OBL‐MABL exchanges are closely correlated with the sea surface temperature (SST) field. The heat fluxes range from 110 W.m−2 over warm waters down to 18 W.m−2 over cold waters. Higher heat fluxes and air‐sea temperature differences are associated with stronger near‐surface winds. This suggests that the MABL is modulated at the synoptic temporal and spatial scale by the strong surface thermal gradients between the (warm) Brazil and the (cold) Malvinas (Falklands) currents.

SST-Induced Surface Wind Variations over the Brazil–Malvinas Confluence: Satellite and In Situ Observations*

Abstract The confluence of the Brazil–Malvinas Currents maintains strong sea surface temperature (SST) fronts in the midlatitude southwestern Atlantic year-round. SST effects on near-surface stability and surface wind variations are examined in this region using satellite and in situ datasets. Satellite observations show strong (weak) surface wind speeds over the warm Brazil (cold Malvinas) Current. A novel feature of this study is the construction of a high-resolution surface meteorological dataset that is based on historical ship observations. Analysis of this new in situ dataset reveals an increased (reduced) sea–air temperature difference over the Brazil (Malvinas) Current, indicating destabilization (stabilization) in the atmospheric boundary layer. These results are consistent with the SST-induced vertical mixing mechanism for wind adjustment. The SST effect on the near-surface atmosphere is observed both in the climatology and on interannual time scales in the Brazil–Malvinas confluence. Along a zonal SST front at 49°S northeast of the Malvinas/Falkland Islands, there is a collocated line of surface wind divergence, with moderate convergence to the north. Vertical mixing does not explain this divergence pattern because the prevailing surface winds are westerly, blowing in parallel with the front. An additional mechanism is proposed for boundary layer wind adjustment.

Patterns in diversity and community structure of epipelagic copepods from the Brazil–Malvinas Confluence area, south-western Atlantic

Diversity and structure of epipelagic copepod communities were investigated using 70 zooplankton samples collected from the top 50 m of the Brazil–Malvinas Confluence area between 35° and 55°S. Biogeographic patterns were investigated using multivariate analysis. Biodiversity patterns were examined using different univariate indexes. Representatives of 35 species of copepods from 23 genera and 13 families were found. Two zones were delimited based on their copepod composition, one in the subtropical waters of the Brazil current, and the other in the subantarctic waters of the Malvinas current. Both environments displayed contrasting patterns of biodiversity depending upon which element is measured (regional species richness as well as mean point species richness were significantly higher in the subtropical group of stations, whereas taxonomic distinctness was significantly higher in the subantarctic community). Based on these contrasting results, we suggest the use of both kind of indexes when defining priority areas for conservation.

Validation of SeaWiFS-derived chlorophyll for the Rio de la Plata Estuary and adjacent waters

The South Atlantic Ocean near the Río de la Plata Estuary is a highly dynamic region that encompasses four different water bodies: the Río de la Plata, the continental shelf and the Brazil and Malvinas currents. Bio-optical measurements obtained during 6–11 November 1999 provided the initial field data for validating Sea viewing Wide Field of view Sensor (SeaWiFS)-derived chlorophyll imagery for this region. The turbid waters of the Río de la Plata Estuary showed the highest variability and complexity in water optical properties, while the offshore waters of the Brazil Current had the lowest. In most cases, the estimates of chlorophyll using the SeaWiFS algorithm were higher than in situ measured values.

Investigation of the Brazil Current front variability from altimeter data

The southwestern Atlantic Ocean is characterized by the confluence of the Brazil and Malvinas Currents, which form very strong surface and subsurface fronts that can be detected from hydrographic and remote sensing observations. Three data sets, consisting of TOPEX/Poseidon‐derived sea height anomalies, the climatologically derived depth of the 10°C isotherm, and reduced gravity, are used in conjunction with a two‐layer dynamical ocean scheme to monitor the Brazil Current front and to investigate its variability during a 6 year period (1993–1998). Results reveal that the fronts exhibit motions that are larger zonally than meridionally, showing strong interannual variability with annual mean amplitudes that range from 1° to 6°. The annual and semiannual components account for more than 75% of the variability of the frontal oscillations. In the annual cycle the frontal motions appear to be related closely to fluctuations in the baroclinic transport of the Brazil Current and are only influenced by the Malvinas Current when the Brazil Current transport is very small.

Comparison of near‐surface mean and eddy flows from two numerical models of the South Atlantic Ocean

The near‐surface mean and eddy flows simulated by two eddy‐permitting models of the South Atlantic (S‐coordinate Primitive Equation Model (SPEM) and Ocean Parallel (OPA) model) are compared. These models are intrinsically different by virtue of their vertical discretizations (σ versus geopotential coordinates, respectively), so that their solutions differ essentially through the representation of current‐topography interactions. The path of the Agulhas Current, the generation process, the characteristics, and the subsequent trajectory of the Agulhas rings are differently affected by topography in the two models. Along the American western boundary, SPEM traps a larger part of the Antarctic Circumpolar Current (ACC) around the Falkland Plateau and consequently produces a stronger Malvinas Current, which overshoots far to the north, inshore of a rather realistic Brazil Current and Confluence regime. In contrast, OPA allows a large part of the ACC to flow across the Scotia Sea, produces a weaker Malvinas Current, traps the Brazil and Malvinas Currents along the American coast, and tends to distort the complex dynamics of the Confluence region and the Zapiola anticyclone. Some of these differences can be attributed to the particularities of each vertical coordinate system and to the representation of topographic slopes (staircases in OPA and facettes in SPEM). More generally, the topographic constraint (trapping of boundary currents and interaction of mean and eddy flows with bathymetry) is stronger in SPEM than in OPA. The topographic smoothing usually performed in sigma coordinate models removes mesoscale topographic roughness and reduces topographic slopes. This treatment is probably responsible for several of the differences observed in the numerical solutions.

Annual cycle of the Brazil‐Malvinas confluence region in the National Center for Atmospheric Research Climate System Model

The objective of this study is to compare the mean and seasonal variability of the circulation in the southwest Atlantic with observations. The results used in the comparison are from the last 200 years of a 300 year control integration of the Climate System Model (CSM). The area of study includes the confluence region between the subtropical and subpolar waters represented by the Brazil and Malvinas Currents. The seasonal variation of transport and its relationship to changes in the wind stress forcing and in the sea surface temperature are examined and compared to available oceanographic observations. This study shows that a coarse resolution climate model, such as the CSM, can successfully reproduce major characteristics of the Brazil‐Malvinas confluence seasonally, although the mesoscale features involving recirculation and meander dynamics are not resolved. The CSM transport values in the region of 38°S are consistent with hydrographically derived values. The transport of the CSM Brazil Current is higher during austral summer and smaller during austral winter. Conversely, the Malvinas Current transport is weaker during austral summer and stronger during austral winter. This is also consistent with observations. The CSM seasonal cycle in transport associated with both the Brazil and Malvinas Currents and its meridional displacement is closely linked to the seasonal variations in the local wind stress curl. However, the displacement is much smaller in the model than in observations. The CSM results show that the latitudinal displacement of the 24°C and 17°C at the South American coast beween austral summer and winter is 20° and 12°, respectively. This is very similar to the displacement seen in observations.

Foraminíferos uniloculares calcáreos del Eoceno medio-superior y Oligoceno inferior, Isla Grande de Tierra del Fuego y plataforma continental adyacente, Argentina

Unilocular calcareous Foraminifera of the middle-upper Eocene and lower Oligocene from Tierra del Fuego Island and adjacent continental platform, Argentina. The calcareous unilocular Foraminifera of the Leticia Formation (upper middle Eocene), Cerro Colorado Formation (upper middle Eocene-upper Eocene) and the Cabo Peña Formation (lower Oligocene) from Tierra del Fuego Island are described. The Leticia Formation yields scarce unilocular foraminifers that are characterized by species described from the Maastrichtian and the lower Eocene, including Conolagena argentina , C . acuticosta and Sipholagena sp. The Cerro Colorado Formation bears 29 species, almost the half of which (both individuals and species) belong to the Genus Fissurina , with Fissurina bisulcata occurring as the most abundant taxon. The formation also contains species characteristic of either Antarctic waters or the Malvinas current, reflecting the temperature decrease in the oceanic waters during the middle-late Eocene. The Cabo Peña Formation, which contains the Spirosigmoilinella-Martinottiella Assemblage typical of the Antarctic region, yielded 19 species; the dominant unilocular species belong to the genera Fissurina and Parafissurina , and the most conspicuous are fimbriate forms, such as F. fimbriata and F. fueguina, which are associated with Antarctic waters.

Transports of the Brazil and Malvinas Currents at their Confluence

Geostrophic transports of the western boundary currents at the Brazil/Malvinas Confluence in the South Atlantic Ocean are estimated from the data set of the Confluence 3 cruise (February 1990) with a nonlinear inverse model which takes into consideration density, current meter and wind data, and dynamical (planetary vorticity, Ekman, mass conservation) constraints. Inversions are carried out with two initial different levels of no motion at 1500 m (Case A) and at 3000 m (Case B). Consistencies of the water volume transports provided by both inversions are analyzed and compared to previous estimates. Current meter constraints are applied in the Malvinas Current region where a total transport of 45 ± 7 Sv (I Sv = 10 6 m 3 s -1 ) is given by both inversions. Within the Brazil Current region, discrepancies between both inversions appear. Case A provides a total transport of 30 ± 7 Sv while case B gives a total transport of 56 ± 8 Sv. In the first two layers (0-1000 m; 1000-2000 m), case B (53 Sv) gives larger transport than case A (32 Sv). North of the Confluence and at the North Atlantic level, water is found to flow northward in case A at a rate of 3.4 ± 2 Sv and southward in case B at a rate of 3 ± 3 Sv. Case B results are more in agreement with our present knowledge of water mass circulation in the Confluence region than case A results. Eastward transports at the Brazil-Malvinas Confluence are estimated to be 20 ± 7 Sv and 30 ± 7 Sv for cases A and B, respectively. Compared to the total transports of the Brazil and Malvinas Currents, these estimates suggest that most of the water supplied to the Confluence area recirculates within the Brazil Current and the Malvinas Current regions.

Food and feeding of the short-finned squid Illex argentinus (Cephalopoda: Ommastrephidae) off southern Brazil

Diet and feeding habits of the short-finned squid Illex argentinus were studied from the analysis of 729 stomachs of juveniles, subadults and adults caught with trawls on the continental shelf and slope off southern Brazil (26 °35′S–34 °31′S) from 1981 to 1992. Food, in different degrees of digestion, was found in 363 stomachs (49.8%). Feeding activity occurred at day and night and seemed to be most intense at dusk and early night. The proportion of stomachs with food increased with size and sexual maturity. Fish occurred in 43.8%, cephalopods in 27.5% and crustaceans in 18.7% of the stomachs with food. Identified prey included the fish Diaphus dumerilii, Maurolicus muelleri and Merluccius hubbsi, the cephalopods I. argentinus, Loligo sanpaulensis, Spirula spirula, Semirossia tenera and Eledone gaucha and the crustaceans Oncaea media and Euphausia sp. The occurrence of fish increased with squid size, but both cephalopods and crustaceans were equally important for all sizes. Cannibalism was observed at all sizes. The overall low proportion of stomachs with food, the high rate of cannibalism and the low frequency of occurrence of crustaceans for the juveniles and subadults in all seasons, but particularly in the summer and autumn, may reflect a limited availability of food in the region. If this is true, the main nursery grounds for the spawners off southern Brazil are likely to be off the Rio de La Plata Front or in the offshore confluence between the Brazil and Malvinas Currents, where primary and secondary production is higher than off southern Brazil.

The Drifting Confluence Zone

Alongshore migration of a western boundary current separation is investigated with a nonlinear inviscid reduced gravity model on an f plane. Separation due to a collision with an opposing current is considered. It is known that for such a case stationary collision and separation is possible only for boundary currents with “balanced” transports, that is, equal near-wall depths. The authors perturb this stationary solution with a small steplike variation of the opposing current transport and focus on the resulting time-dependent flow. Two different analytical methods to compute the migration rate are used. The first method involves integrated balances, and the second involves the path equation for the separated flow. Using the fist approach, it is found analytically that the flow consists of one current intruding into the area occupied by the other. After an initial adjustment period the intrusion becomes steadily propagating. The width of intrusion is much greater than the width of boundary currents, whereas the migration speed is much smaller than the speed of the currents. The speed of the opposing current intrusion into the main western boundary current is given approximately by the formula C ≈ (g′H)1/2(D21 − D22)2/4D3/22, where H is the undisturbed depth of the main poleward flowing current, D1 is its depth near the wall (nondimensionalized by H), and D2 is the opposing current near-wall (nondimensional) depth. Due to the nonlinearity of the process, the expression for the opposite speed (i.e., the main western boundary current intrusion into the opposing current) has a similar looking but different form. Using the second approach, the original initial value problem is reduced to a time-dependent path equation for the separated current. It is shown analytically that, as should be the case, in the limit t → ∞ the path equation solution is identical to the earlier solution for the steadily propagating intrusion. The migration process exhibits an hysteresis; that is, the progression of the separated currents differs from its corresponding regression. Application of the theory to the collision and separation of the Brazil and the Malvinas Currents is discussed. It is suggested that the observed migrations of the separation latitude may be caused by seasonal changes of the Malvinas Current transport.