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The Brazil current as represented by the Mariano Global Surface Velocity Analysis (MGSVA). The Brazil current is the western boundary current of the South Atlantic subtropical gyre. It transports warm water polewards. Near 22 S, the Brazil current splits; one component flows eastward and the other component hugs the coast and flows toward the southwest and interacts with the colder Malvinas Current. Click here for example plots of seasonal averages. | |
The Brazil Current is a weak western boundary current carrying warm subtropical water, which runs south along the coast of Brazil from about 9°S to about 38°S and is generally confined to the upper 600m of the water column. Its origin begins where the westward flowing trans-Atlantic South Equatorial Current (SEC) bifurcates (or splits) as it approaches the continental shelf off of Cabo de Sao Roque, Brazil (Stramma et al., 1990; Podesta et al., 1991). SEC water flowing north becomes the North Brazil Current, and the branch flowing south becomes the Brazil Current (BC). Although Isaaci Vossius is given credit as the first to recognize and describe the Brazil Current in 1663 in "A Treatise Concerning the Motion of the Seas & Winds," it was James Rennell in 1832 who worked out details of the actual flow, and dubbed the current the Brazil Current. He had been a member of the British Royal Navy and later joined the East India Company, making several trips across the Atlantic under the supervision of John Purdy. In addition to his detailed descriptions of these currents, it was Rennell who first determined that the BC was weaker than the North Brazil Current and was among the first to map out the surface currents along the New World continents (Peterson et al., 1996).
The Brazil Current begins at about 10°S, separating slightly from the coast near 12°S where the continental shelf becomes wider (Peterson & Stramma, 1991; Stramma et al., 1990). Satellite images taken over three years (1984-1987) show that the actual point at which the BC separates from the continental shelf varies anywhere between 33°S-38°S, with the average being about 36°S (Olson et al., 1988; Podesta et al., 1991). The BC continues to flow south off the Brazilian coast until it reaches about 33-38°S, when it collides with the north-flowing Malvinas (Falkland) Current. The BC is then, in part, deflected to the east offshore of Rio de la Plata, a region known as the Brazil-Malvinas Confluence Zone (BMC), one of the most energetic regions in all the oceans (Sarceno et al.,2004). Gordon and Greengrove (1986) were the first to label this region the Confluence. The latitude of confluence, which determines where the BC will separate from the continent, is farther north during austral winter and spring. This seasonality is presumed to be related to the general seasonal shift of wind systems and seasonal meridional shift of the subtropical gyre (Peterson & Stramma, 1991).
The transport of the Brazil Current is considered small when compared to that of the Gulf Stream, its counterpart in the Northern Atlantic. The problem when estimating transport of the BC is that in its northern region, it is shallow and closely confined to the continental shelf. Transport values between 5 Sv and 6.5 Sv have been observed near surface waters (upper 500m) of the BC around 20°S (Peterson and Stramma, 1990; Stramma et al., 1990). At about 20.5°S, the current encounters the Vitoria-Trindade Ridge, a zonal seamount chain where it has been observed to flow through the inshore passage rather than the passages farther east. In this region, a cyclonic gyre seaward of the Brazil Current, centered at about 17°S and 34°W has been observed and attributed to the southernmost meanders of the South Equatorial Current that are reflected northward by this same seamount chain (Memery et al., 2000; Stramma et al., 1990). At about 20.5°S, near the seamount chain, the current flows at about 50-60 cm s-1 as estimated by Evans and others (1983).
As the Brazil Current flows south of 24°S, its flow intensifies by about 5% per 100km, which is similar to the growth rate in the Gulf Stream, although transport values in the BC are considerably less (Peterson and Stramma, 1991). Thus, at about 33°S the total transport (which includes a recirculation cell in the upper 1400m) is about 18Sv, and reaches values from 19-22 Sv at about 38°S, where it encounters the Malvinas (Falkland) Current (Olson et al., 1988; Peterson and Stramma, 1991). The mean latitude of the BC's separation from the shelf break is about 35.8°S ± 1.1° and for the Malvinas Current, the mean latitude of separation is 38.9°S ± 0.9°. The coastal ranges of the separation positions are at 950km and 850km respectively (Olson et al., 1988).
The combined flow of the two currents causes a strong thermohaline frontal region, called the Brazil-Malvinas Confluence (BMC) in which the BC breaks off into two branches, one turning to the north forming a recirculation cell, while the other continues southward and veers northeast at about 45°S, becoming the South Atlantic Current (Boebel et al., 1999; Saraceno et al., 2004). The mean transport in this region has been measured to be about 11Sv (Garzoli and Bianchi, 1987). Maximum velocities at the confluence (at about 38°S) reach 55 cm s-1 with the average value of 35 cm s-1 with transports of 18 and 11 Sv respectively. Flow can increase up to 23 Sv at the Brazil-Malvinas Confluence (Garzoli, 1993) Mean conditions of circulation vary significantly, and more recent evidence shows that it is likely related to meteorological anomalies (Assireu et al., 2003). Some short term variability in the southward extent of the BC has also been observed. Occasionally, when a BC meander that has extended unusually far south retreats, it can shed a series of warm core eddies that migrate into the Antarctic Circumpolar Current (Partos and Piccolo, 1988). Values also vary according to measurement method and depth. A comprehensive overview of literature on BC transport estimates prior to 1991 can be found in Table 2 of Peterson and Stramma (1991).
The range of the Confluence oscillate between about 54°W and 45°W, a total distance of about 770 km (at 38°S). The meanders appear to occur on a twelve month cycle and are likely correlate to changes in the separation latitude of the Brazil Current (Boebel et al., 1999; Garzoli and Bianchi, 1987; Goni & Wainer, 2001; Maamaatuaiahutapu et al., 1999; Zavialov et al., 1999). The mean speed of the front is estimated to be about 14 cm s-1. The front oscillates around its mean seasonal position (farther north and east during austral winter and farther south and west during austral summer) within a period of about one month and an amplitude that varies from 10-50 km per day. The mean velocity of the displacement of the front reaches values up to 10 km/day (Garzoli and Bianchi, 1987). This area is also rich in eddies, more often called Brazil Current Rings, averaging to about 7-9 rings per year. These elliptical rings can vary in size from about 56 to 225 km along the semi-major axis, and 23 to 108 km for the semi-minor axis. These anticyclones have a mean lifetime of about 35 days and translational speeds of anywhere between 4-27 km per day (Lentini et al., 2002).
On average, the temperature in the Brazil Current is about 18°C-28°C, with essentially three meridional zones that experience several degrees of distinctly different annual temperature fluctuations, which corresponds to their proximity to shore. The first zone is located over the shelf and experiences temperature variability of 7-10 degrees, which is controlled by both winter invasions of subantarctic water from the Malvinas Current and discharges from Rio de la Plata and Patos-Mirim. The second or central portion, closer to the eastern margin of the continental shelf, experiences a 5-7 degree variance. The third, on the seaward-most zone, shows little fluctuation until the Confluence (Memery, et al., 2000; Zavialov et al., 1999). Temperatures in the southern section of the current, near the Confluence, can change by 5-13 degrees, with the cooler temperatures occurring around August-September and the warmer values observed in February (Boebel et al., 1999; Podesta, et al., 1991). Almost yearly temperature anomalies of warm and cold fronts occur that seem to be related to the El Nino-Southern Oscillation (ENSO) events. Anomalous cold water extensions to the north occur on the shelf generally one year after every warm ENSO event, and anomalous warm water extensions occur generally one year after every cold ENSO (Lentini et al., 2001). Surface salinities indicative of Brazil Current waters range from 35.1 to 36.2, with the maximum commonly found at around 20°S, where it can reach a salinity of 37.3 (Memery et al., 2000; Wilson & Rees, 2000).