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The South Equatorial current (SEC) as represented by the Mariano Global Surface Velocity Analysis (MGSVA). The SEC is a broad westward flow that is the primary basin-scale surface ciculation in the equatorial Atlantic. Click here for example plots of seasonal averages. | |
The South Equatorial Current can be divided into three branches: The Southern South Equatorial Current (SSEC), the Central South Equatorial Current (CSEC), and the Northern South Equatorial Current (NSEC). According to Stramma (1999), the CSEC could be further separated into two other branches. The southern branch of the westward flowing South Equatorial Current is located between the South Equatorial Counter Current and is found from about 8°S to 25°S. The SSEC is fed by the Benguela Current, which crosses the Greenwich Meridian south of 20°S, and flows westward to the premonitory of South America, Cabo de Sao Roque. Here, at 30°W the SSEC turns northward. A small part of the water turns polewardsouth of 10°S to form the Brazil Current, whereas the bulk of the flow contributes to the North Brazil Current and the South Equatorial Counter Current (SECC). The poleward boundary of the current southern branch is at 25°S through the whole thickness of the subtropical gyre, but the latitude of the northern boundary varies from 7°30' at the surface and in the western South Alantic, to 27°S at 1400 m depth and in the eastern basin (Wienders et al., 2000). The estimated latitude of its bifurcation into the Brazil Current and North Brazil Current also varies downward from about 14°S at the surface to 18°S at a depth of 600 m.
The SSEC is a broad sluggish flow with an average westward surface speed of 11.3 cm s-1. The highest average surface velocities were found between 9° and 10°S reaching 16.4 cm s-1. Several measurements at 30°W show the broad expansion of the southern band of the South Equatorial Current between 8°S to 10°S, while further east the SSEC is found between 11°S to 17°S. The average transport from the southern band of the South Equatorial Current is about 20 Sv for the upper 500 m, and in the upper and intermediate layers, the westward transport was measured to be 49 Sv. The variability transport observed might be related to a possible seasonal or annual variability, as well as, mescoscale variability.
The Central South Equatorial Current (CSEC) is located between the South Equatorial Undercurrent (3°S-5°S), and the South Equatorial Counter Current (6°S-9°S) (Molinari et al., 1981; Peterson and Stramma, 1990; Stramma, 1991). The average transport of the CSEC in the upper 200 m is about 17 Sv with typical variation between 7 to 26 Sv (Molinari, 1982), and a maximum of 29 Sv. The CSEC bifurcates at about 5°-6°S and separates into two portions. The northern portion transports about 15 Sv Northwestward, and north of 5°S, the CSEC coalesces with the North Brazil Current to form a 300 km wide current that transport more than 36 Sv equatorward (Da Silvera and Al., 1994). Part of the northern portion of the CSEC turns north-eastward and supplies the South Equatorial UnderCurrent (SEUC). The southern branch of the CSEC transport 14 Sv south-westward until about 9°30'S, where it makes a cyclonic turn to the north and merges with northward flow from the southernmost branch of the SSEC (Da Silvera et al., 1994).
The westward flowing Northern South Equatorial Current is situated between 1°N and the SEUC at 3°S-5°S (Peterson and Stramma, 1990), and extends farther north during the boreal winter-spring. Its velocity rarely exceeds 30 cm s-1 and its transport in the upper layer is about 12 Sv, with a seasonal variation between 6 to 23 Sv according to Peterson and Stramma (1990). However, measurements show a westward transport of 12.5 Sv in September, 8.5 Sv in February, 7.7 Sv in March and 4.5 Sv (as a minimum) in April (Bourles and al., 1991).
The SEC was found to divide seasonally near the eastern tip of Brazil where residual alongshore velocities are northward for half the year (peaking during May and June) and southward for the other half of the year. Seasonal changes of dynamic topography on the sea surface relative to 500 dbar between 20°S and 30°N were studied by Merle and Arnault (1985). They found only a small variability in dynamic topography in the central and western South Atlantic north of 20°S. More recent results on the Equatorial Atlantic annual mean ship drift, the surface geostrophic current relative to 500 dbar, and the Ekman drift have been presented by Arnault (1987). Large differences exist between thesurface drift found in ship drifts or surface drifters and in geostrophic surface currents. Between latitudes 2°S and 8°S, Arnault (1987) found an averagewestward ship drift at all longitudes and for all months. However, the surface geostrophic currents were eastward, east of 8°W, except for the period between June and September, when they were westward. The geostrophic component of the SEC is strongest during the austral winter and weakest during the austral summer (Molinari, 1982).
The southern branch of the SEC is situated at the formation region of the Salinity Maximum Water (8°S to 25°S), which is formed at the surface through intense evaporation. The average salinity of the SSEC was measured between 37.00 and 37.20. The CSEC and NSEC show lower salinity, between 36.00 and 36.20 at the surface, and less than 36.0 at 100 m depth (Wienders and al., 2000). The average temperature of the South Equatorial Current is about 26°-28°C near the surface and 22°-24°C at 100 m depth (Mayer and al., 1998). The annual cycle of temperature is relatively stable between March and September, the two seasonal extremes, with a variability of about 4°C.
The South Equatorial UnderCurrent (SEUC) is, on the average, an eastward flowing current forced by the trade winds. The SEUC is located between 3°S and 5°S, and is a sub-surface intensified eastward flow. Its flow is maximum in boreal fall (September to early October) and minimum in boreal spring (April-early June) around 30°W. The SEUC contains oxygen-enriched water of southern origin that indicates that it is supplied by the North Brazil Current retroflection. However, measurements show that the SEUC is also fed by SSEC oxygen-poor water. Surface flow above the SEUC is to the east during austral summer (November to early March) and to the west during July and August. At 3°S, the SEUC is found as a subsurface current between 100 m and 1000 m while the surface flow is directed eastward again. Its eastward transport is estimated to be 15 Sv (Cochrane and al., 1979), with a variation between 2 and 23 Sv. More precise measurements estimate the transport to be 3.1 Sv in February, 2.2 Sv in April, and the current is located between 200 m and 300 m depth during this period. In March, the SEUC appears centered around 4S, with a velocity greater than 20 cm s-1. Its temperature usually varies between 24°C at 36°W and 20°C at 20°W at 100 m depth and is 12°C at 300 m depth with a typical 4°C seasonal variability for depths between 100 and 200 m. The salinity of the SEUC is less than 36.0, measurements indicate about 35.6 at 100 m depth, 35.2 at 200 m, and 35.0 at 300 m depth (Wienders and al., 2000).
The South Equatorial Counter Current (SECC) is an eastward flowing current extending from the surface down to at least 400 m depth. Its formation region is located near 30W (Stramma and Schott, 1996). The SECC seems to cross the entire South Atlantic at about 7°S to 9°S to at least the Greenwich Meridian but part of the water might get entrained and redirected by the central branch of the SEC flowing westward at about 5°S. The SECC has a sea surface expression at 7°S to 8°S, with an intensified subsurface velocity core (20 cm s-1) at an approximate depth of 300 m. This subsurface maximum in velocity seems to be connected to the subsurface velocity core of the South Equatorial Undercurrent Current (SEUC) at 3°S. However, a second velocity core is present around 40 m depth, separated from the deeper velocity core by a westward flow (with significant westward velocity of about 9 cm s-1 near 7°S around 110 m depth. According to Molinari (1982; 1983), the westward surface flow events usually occur in boreal winter (October-early March) above the subsurface SECC. He suggests that while the SECC is primarily geostrophically driven, strong southeast trade winds can induce westward reversals of the SECC during this period of the year. Its eastward transports was observed between 3 to 7 Sv in the upper 1000 m (Molinari, 1982), and between 4.6 and 10.4 Sv in the upper 500 m. The weak surface expression and strong seasonal variations in flow directions makes it difficult to detect the SECC in maps of average surface velocity.
From temperature and salinity distributions, Molinari (1982; 1983) indicates that the SECC is supplied, in the surface layers, with warm and salty subtropical waters by the southern branch of the SEC. It also exhibits high oxygen concentrations over the whole water column, while low oxygen values measured within the SECC at temperatures lower than 19°C indicate that the SECC is also fed with Equatorial Atlantic water below the thermocline. Its salinity varies between 36.2 at the surface, to 34.8 at 400 m depth (Wienders and al., 2000). Its temperature varies between 26°C at the surface to 12C near 400 m depth, with a seasonal variation of about 3-4C in the 300 m upper layer.
The Equatorial Under-Current (EUC) is a strong subsurface current flowing eastward at the equator in the Atlantic, located between 2°N and 2°S. It is fed by the North Brazil Current which crosses the equator and retroflects about 5°N, and by the North Equatorial Countercurrent. The EUC is clearly visible with a core at about 100 m depth, which moves downward in boreal summer-fall and upward in boreal winter-spring, following the vertical migration of thermocline. Hydrographic measurements show that the depth of the core of EUC coincides with the depth of the thermocline of the tropical Atlantic (Schott et al. 1998). The tropical thermocline waters are fed by a meridional circulation that consist of downwelling about 20000 km off the equator and a shallow adiabatic subsurface equatorward flow. Ekman-driven, divergent flow, at the equator, forces equatorial upwelling and polward surface flow to form a closed shallow subtropical cell. The eastward flowing EUC is characterised by a strong velocity core reaching 90 cm s-1. During every hydrographic cruise, an eastward near-surface flows is present above the EUC (at least up to the first ADCP measurements depth, either 16 or 28 m), with a velocity up to 40 cm s-1. In boreal winter-spring, eastward equatorial near-surface currents have been observed and are explained by the relaxation of the wind forcing and an eastward pressure gradient (Katz and al., 1981).
The largest EUC transport, including the eastward near-surface flow, measured in April was 28.3 Sv, and the weakest transport of 22.5 Sv was measured in February. Results show that, if a seasonal cycle exists, it is strongly masked in boreal spring by wind trade relaxing, yielding to near-surface eastward jets and hence to strong eastward transport events. By only considering the subthermocline layer, the EUC flow does not indicate significant seasonal variations. The core of the EUC has temperatures around 20°C, which is about 5°C cooler than the surface waters in the equatorial region. In September and April, high South Atlantic Water (SAW) salinity values (greater than 36.6) present within the EUC core agree with a direct SAW supply through the NBC retroflection. However, relatively fresher waters (less than 36.4) are also observed along its poleward boundaries. This indicates that Equatorial Atlantic Water (EAW) with its relatively low salinity waters may also feed the EUC. That point, along with the presence of SAW within the northern SEC, supports the existence of recirculation and water mass exchanges between the Equatorial Undercurrent and the South Equatorial Current, flowing in opposite directions.