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The North Brazil current as represented by the Mariano Global Surface Velocity Analysis (MGSVA). The N. Brazil current transports a significant amount of water, including freshwater from the Amazon, northwestward along the coast of northern Brazil, French Guiana, and Suriname. It is fed by the S. Equatorial Current and part of it becomes the Guiana Current. Depending on the season, the N. Brazil current retroflects and feeds the N. Equatorial counter current. This retroflection is strongest during the summer months. Click here for example plots of seasonal averages. | |
Beginning at about 10°S, the mass of the northward warm water flow that is to become the NBC follows the coast as the North Brazil Undercurrent (NBUC), carrying around 23 Sv above 1000 m depth. The velocity of the NBUC is about 80 cm s -1 at 200 m but reducing in speed and sometimes even moving southward near the surface, giving it its "undercurrent" quality in this region. Very little inflow is gained between 10°S and 5°S, where a constant flow of about 21 Sv was found by da Silveira and others (1994) off the continental slope of Brazil. Transport over the shelf ranges from 3-5 Sv (Johns et al, 1998). However, measurements taken at about 4°S in the upper 300 m showed that the NBC has a significant annual cycle in this area, ranging from a maximum transport of about 36 Sv in July-August to a minimum of 13 Sv in April-May, with an annual mean transport of about 26 Sv (Johns et al, 1998). At about 5°S, zonal inflow from the east by the surface-intensified SEC enters the NBC system, adding transport northwest of Cabo Sáo Roque at about 35°W, and transforming the NBUC as it moves towards the equator into a surface-intensified current, the North Brazil Current (Chepurin and Carton, 1997; Schott et. al., 1998). At this point, at about 44°W, the NBC is a 300 km wide swath transporting about 35-36 Sv equatorward (includes the NBUC transport), with a seasonal amplitude of about 3 Sv (Bourles et. al., 1999; Johns et. al., 1998; da Silveira et al, 1994; Schott et. al., 1998).
The total meridional warm water flow in the near-equatorial boundary current substantially exceeds the required Atlantic meridional overturn of some 17 Sv produced by basin-scale wind forcing alone. The reason for this is that three cells exist in this region, contributing to the transport volume: (1) the top-to-bottom global "conveyor belt" circulation (2) the shallow, tropical-subtropical cell (STC) which connects the subduction regions of the eastern subtropics with the Equatorial Under Current and equatorial upwelling, from where the water returns poleward in the tropical interior (3) the tropical cell (TC) in which part of the equatorial divergence is subducted at near-equatorial latitudes by the wind-forced Ekman convergence (Johns, et al, 1998; Memery et. al. 2000; Schott et al, 1998). Cells (1) and (2) are predominately driven by thermohaline processes, but the effects of wind-forcing can not be ignored and have not been isolated.
The NBC appears as a surface-intensified flow with maximum speeds located above the 24.5 isopyncnal surface on the southern edge of all sections. Maximum speeds are typically found in the southern portion. Peak speeds of 110 cm s-1 have been recorded, although generally the current flows between 60-100 cm s-1 (Arnault et. al., 1999; Bourles, et. al., 1999). At about 5°S and 35°W, a salinity maximum of 37.1 was measured declining to 36.7 at the equator (Memery et. al., 2000). However, salinity on average ranges anywhere from 35 to 36.75 (Bourles et al, 1999). Average temperatures of the NBC range from 22°C to 29°C. Flagg et al. (1986) measured surface temperatures between 27°C and 28°C in December of 1980. Near surface waters in this region show enhanced nutrient content (phosphate, silicate and nitrate) and their distribution confirm meanders of the NBC deduced from drifter experiments. These meanders are generated by the retroflection of the NBC in the western area, which then feeds the North Equatorial Countercurrent. (Oudot et. al., 1998).
A significant seasonal variability of the NBC boundary regime exists in this area north of the equator. In boreal spring, the North Equatorial Countercurrent is absent or even flowing westward. Schott and others (1998) noted a northwestward long-shore flow of about 10 Sv of water having properties consistent with that of the southern hemisphere origin along the Guiana boundary. It is still not clear where the remainder of the cross-equatorial NBC flows during this period, however it is suspected that some of the water continues northwestward in the subthermocline layer during short periods throughout this time (Bourles et. al., 1999; Schott et. al., 1998). However, from June to January, the upper North Brazil Current connects with the eastward North Equatorial Countercurrent through a retroflection zone, often referred to as the NBC Retroflection Zone, where eddies called NBC Rings are formed (Condie, 1991; Memery et. al., 2000; Schott et. al., 1998). Before satellite observations and based on limited in-situ observations, it was thought that there was a quasi-permanent eddy, the Demarara Anticyclone, that seasonally migrated up and down the coast (Flagg et al., 1986). The analysis of satellite-based thermal, altimetric, and ocean color data has shown that a significant number of NBC rings are formed and translate through this area.
The North Brazil Current Rings are a significant contributor to transporting water across current gyres and between hemispheres in the tropical Atlantic. On the average, North Brazil Current Rings form 5-6 times per year, propagate at 14 km/day with a range of 8-30 km/day, and have a radius on the order of 100-200 km (Johns et al., 1990; Goni and Johns, 2001). Each ring transports about 1 Sv of water, a significant fraction (1/3) of the interhemispheric meridional overturning cell, along a path that closely parallels the 500 m isobath (Goni and Johns, 2001). Recently it was determined by Wilson and others (2002) that the vertical structures of the North Brazilian current rings are highly variable, carrying implications for assessing their role in these cross-gyre exchanges. High resolution shipboard surveys made by Wilson and others (2002) documented three types of eddies in this current system: The first is a shallow, surface-trapped structure with velocities confined to the top 200 m; the second is a deep-reaching structure with significant swirl velocities (about 0.2 m/s) and depths up to 2000 m; and finally, a thermocline-intensified structure with only a very slightly detectable surface signature. Annually, one to three intact rings migrate into the Lesser Antilles from their formation region at about 50°W, and will seep into the Caribbean Current system (Arnault et. al., 1999; Fratantoni et. al., 1995; Johns et al, 2002). These rings will usually disintegrate after approximately 100 days. The NBC rings may be responsible for 3-4 Sv of direct mass transport across the equatorial-tropical gyre boundary (Fratantoni et al, 1995).
The vertical structure of circulation of the NBC is a well-studied and well-understood phenomenon and clearly summarized by Bourles et al (1999): The NBC and its subsurface component, the NBUC advect southern hemispheric waters across the equator and feed subsurface eastward currents at different latitudes and different depths. These zonal currents include the North Equatorial Countercurrent, located in the near-surface layer, the Equatorial Undercurrent, centered at the thermocline, and the North Equatorial Undercurrent (NEUC), located below the thermocline. A southeastward undercurrent, the Western Boundary Undercurrent, has been observed flowing along the S. American coast from the eastern Caribbean, eventually joining the NEUC (Bourles et. al., 1999; Metcalf and Stalcup, 1967).