The Brazil - Malvinas Confluence

Alberto R. Piola ¹, Alejandro A. Bianchi ¹, Andres L. Rivas ², 
Elbio D. Palma³, Ricardo P. Matano⁴ and  Rainer Bleck5 

1. Departamento Oceanografia, Servicio de Hidrografia Naval, Buenos Aires, Av. Montes de Oca 2124, 1271 Buenos Aires,
 and Departamento de Ciencias de la Atmosfera y los Oceanos, Universidad de Buenos Aires, Argentina
2. Centro Nacional Patagonico, 9120 Puerto Madryn, Chubut, Argentina
3. Departamento de Física, Universidad Nacional del Sur, 8000 Bahía Blanca, Argentina
4. College of Oceanic and Atmospheric Science, Oregon State University, Corvallis, Oregon 97331, USA
5. Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

contact: Alberto Piola - apiola@hidro.gov.ar

1. Introduction

2. Water Masses and Cross-Frontal Mixing

3. Circulation and Volume Transport

4. Variability

5. Acknowledgements

6. References

Forum

1. Introduction
The Brazil Current is the western limb of the South Atlantic subtropical gyre.  It carries warm and salty waters poleward along the continental slope of South America and near 39° S it collides with a northward branch of the Antarctic Circumpolar Current (ACC): the Malvinas (Falkland) Current (MC).  The MC transports cold and relatively fresh subantarctic waters equatorward.  The collision between these distinct water masses generates one of the most energetic regions of the world ocean: the Brazil/Malvinas Confluence (BMC, hereafter).

Figure 1:  January snapshot of the upper layer horizontal velocity and temperature fields derived from the Miami Isopycnal Coordinate Model (Micom).  The red and blue lines show schematically the regions of higher velocity and where the flow is relatively well organized.   The thick blue line represents the Malvinas Current and the Malvinas Return and the thick red line represents the Brazil Current.  Note the tight recirculation cells in both, poleward and equatorward western boundary currents..  The thick white lines indicate two hydrographic sections (Figures 2a and 2b) occupied in September 1997.  The line near 38°S also indicates the position of the geostrophic velocity section shown in Figure 4.  The large dots indicate the location of two inverted echosounders (see Figure 7).  The thin white contour is the 200 m isobath.

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2. Water Masses
The poleward penetration of subtropical waters associated to the Brazil Current and the equatorward penetration of subantarctic waters associated to the Malvinas Current  lead to the complex vertical structure of water masses observed in western Argentine Basin.

Figure 2a: Potential temperature (q), salinity and dissolved oxygen sections in a snapshot taken across the BMC (37°S). The property extremes mark the cores of the following water masses: Tropical Water (TW), South Atlantic Central Water (SACW), Antarctic Intermeadiate Water (AAIW), Upper Circumpolar Deep Water (UCDW), North Atlantic Deep Water (NADW), Lower Circumpolar Deep Water (LCDW) and Weddell Sea Deep Water (WSDW).  TW and SACW  are only apparent in the  eastern end of the section where q >16°C and S > 35.5 (Piola and Matano, in press).

Further south, near 46°S (Figure 2b), the upper layers of the Brazil Current turn eastward and only the northward flowing subantarctic water  is found.  However, traces of NADW are present at 2500m. (S > 34.75, O₂>210 µmol/Kg).  Also the layer of WSDW (q<0°C) extends about 700m above the seafloor.

Figure 2b: As in Figure 2a, at 46°S

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Cross-frontal Mixing
CTD observations from high spatial resolution surveys at the BMC reveal strong interleaving, thermohaline staircases, and evidence of double diffusion at the interface between SACW and AAIW.  Low density ratios (Rr < 1.7) and Turner angles (TU) in the range of 67 to 90° are strong indications of saltfingering (Figure 3).

Figure 3: Temperature and salinity profiles at the BMC (upper panels), Turner angle (TU, lower left), and density ratio (lower right).  Thermohaline steps are indicated as a, b, c and e.  The shaded region 45°< TU<90° marks the saltfinger regime.  Where TU<-45°, i.e.  near 260-280 m, at the base of the cold-low salinity intrusion (step e), double diffusion convection occurs.  Very large buoyancy fluxes are associated to these interfaces (Bianchi et al., in press). 

Model derived vertical salt fluxes induced by salt fingers  are similar to estimates in a Mediterranean salt lens embedded within NADW.  In  the warm- salty side of the BMC, estimates of  cross-front integrated salt fluxes using a statistical model are nearly balanced by salt finger integrated vertical fluxes (Bianchi et al., 1993).

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3. Circulation and Volume Transport

The confluence of subpolar and subantarctic waters at the BMC induces a strong baroclinic jet with surface velocities frequently higher than 1m/s (e.g. Figure 4).

Figure 4: Geostrophic velocity across the BMC near 38°S (see Figure 1). Positive values (blues shading) indicate northward flow and negative values (green and red shading) indicate southward flow.   The contour interval is 10 cm/s except for the dashed contours which correspond to -3cm/s and 3cm/s.  The heavy dotted lines are the 25.5, 27.1, 27.4, and 27.7 kg/m³ isopycnals, which mark the transition between TW, SACW, AAIW and UCDW.  The net relative geostrophic volume transports in this section are 10.6 Sv (1 Sv = 10⁶ m³/s) northward, associated to the Malvinas Current and 35.1 Sv southward, associated to the Brazil/Malvinas Confluence.

The use of velocities derived from surface drifters  (Peterson et al., 1996) and deep floats (800-1000m) in the geostrophic calculation increases the volume transport of the Malvinas Current at 38°S to 25.3 Sv.  At 43°S the adjusted transport increases to 47 Sv, which is in close agreement with 41.5 ± 12 Sv mean transport derived from 8 months of direct current measurements at 40°S (Vivier and Provost, 1999) but is substantially lower than the 75 Sv at 42°S based on mass conservation arguments (Peterson, 1992) .

The surface speed field (Figure 5) shows a region of speeds greater than 60 cm/s in the southward extension of the Brazil Current, and further south, along the Malvinas return (see Figure 1 and Figure 6). This high speed region resembles the C-shaped region of high eddy kinetic energy (Piola et al., 1987) and sea surface height variability characteristic of the western Argentine Basin (Chelton et al., 1990).  Surface speeds within the core of the Mavlinas Current, found close to the location of the 1500 m isobath, vary from 50 to 75 cm/s. 

Figure 5: Surface speed distribution in the western South Atlantic derived from 87 WOCE SVP drifters between 1990 and 1998 (15449 drifter days).  The inset shows the number of drifter days per year.  For reference, also shown here are the tracks of seven drifters deployed in the Malvinas Current (in blue) and in the Brazil Current (in red).  The dashed lines indicate the 200 m, 1000 m,  2000 m and 3000 m isobaths.

An eddy resolving numerical model of the BMC and the adjacent continental shelf is being developed (see Figure 6).  The model is based on the Princeton Ocean Model (POM) and uses a variable horizontal grid (4 km near the coast to 40 km offshore) and 25 vertical levels and uses GTOPO30 bathymetry.  The model is being initialized by the Levitus annual mean temperature and salinity and is forced by the mean annual winds of Hellerman and Rosenstein (1983).   Barotropic transports and sea surface elevation are interpolated from POCM_4 runs (annual mean) and prescribed at the three open boundaries".

Figure 6: Sea surface temperature (left panel, °C) and sea surface height (right panel, cm) from the regional POM model runs, day 180.  These preliminary model runs show the development of  warm and cold core eddies, meanders and filaments.

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4. Variability

The BMC experiences meridional fluctuations of several hundred kilometers (Olson et al., 1988; Bianchi and Garzoli, 1997).  At the annual time scale the BMC variability is characterized by an equatorward displacement of the front during the austral winter and a poleward displacement during the austral summer.  Observations and models indicate that the seasonal displacements of the front might be linked to variations in the poleward transport of the Brazil Current (Matano et al., 1993). The seasonal cycle observed in the equatorward transport of the Malvinas Current appears to be too small to have any significant impact on the displacements of the front (Vivier and Provost, 1999).  It is expected that variations in the Malvinas transport will play an important role in the variability of the front at intra- and interannual time scales.

Migrations of the BMC cause sea surface temperature anomalies (SSTA) of several degrees, which are thought to impact on the regional climate.  Time series of the mean temperature of the  upper 500m, constructed from an array of IES, also reveal large variations associated to northward excursions of the Malvinas Current with a persistence of 15 to 60 days (Figure 7) and frontal motions of 0.2 m/s.  Rainfall anomalies over Uruguay, which setup late in El Niño years, are also associated to SSTA in the western Argentine Basin.  The precipitation record over Uruguay and the SSTA over the western South Atlantic are coherent (>0.8) and 90° out of phase (Podestá et al., unpublished, see Garzoli et al., 1996). 

Figure 7: Mean temperature in the upper 500m derived from two IES located at 37°30'S (blue, IES 1) and 36°30'S (red, IES 9) on the 2000m isobath.  The low temperature periods observed in December 1988 and May-September 1989 are associated to northward penetrations of the Malvinas Current and offshore displacements of the BMC while in January, late February and June-July 1989 the front displaced shoreward and southward, past the array (from Bianchi and Garzoli, 1997).  The gray shading is the first principal component of the rainfall anomaly over Uruguay (from Podestá et al., unpublished).

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5. Acknowledgements

This work is supported by the Inter-American Institute for Global Change Research, Agencia Nacional de Promoción Científica y Tecnológica, Argentina (ARP, ALR and EDP) and NSF grant OPP 95-27695 and JPL contract 1206714 (RPM).

6. References

Bianchi, A.A., C.F. Giulivi and A.R. Piola, 1993, Mixing in the Brazil/Malvinas Confluence, Deep-Sea Res., 40, 1345-1358.

Bianchi, A.A. and S.L. Garzoli, 1997, Variability and motion of the Brazil-Malvinas front, Geoacta, 22, 74-90.

Bianchi., A.R. Piola and G. Collino, Evidence of double diffusion  in the Brazil/Malvinas Confluence, Deep-Sea Research, in press.

Chelton, D. B., M. G. Schlax, D. L. Witter, and J. Richman, 1990, Geosat altimeter observations of the surface circulation of the southern ocean. J. Geophys. Res., 95, 17,877-17,903.

Garzoli, S.L., D.B. Olson, E. Chassignet, R. Matano, H. Berbery, E. Campos, J. Miller, A. Piola, G. Podesta, R. Fine and R. Molinari, 1996, SACC: South Atlantic Climate Change, Draft Document, unpublished.

Hellerman, S., and M. Rosenstein, 1983, Normal monthly wind stress over the world ocean with error estimates. J. Phys.Oceanogr. 13, 1093-1104.

Matano, R.P., M.G. Schlax and D.B. Chelton, 1993, Seasonal variability in the Southwestern Atlantic, J. Geophys. Res., 98, 18027-18035.

Olson, D. B., G. P. Podestá, R. H. Evans, and O. Brown, 1988: Temporal variations in the separation of the Brazil and Malvinas currents, Deep-Sea Res., 35, 1971-1990.

Peterson, R. G., 1992, The boundary currents in the western Argentine Basin, Deep Sea Res., 39, 623-644.

Peterson, R.G, C.S. Johnson, W. Krauss and R.E. Davis, 1996, Lagrangian measurements in the Malvinas Current, in: The South Atlantic: Present and past circulation, Springer-Verlag, Berlin, 239-247.

Piola, A.R., H.A. Figueroa and A.A. Bianchi, 1987, Some aspects of the surface circulation south of 20°S revealed by First GARP Global Experiment Drifters, J. Geophys. Res., 92, 5101-5114. 

Piola, A.R. and R.P. Matano, Atlantic Western Boundary - Brazil Current/Falkland (Malvinas) Current, in: Encyclopedia of Ocean Sciences, Academic Press, in press.

Vivier, F. and C. Provost, 1999, Volume transport of the Malvinas Current: Can the flow be monitored by Topex-Poseidon?, J. Geophys. Res., 104, 21105-21122.

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