Research

Brazil / Malvinas Confluence Research


Ana AngeleriSeptember 13, 2017

Apr – 4 – 2011
Brazil / Malvinas Confluence Research
The Brazil / Malvinas Confluence is one of the most energetic regions of the World Ocean. The primary goal of this research is to determine the time-space variability of water mass properties and mass transport within the continental shelf, the core of the Malvinas Current and the Brazil/Malvinas Confluence. As part of this effort we have conducted a series of oceanographic cruises (see Gallery. Figure 1). This work was conducted in collaboration with Instituto Nacional de Investigación y Desarrollo Pesquero http://www.inidep.edu.ar/ (INIDEP, Raul Guerrero, guerrero@inidep.edu.ar), Mar del Plata, Argentina and Scripps Institution of Oceanography http://www-sio.ucsd.edu/ (SIO, R. Peterson), La Jolla, CA, USA.

 

Field Work
As part of the World Ocean Circulation of Experiment (WOCE) the group at Servicio de Hidrografía Naval (SHN, A.Piola, apiola@hidro.gov.ar) has studied the northern extension of the Malvinas Current in the western Argentine Basin and its interaction with the outer continental shelf and the Brazil / Malvinas Confluence.

Figure 2 in Gallery represents Averaged chlorophyll a distribution on 25 and 26 September 1997 from SeaWifs (Sea-Viewing Wide Field-of-View Scanner). There are various elongated high chlorophyll bands (2-5 mg/m3) in the mid-shelf and along the shelf-break front. The later extends offshore and southeast along the Brazil/Malvinas Confluence. The low chlorophyll concentration (< 0.5 mg/m3) to the east and south of the shelf-break maximum is associated to the Malvinas and Malvinas return currents.

The geostrophic volume transport relative to 2000 meters associated to the Brazil/Malvinas Confluence at 38°S (Figure 3, Gallery) is appoximately 10.6 Sv (1 Sv = 106 m3 s-1) northward, associated to the Malvinas Current and 35.1 Sv southward, associated to the Brazil/Malvinas Confluence. Within the core of the Malvinas Current the geostrophic velocity decreases from 30 cm s-1 at the sea surface to 20 cm s-1 at 500 m. In contrast, in the core of the Brazil/Malvinas Confluence, the velocity decreases from 115 cm s-1 to 29 cm s-1 in the same depth range. Due to the relatively low baroclinicity, the Malvinas Current transport based on hydrographic data is underestimated.

Combining hydrographic data with suburface drifter (Alace) data it is possible to estimate the total geostrophic flow field. The surface velocities derived from this calculation are in good agreement with surface velocities estimated from surface drifters (see dissertation by Lifschitz, 2007 in http://sacc.coas.oregonstate.edu/~sacc/~sacc/documents.php?cat=Disertations_and_Theses)

Ongoing research suggests that the Malvinas Current (MC) is characterized by at least two narrow and energetic high-velocity cores separated by regions of relatively lower velocity. This result is based on the analysis of hydrographic (e.g. a two core structure is suggested in Fig. 3, Gallery), underway Acoustic Doppler Current Profiler, surface drifters and satellite altimetry data. These data indicate that the main MC jet, which concentrates about 30% of the MC transport is located over a relatively flat portion of the upper slope, where the depth is about 1400 m.

At 38°S, adjustment of the relative geostrophic velocities to surface drifter (Peterson et al., 1996 , in: The South Atlantic: Present and past circulation, Springer-Verlag, Berlin, 239-247) and deep float (800-1000m) velocities increases the volume transport of the Malvinas Current to 25.3 Sv. At 43°S the adjusted transport increases to 47 Sv, which is in close agreement with the 41.5 ± 12 Sv mean transport derived from 8 months of direct current measurements at 40°S (Vivier and Provost, 1999, J. Geophys. Res., 104, 21105-21122.)

Numerical Experiments
In cooperation with E. Palma (Universidad del Sur, Argentina, uspalma@criba.edu.ar) and R. Matano (Oregon State University, USA, rpm@oce.orst.edu) we are conducting a series of numerical simulations based on a high resolution Princeton Ocean Model application. These simulations include the study of the effects of tides, wind and continental runoff variability on the ocean circulation. The model domain is the region bounded by 55°S to 23°S and 70°W to 40°W. The horizontal model grid is designed using an orthogonal coordinate transformation with 250 (along shelf) x 150 (cross shelf) grid points, which provides a horizontal resolution of about 7.5 km on average in the cross shelf direction and 10 km in the along-shelf direction. The total number of sigma levels is 25.

Figure 4, in Gallery: Sea surface elevation snapshot of the Confluence region after a 3 year simulation including tides (Egbert et al 1994), Trenberth’s ECMWF wind climatology (Trenberth et al., 1990), and continental runoff. The contour interval is 5 cm. The separation of the western boundary currents occurs near 36°S where a strong SE jet develops. In agreement with observational evidence, further downstream the meandering nature of the Confluence is apparent. An anticyclonic eddy centered at 43°S is observed east of the Malvinas return current.

In addition, in cooperation with A.Rivas (Centro Nacional Patagónico, Argentina, andres@cenpat.edu.ar), R. Bleck (Los Alamos National Laboratory, USA, bleck@rsmas.miami.edu) and R. Matano we are analyzing outputs of the Miami Isopycnal Coordinate Model from the western South Atlantic.

Figure 1: Activities conducted during the Talud 3 cruise 7-19 September 1997 on board R/V Figure 2: Averaged chlorophyll a distribution on 25 and 26 September 1997 from SeaWifs (Sea- Figure 3: Relative geostrophic velocity section of the upper 2000 decibars, across the Figure 4: Sea surface elevation snapshot of the Confluence region after a 3 year simulation