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South-western Atlantic Ocean during 1993-94
Introduction
The South-western region of the Atlantic Ocean comprises one of the most dynamically active regions of the World Ocean, the Brazil-Malvinas (Falkland) Confluence (BMC) region. The BMC comprises territorial waters of Brazil, Uruguay and Argentina, and is an oceanographic front between the Brazil Current (BC) and the Malvinas (Falkland) Current (MC), where surface cold waters of subantarctic origin carried by the MC meet warm waters of tropical origin carried by the BC. The BMC is the western part of the Subtropical Front, the region where the subsurface South Atlantic Central Water is formed and where the South Atlantic Current flows eastwards as part of the South Atlantic subtropical gyre.
A complete understanding of the physical aspects of the BMC is far from being achieved. Reasons for that lie in the fact that the South Atlantic remained for many years one of the regions of the World Ocean where there was a remarkable lack of in situ measurements, especially current measurements. The complex physical processes of water mass mixture occurring in the BMC region and its vicinity, together with a strong seasonal oscillation of the BMC, make the situation even more complicated.
Because of the lack of integration between the neighbour countries in Southern South America and the difficulty in obtaining direct measurements in the currents present in the BMC region as a whole, several primary questions regarding the kinematics and dynamics of this region remain to be answered. For instance, questions remain about the mechanisms involved in the BMC oscillation, its interconnections with the large-scale South Atlantic atmospheric systems and with current systems like the Antarctic Circumpolar Current and the South Equatorial Current. More basic still, a good description of the fronts and other mesoscale features like meanders and eddies in the BMC is far from complete.
The majority of the reported eddy activity in the SWA is related to the BMC region (e.g. Legeckis and Gordon, 1982; Olson, Podestá, Evans and Brown, 1988, Willson and Rees, 2000). Nevertheless, the characterisation of the BMC eddies is generally restricted only to their sizes and location. Given the relatively few in situ data, most of the previous works have described these structures using only remote sensing images. To our knowledge, very few references are available at the present describing other properties such as rotational periods or tangential velocities of the BMC eddies, for instance. These are important parameters for describing the eddy field more realistically and can be used directly on environmental monitoring of the region or even as input parameters for global circulation models (GCMs), for example.
With respect to the South Brazilian Continental Shelf (SBCS ), the region where in wintertime a strong lateral front occurs between BC and the northwardly coastal current BCC (Souza, 2000), the situation is a little worse than that of the BMC region. Some previous descriptions of the eddies in the BC along the BC/BCC front can be found in the works made by Garfield (1990) and Schmid, Schafer, Podestá and Zenk (1995), for example. Unfortunately, these individual efforts fail to provide a general description of the eddies across the SBCS or at the BC/BCC front and over long periods of time. Some eddy activity in the SBCS was also described in the first results from the Oceanic Circulation in the Western Region of the South Atlantic (COROAS) project (e.g. Lorenzzetti, Stevenson, Silva Jr. and Souza, 1994; Campos, Ikeda, Castro, Gaeta, Lorenzzetti and Stevenson, 1996; Stech, Lorenzzetti, de Souza and Araujo, 1996). COROAS is the Brazilian counterpart to the World Ocean Circulation Experiment (WOCE). The initial COROAS papers, although using some of the data used here, have generally dealt with specific events, neither addressing the overall COROAS experimental phase nor making any comparison to other eddies found elsewhere in the South-western Atlantic Ocean (SWA), as it will be presented here.
More recently, as a result of new oceanographic programmes conducted in Brazil (e.g. Lorenzzetti and Kampel, 1998), drifting buoys are being used to study the BC and eddy activity is being registered in this current. Up to the present, however, no new insights about the eddy fields of the SWA are encountered in the international scientific literature resulting from the Brazilian programmes.
This paper investigates the characteristics of the eddies found in the SWA in 1993 and 1994 as they were observed by the combination of surface drifters and high-resolution satellite data obtained during COROAS project. The characterisation of the eddies is made according to their sizes (perimeters or diameters), rotational periods and velocities, Rossby numbers and relation to the local internal Rossby radius of deformation. Empirical relationships found between the eddies’ sizes and their rotational periods or tangential velocities are also discussed here. The relations can be used in environmental monitoring activities when in situ data are lacking and only remote sensing images are available.
At the present,
eddy properties are being investigated for the World Ocean in order to
assess the importance of such structures in the world ocean’s climate and
heat and mass balance. The eddies’ relative importance for the ocean’s
circulation is being investigated through global circulation numeric models
with eddy-resolving properties like the one developed by Semtner Jr. and
Chervin (1988) for the WOCE. This work provides several new insights on
the properties of the BMC and SBCS eddies, offering new material and descriptions
that are potentially useful for future work which ought to investigate
the eddy field and its impact in the SWA environment and physics.
rbds@soc.soton.ac.uk (c) April 2001, all rights reserved.