A region where the Brazil Current meets the Malvinas Current at around 38°S. This collision of subtropical and subantarctic waters produces one of the most spectacular of the oceanic fronts and complex SST fields seen in the world ocean. At the BMC, subantarctic surface waters meet subtropical thermocline water in a front that can have a gradient as strong as 8°C per km. It is frequently marked by a ribbon of warm, low salinity water of Rio de la Plata origin that has folded over the northern tip of the cyclonic trough formed by the Malvinas Current and its return to the south. A warm, low salinity cap tens of meters thick often covers the western segments of the warm subtropical water. It is derived from the continental shelf north of the Rio de la Plata.

The specific configuration of the BMC at any given time is thought to depend on the relative strengths of the baroclinic and barotropic fields of the Malvinas and Brazil Currents. Its variability occurs over time scales ranging from the intra-annual to the inter-annual, with the spatial characteristics including changes in the latitudes of separation of the western boundary currents from the continental margin, and changes in the geometry of their extensions in the offshore region. On intra-annual time scales, the variability is dominated by the periodic production of transient, cold-core eddies from the Malvinas Current and warm-core eddies from the Brazil Current. These have associated SST anomalies that can be as large as 10°C on space and time scales of 1000 km and two months, respectively.

On annual time scales the variability of the western south Atlantic is dominated by the seasonal displacements of the BMC. It is found farther north during austral winter (July-September) than during the summer. Besides the annual cycle, thought to be driven by variations in the strengths of the Malvinas and Brazil Currents, there is a semi-annual component of variability with near zero amplitude at 30°S increasing to nearly half the magnitude of the annual signal at 50°S. This is probably a response to the semi-annual cycle in zonal winds over the Southern Ocean.