Alejandro A. Bianchi

Mixing in the Brazil-Malvinas confluence

Conductwity-temperature-depth profile data from the Western Argentine Basin collected from 1984 to 1989 are used to quantify the cross-front heat and salt transfers associated with the vertical finestructure across the Brazil-Malvmas Confluence The fluxes are estimated following the statistical model of JoYcE (Journal of Physical Oceanography, 7,626-629, 1977). The data indicate that the upper ocean cross-front structure of the large-scale temperature and sahnlty fields is constant The medium-scale finestructure mtenslty is quantified by the variance of the vertical temperature and sahmty gra&ents m the 10-100 m wavelength band Due to the abundance of intrusions, the upper layer (0--1000 m) variances increase by a factor of four at &stances <20 km from the front Heat and salt flux estimates associated with medium-scale mixing in the upper ocean are of the order 10-2°C m s -I and 10 -3 m s -1 respectively. These fluxes are an order of magnitude greater than available estimates for other frontal regions. The medium-scale finestructure may therefore play a key role in the &ssipatlon of eddies and intrusive lenses in the region. Heat and salt fluxes between North Atlantic Deep Water and Circumpolar Deep Water are 6.5 x 10 -4 °C m s -1 and 1.8 × 10 -4 m s -i, and agree w~th ex~stmg estimates Extrapolation of upper layer Brazil-Malwnas Confluence cross-frontal fluxes to the Subtropical Convergence across the South Atlantic suggests that the medmm-scale southward heat flux is about 20% of the oceamc northward heat flux at 30°S Similarly, the freshwater flux balances 20% of the excess evaporation north of 30°S.

Evidence of double diffusion in the Brazil–Malvinas Confluence

Observation of thermohaline staircases and low-density ratios in the Brazil–Malvinas Confluence (BMC) suggests salt-fingering activity between the high salinity South Atlantic Central Water and low salinity Antarctic Intermediate Water. Vertical salt-finger induced salt fluxes are estimated in the BMC using a model for fastest growing fingers. Maximum salt-finger fluxes of 39.9 � 10 � 10 Wk g � 1 (5.3 � 10 � 7 ms � 1 ) were estimated, similar to existing estimates in a Mediterranean salt lens embedded within the North Atlantic water. The fluxes normalized by the large-scale property gradients lead to fairly large estimates of diapycnal haline and thermal diffusivities, of up to 0.74 � 10 � 4 and 0.34 � 10 � 4 m 2 s � 1 , respectively, showing the enhancement of the haline diffusivity due to salt fingers. Estimated diffusive-convection fluxes based on flux laws derived from laboratory experiments are two orders of magnitude larger than salt-finger fluxes. These results suggest that, where cross-front interleaving leads to layering of relatively cold-fresh water over warm-salty water, diffusive upward convection dominates the vertical property fluxes in the frontal region. It appears that on the warm-salty side of the BMC, away from the narrow band dominated by interleaving, salt-finger integrated vertical fluxes nearly balance the cross-front lateral integrated fluxes. The effect of baroclinicity on the vertical property fluxes was evaluated and it was found that, in the BMC, the baroclinicity enhances the interleaving. r 2001 Elsevier Science Ltd. All rights reserved.

Physical Oceanography of the SW Atlantic Shelf: A Review

The continental shelf of the western South Atlantic is characterized by three regions subject to distinct oceanographic regimes. The wide subantarctic shelf, south of approximately 35°S, is occupied by cold, low-salinity waters derived from the Subantarctic Zone and further diluted by the inflow of additional low-salinity waters, primarily from the Magellan Strait. Farther north, the shelf narrows considerably and is subject to the influence of large freshwater discharges and warm-salty intrusions of subtropical waters from the Brazil Current. Intense frontal transitions at various near shore locations and along the shelf break promote vertical circulations that inject nutrients into the upper layer. This nutrient injection leads to enhanced growth of phytoplankton, and, in some regions, to a significant uptake of atmospheric CO2. While the subantarctic shelf is under the influence of strong westerlies and high-amplitude tides, most of the subtropical shelf undergoes seasonally reversing winds and a micro-tidal regime. The shelf characteristics are also influenced by the offshore circulation, which is dominated by the equatorward flow of cold, nutrient-rich waters of the Malvinas Current in the south and the poleward flow of warm, salty, and oligotrophic waters of the Brazil Current in the north. There is a convergent large-scale mean circulation toward the transition between subantarctic and subtropical shelf waters near 34°S, which is balanced by export of shelf waters to the deep ocean. This article describes the contrasting water masses, frontal features, and circulation patterns of this region.