Deep-Sea Research I 49 (2002) 41–52 Evidence of double diffusion in the Brazil–Malvinas Confluence Alejandro A. Bianchi a,b, *, Alberto R. Piola a,b , Gerardo J. Collino a a Departamento Oceanograf ! ıa, Servicio de Hidrograf ! ıa Naval, Av. Montes de Oca 2124, 1271 Buenos Aires, Argentina b Departamento Cs. De la Atm ! osfera y los Oc ! eanos, FCEYN, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina Received 16 November 1999; received in revised form 14 February 2001; accepted 11 June 2001 Abstract 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 Wkg 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. Keywords: Double diffusion; Salt finger; Mixing; Brazil/Malvinas Confluence; Southwestern Atlantic 1. Introduction 1.1. Upper layer water mass stratification in the Argentine Basin In the Argentine Basin, the subtropical upper layer and thermocline waters, usually referred as South Atlantic Central Water (SACW, Sverdrup et al., 1946) are advected southward by the Brazil Current and into a strong thermohaline front referred to as the Brazil–Malvinas Confluence (BMC) (Gordon, 1981; Roden, 1986; Gordon, 1989). The SACW is characterized by tempera- tures higher than 101C (up to 231C, near 381S in the surface layer) and salinities between 35 and 36 (Bianchi et al., 1993). The Subantarctic Water (SAAW) is advected northward in the upper 500m of the Malvinas Current, which has temperatures lower than 101C and salinities lower than 34.3. At *Corresponding author. Fax: +54-11-4303-2299. E-mail address: abianchi@hidro.gov.ar (A.A. Bianchi). 0967-0637/01/$-see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII:S0967-0637(01)00039-5