Evidence of double diffusion in the Brazil–Malvinas Confluence

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 W kg −1 (5.3×10 −7 m s −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 of 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.