The Pathways of Mixing

 

During their journey through the shelf the Rio de La Plata waters are subject to intense mixing with ambient waters so that by the time these waters reach the deep ocean, their physical and chemical properties have been substantially altered. To evaluate the impact of this mixing on the SSS characteristics, we released neutrally-buoyant floats at the mouth of the estuary and tracked their salinity variations as they drift towards the deep ocean (Movie 2).

To illustrate the SSS evolution along the two most extremes cases we chose two sub-groups of float trajectories representing the downstream pathway (floats that went past 28˚S), and the upstream pathway (floats that crossed the shelfbreak south of 36˚S) (Figure). The upstream pathway represent summer conditions, when upwelling favorable winds move the floats southward along the coast of Argentina until an opposing flow from the Patagonian shelf displaces them towards the shelfbreak, where the BMC circulation rapidly funnels them into the deep ocean (Figure). The downstream pathways represent the winter conditions, when the winds reverse direction moving the floats in the downstream direction along the coasts of Uruguay and Brazil, whence they are funneled into the Brazil Current and advected towards the BMC (Figure 1). 

Movie 2: Float trajectories.

Figure 1: Floats trajectories.

Figure 2: SSS variations along the floats trajectories.

The travel time of the along-float trajectories are normalized their  by the time that it took to each float to reach the shelfbreak. Thus, a normalized time > 1 corresponds to floats that are in the deep ocean while those with a normalized time < 1 are over the shelf (regardless of the speed at which the float moved). The superposition of all the trajectories generate a spaghetti diagram that highlights the wide range of SSS variability captured by the floats during their journey towards the deep ocean (Figure 2). The thick lines superimposed on this diagram correspond to the mean SSS variations along the upstream (blue) and the downstream (red) pathways. On average, a float following the downstream path takes approximately 284 days to reach the shelfbreak while a float following the upstream path takes 123 days. At the shelfbreak there is a ~3 PSU difference between the two pathways.