Rio de la Plata estuary response to wind variability in synoptic to intraseasonal scales: 2. Currents' vertical structure and its implications for the salt wedge structure

Fil:Simionato, C.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Meccia, V.L. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Dragani, W.C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Nuñez, M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Lagrangian characterization of the southwestern Atlantic from a dense surface drifter deployment

The Southwestern Atlantic (SWA) is characterized by its large Eddy Kinetic Energy as the result of the confluence of two major western boundary currents, the northward flowing Malvinas Current (MC) and the southward flowing Brazil Current. The SWA study was addressed in the literature based on altimetry data, in situ measurements, regional models and ocean reanalysis. The present study constitutes the first effort to sample a portion of the SWA, with a dense drifter array (N = 62) deployment. The drifters, drogued at 15 m depths, were deployed across the MC and the Argentine Continental Shelf along two zonal transects located at 47°S and 47.25°S, between the 8th and the September 9, 2021. Drifters were set to deliver their position every 10 and 60 min, providing accurate Lagrangian trajectories that provide information on a large range of space and time scales of the surface currents. Three regions are clearly identified based on the analysis of the speed of the drifters, of their trajectories and of the spectral density of their velocities: the continental shelf, the slope and the open ocean. The comparison of the trajectories of the drifters with satellite altimetry images shows that, in general, drifters follow mesoscale features that are detectable in satellite altimetry maps. The analysis of the drifter trajectories also allowed us the study of submesoscale features of the flow (1–10 km) that are not observable in satellite altimetry data. Comparison with cloud-free, high-resolution color images, shows that drifter trajectories organized by the mesoscale flow might also locally follow sub-mesoscale features. In frontal regions it was found that drifter velocities double satellite altimetry geostrophic velocities, which suggests that the dynamics at those regions is largely dominated by ageostrophic components. The ageostrophic Ekman component might explain the direction of the drifters when strong winds from a given direction prevail for several days and the drifters are not in a region with large sea surface height (SSH) gradients. The joint analysis of drifters’ trajectory and SSH clearly depicts that mesoscale features on the open ocean region control the cross-shelf exchanges between the MC and open ocean regions as well as the strength and width of the MC. Finally, the spatial density distribution of the drifters during the first hours after deployment and within a small eddy also allowed us to characterize the flow in terms of its divergence, vorticity and strain, indicating that the MC is geostrophic and has a jet-like behavior while the eddy is largely ageostrophic and has a dominant vorticity component over strain. We conclude observing that the analysis of a dense array of drifters provides valuable information of the flow that cannot be attained solely based on satellite data. A dedicated experiment based on the deployment of 62 surface drifters, an array of unprecedented dimension for the region, was devised to improve our understanding of the surface circulation of the Argentine Continental Shelf (ACS), the Patagonian Slope and, generally, the Southwestern Atlantic Ocean. The main highlights derived from the analysis are.•Three regions were clearly identified based on the analysis of the speed of the drifters, their trajectories and the spectral density of their velocities: the continental shelf, the slope and the open ocean.•Within the open ocean region, drifters might exceed 1.8 m s−1 when trapped by the Brazil-Malvinas Confluence (BMC), along the northern branch of the Subantarctic Front, or between large mesoscale eddies of different polarities.•Within the slope region, thanks to the deployment strategy adopted, fine scale details of the Malvinas Current (MC) have been unveiled for the first time: large shears between jets of the MC (up to 20 cm s−1 between drifters separated by only 2.5 km) and the clear modulation of the trajectories of the drifters due to tidal currents, despite the dominant MC.•Over the ACS, drifters showed how tides are, as expected, the main forcing of the shelf circulation. However, small scale patterns may also modulate shelf currents. The origin of these patterns should be further investigated.•Clear examples of the accuracy of the geostrophic currents predicted by satellite altimetry in the different regions are provided. In general, currents in the slope and open ocean region are very well represented by the satellite altimetry estimations, at least for features at the resolved mesoscale.•The analysis of drifter trajectories also allowed the study of submesoscale features of the flow (1–10 km) that are not observable in satellite altimetry data but are visible in color images.•In frontal regions it was found that the recorded drifter velocities double satellite altimetry geostrophic velocities, suggesting that the dynamics at those regions is largely dominated by unresolved components.•First in situ evidence that mesoscale features on the open ocean region control the cross-shelf exchanges between the slope and open ocean regions as well as the strength and width of the MC, is provided.•The analysis of drifter triplets allowed us to characterize the flow in terms of its divergence, vorticity and strain. The analysis confirms that within the MC strain dominates, indicating that the MC is in geostrophic balance and has a jet-like behavior. Within a small mesoscale cyclonic eddy that trapped three drifters for more than 20 days, negative vorticity prevailed.•The potential role of Ekman transport to promote exchanges between the slope and adjacent regions is discussed. It is proposed that wind may impact drifter trajectories only in regions exhibiting small Sea Surface Height (SSH) gradients