Stability of the Malvinas Current

Deterministic and probabilistic tools from nonlinear dynamics are used to assess enduring near-surface Lagrangian aspects of the Malvinas Current. The deterministic tools are applied on a multi-year record of velocities derived from satellite altimetry data, revealing a resilient cross-stream transport barrier. This is composed of shearless-parabolic Lagrangian coherent structures (LCS), which, extracted over sliding time windows along the multi-year altimetry-derived velocity record, lie in near coincidental position. The probabilistic tools are applied on a large collection of historical satellite-tracked drifter trajectories, revealing weakly communicating flow regions on either side of the altimetry-derived barrier. Shearless-parabolic LCS are detected for the first time from altimetry data, and their significance is supported on satellite-derived ocean color data, which reveal shapes that quite closely resemble the peculiar V shapes, dubbed `chevrons,' that have recently confirmed the presence of similar LCS in the atmosphere of Jupiter. Finally, using in-situ velocity and hydrographic data, conditions for symmetric stability are found to be satisfied, suggesting a duality between Lagrangian and Eulerian stability for the Malvinas Current.Comment: Submitted to Scientific Report

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

Multiple Lagrangian Jet-Core Structures in the Malvinas Current

The Malvinas Current (MC) in the Southwestern Atlantic is characterized as a system of multiple Lagrangian jets. This is done by applying geodesic theory of Lagrangian Coherent Structures (LCSs) on altimetry-derived velocity. The number and spatial disposition of jet-cores organizing the MC are found to vary in time influenced by topographic waves propagating along the current. The Lagrangian analysis reveals that permanent jet-cores tend to approach one another upstream, yet do not merge into a single jet as the Eulerian analysis of the data suggests. Independent support for the existence of multiple jets is provided by trajectories of surface drifters from a deployment of unparalleled dimensions for the region and ocean color images. The drifters develop the characteristic boomerang-shaped patterns into which passive tracers straddling jet-cores deform and exhibit Lagrangian metrics that are largely consistent with those derived from altimetry data. Ocean color imagery reveals similar "chevrons" and suggest upwelling along the jets, which is consistent with a new solution of arrested topographic wave dynamics. This observation and the finding that the MC jet system is influenced by propagating waves suggests that the MC variability may have major implications in the modulation of primary productivity on synoptic timescales. Drifters flowing into the open ocean, exhibit organizing patterns that are largely shaped by mesoscale circulation and essentially differ from those observed along the MC. Evidence of this observation is provided on the basis of LCS analysis and the calculation of a quasi-objective Lagrangian metric derived directly from drifter trajectories. The ocean circulation exhibits complex motions ranging across a large span of spatio-temporal scales. Despite this complexity, distinguished material curves known as Lagrangian Coherent Structures (LCSs) may underlie the system dynamics and provide the backbones of flow organization. These structures may represent the centerpieces of passive tracer filaments, they may delineate the axes of Lagrangian jets, or they may provide boundaries to material eddies. In this work, we apply LCS methods to extract the organizing structure of the ocean currents in the Southwestern Atlantic, where the Malvinas Current (MC) flows. Surface Lagrangian dynamics, as resolved by satellite altimetry data, are interpreted together with trajectories of surface drifting buoys from a deployment of unprecedented dimensions for the region and ocean color imagery. The main finding of this multidata approach is that the MC is structured as a time-varying system of multiple Lagrangian jets. These structures are found to be modulated by trapped topographic waves characteristic of synoptic timescales. A minimal model of the MC associates upwelling to these jets, which suggests potential implications for primary productivity in the region. Original evidence derived from a dedicated drifter experiment characterizes the Malvinas Current as a system of multiple Lagrangian jets The longitudinal position of the jets are modulated by trapped topographic waves propagating along the current Ocean color suggests upwelling along the jets, consistent with arrested topographic wave dynamic