Control of oceanic circulation on sediment distribution in the southwestern Atlantic margin (23 to 55º S)

In this study, we interpret the role played by ocean circulation in sediment distribution on the southwestern Atlantic margin using radiogenic Nd and Pb isotopes. The latitudinal trends for Pb and Nd isotopes reflect the different current systems acting on the margin. The utilization of the sediment fingerprinting method allowed us to associate the isotopic signatures with the main oceanographic features in the area. We recognized differences between Nd and Pb sources to the Argentinean shelf (carried by the flow of Subantarctic Shelf Water) and slopes (transported by deeper flows). Sediments from Antarctica extend up to the Uruguayan margin, carried by the Upper and Lower Circumpolar Deep Water. Our data confirm that, for shelf and intermediate areas (the upper 1200 m), the transfer of sediments from the Argentinean margin to the north of 35∘ S is limited by the Subtropical Shelf Front and the basin-wide recirculated Antarctic Intermediate Water. On the southern Brazilian inner and middle shelf, it is possible to recognize the northward influence of the Río de la Plata sediments carried by the Plata Plume Water. Another flow responsible for sediment transport and deposition on the outer shelf and slope is the southward flow of the Brazil Current. Finally, we propose that the Brazil–Malvinas Confluence and the Santos Bifurcation act as boundaries of geochemical provinces in the area. A conceptual model of sediment sources and transport is provided for the southwestern Atlantic margin

The turbulent and wavy upper ocean: transition from geostrophic flows to internal waves and stimulated generation of near-inertial waves

We study the mesoscale to submesoscale (10-300 km) dynamics of the upper ocean, with particular attention to the partitioning between geostrophic flows and internal waves, and the interaction between these two types of flow. Using 13 years of shipboard ADCP transects in Drake Passage, we show that internal waves account for more than half of the upper-ocean kinetic energy at scales between 10-40 km; a transition from the dominance of geostrophic flow to inertia-gravity waves occurs at 40 km. We further show that a global numerical model with embedded tides reproduces this partitioning between upper-ocean geostrophic flows and inertia-gravity waves. Using the output of this model, we show that in the Kuroshio Extension upper-ocean submesoscale (10-100 km) geostrophic flow and inertia-gravity waves undergo vigorous seasonal cycles that are out of phase: geostrophic flows peak in late winter/early spring, while the projection of inertia-gravity waves at the surface peaks in late summer/early fall.The observational and modeling evidence of the importance of both geostrophic flows and internal gravity waves at mesoscales to submesoscales hints on the interaction between these two types of flow. To better understand these interactions, we analyze a simple model that couples barotropic quasi-geostrophic flow and near-inertial waves. There are two mechanisms of energy transfer from geostrophic flow to externally forced near-inertial waves: the refractive convergence of the wave action density into anti-cyclones (and divergence from cyclones); and the enhancement of wave-field gradients by geostrophic straining. Unforced inviscid numerical solutions of this reduced model reveal that geostrophic straining accounts for most of stimulated generation, which represents 10-20$\%$ of the decay of the initial balanced energy. Consideration of the dissipative problem reveals that wave dissipation generates both quasi-geostrophic potential vorticity locally and geostrophic kinetic energy. And this wave streaming mechanism is non-negligible in forced-dissipative solutions, which equilibrate even without bottom drag.In a separate study, we derive a Galerkin approximation for the surface-active quasi-geostrophic system using standard vertical modes. While the Galerkin expansions of streamfunction and potential vorticity do not satisfy the inversion relation exactly, the series converge with no Gibbs oscillations. With enough modes, the Galerkin series provide a good approximation to the streamfunction throughout the domain, which can be used to advect potential vorticity in the interior and buoyancy at the surfaces

The turbulent and wavy upper ocean: transition from geostrophic flows to internal waves and stimulated generation of near-inertial waves

We study the mesoscale to submesoscale (10-300 km) dynamics of the upper ocean, with particular attention to the partitioning between geostrophic flows and internal waves, and the interaction between these two types of flow. Using 13 years of shipboard ADCP transects in Drake Passage, we show that internal waves account for more than half of the upper-ocean kinetic energy at scales between 10-40 km; a transition from the dominance of geostrophic flow to inertia-gravity waves occurs at 40 km. We further show that a global numerical model with embedded tides reproduces this partitioning between upper-ocean geostrophic flows and inertia-gravity waves. Using the output of this model, we show that in the Kuroshio Extension upper-ocean submesoscale (10-100 km) geostrophic flow and inertia-gravity waves undergo vigorous seasonal cycles that are out of phase: geostrophic flows peak in late winter/early spring, while the projection of inertia-gravity waves at the surface peaks in late summer/early fall.The observational and modeling evidence of the importance of both geostrophic flows and internal gravity waves at mesoscales to submesoscales hints on the interaction between these two types of flow. To better understand these interactions, we analyze a simple model that couples barotropic quasi-geostrophic flow and near-inertial waves. There are two mechanisms of energy transfer from geostrophic flow to externally forced near-inertial waves: the refractive convergence of the wave action density into anti-cyclones (and divergence from cyclones); and the enhancement of wave-field gradients by geostrophic straining. Unforced inviscid numerical solutions of this reduced model reveal that geostrophic straining accounts for most of stimulated generation, which represents 10-20$\%$ of the decay of the initial balanced energy. Consideration of the dissipative problem reveals that wave dissipation generates both quasi-geostrophic potential vorticity locally and geostrophic kinetic energy. And this wave streaming mechanism is non-negligible in forced-dissipative solutions, which equilibrate even without bottom drag.In a separate study, we derive a Galerkin approximation for the surface-active quasi-geostrophic system using standard vertical modes. While the Galerkin expansions of streamfunction and potential vorticity do not satisfy the inversion relation exactly, the series converge with no Gibbs oscillations. With enough modes, the Galerkin series provide a good approximation to the streamfunction throughout the domain, which can be used to advect potential vorticity in the interior and buoyancy at the surfaces

On the Mechanisms Driving Latent Heat Flux Variations in the Northwest Tropical Atlantic

International audienceAbstract The Northwest Tropical Atlantic (NWTA) is a region of complex surface ocean circulation. The most prominent feature is the North Brazil Current (NBC) and its retroflection at 8°N, which leads to the formation of numerous mesoscale eddies known as NBC rings. The NWTA also receives the outflow of the Amazon River, generating freshwater plumes that can extend up to 100,000 km 2 . We show that these two processes influence the spatial variability of the region's surface latent heat flux (LHF). On the one hand, the presence of surface freshwater modifies the vertical stratification of the ocean, the mixed layer heat budget, and thus the air‐sea heat exchanges. On the other hand, NBC rings create a highly heterogeneous mesoscale sea surface temperature (SST) field that directly influences the near‐surface atmospheric circulation. These effects are illustrated by observations from the ElUcidating the RolE of Cloud‐Circulation Coupling in ClimAte ‐ Ocean Atmosphere (EUREC 4 A‐OA) and Atlantic Tradewind Ocean‐Atmosphere Interaction Campaign (ATOMIC) experiments, satellite and reanalysis data. We decompose the LHF budget into several terms controlled by different atmospheric and oceanic processes to identify the mechanisms leading to LHF changes. We find LHF variations of up to 160 W m 2 , of which 100 W m 2 are associated with wind speed changes and 40 W m 2 with SST variations. Surface currents or heat release associated with stratification changes remain as second‐order contributions with LHF variations of less than 10 W m 2 each. This study highlights the importance of considering these three components to properly characterize LHF variability at different spatial scales, although it is limited by the scarcity of collocated observations