Physical Controls on Oxygen Distribution and Denitrification Potential in the North West Arabian Sea

At suboxic oxygen concentrations, key biogeochemical cycles change and denitrification becomes the dominant remineralization pathway. Earth system models predict oxygen loss across most ocean basins in the next century; oxygen minimum zones near suboxia may become suboxic and therefore denitrifying. Using an ocean glider survey and historical data, we show oxygen loss in the Gulf of Oman (from 6–12 to <2 μmol/kg−1) not represented in climatologies. Because of the nonlinearity between denitrification and oxygen concentration, resolutions of current Earth system models are too coarse to accurately estimate denitrification. We develop a novel physical proxy for oxygen from the glider data and use a high‐resolution physical model to show eddy stirring of oxygen across the Gulf of Oman. We use the model to investigate spatial and seasonal differences in the ratio of oxic and suboxic water across the Gulf of Oman and waters exported to the wider Arabian Sea

Breaking of internal waves and turbulent dissipation in an anticyclonic mode Water Eddy

A four-month glider mission was analyzed to assess turbulent dissipation in an anticyclonic eddy at the western boundary of the subtropical North Atlantic. The eddy (radius ≈ 60 km) had a core of low potential vorticity between 100–450 m, with maximum radial velocities of 0.5 m s−1 and Rossby number ≈ −0.1. Turbulent dissipation was inferred from vertical water velocities derived from the glider flight model. Dissipation was suppressed in the eddy core (ε ≈ 5×10−10 W kg−1) and enhanced below it (> 10−9 W kg−1). Elevated dissipation was coincident with quasi-periodic structures in the vertical velocity and pressure perturbations, suggesting internal waves as the drivers of dissipation. A heuristic ray-tracing approximation was used to investigate the wave-eddy interactions leading to turbulent dissipation. Ray-tracing simulations were consistent with two types of wave-eddy interactions that may induce dissipation: the trapping of near-inertial wave energy by the eddy’s relative vorticity, or the entry of an internal tide (generated at the nearby continental slope) to a critical layer in the eddy shear. The latter scenario suggests that the intense mesoscale field characterizing the western boundaries of ocean basins might act as a ‘leaky wall’ controlling the propagation of internal tides into the basins’ interior

Galápagos upwelling driven by localized wind–front interactions

The Galápagos archipelago, rising from the eastern equatorial Pacific Ocean some 900 km off the South American mainland, hosts an iconic and globally significant biological hotspot. The islands are renowned for their unique wealth of endemic species, which inspired Charles Darwin’s theory of evolution and today underpins one of the largest UNESCO World Heritage Sites and Marine Reserves on Earth. The regional ecosystem is sustained by strongly seasonal oceanic upwelling events—upward surges of cool, nutrient-rich deep waters that fuel the growth of the phytoplankton upon which the entire ecosystem thrives. Yet despite its critical life-supporting role, the upwelling’s controlling factors remain undetermined. Here, we use a realistic model of the regional ocean circulation to show that the intensity of upwelling is governed by local northward winds, which generate vigorous submesoscale circulations at upper-ocean fronts to the west of the islands. These submesoscale flows drive upwelling of interior waters into the surface mixed layer. Our findings thus demonstrate that Galápagos upwelling is controlled by highly localized atmosphere–ocean interactions, and call for a focus on these processes in assessing and mitigating the regional ecosystem’s vulnerability to 21st-century climate change

Deep-ocean mixing driven by small-scale internal tides

Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earth’s climate. In the deep ocean, tides supply much of the mechanical energy required to sustain mixing via the generation of internal waves, known as internal tides, whose fate—the relative importance of their local versus remote breaking into turbulence—remains uncertain. Here, we combine a semi-analytical model of internal tide generation with satellite and in situ measurements to show that from an energetic viewpoint, small-scale internal tides, hitherto overlooked, account for the bulk (>50%) of global internal tide generation, breaking and mixing. Furthermore, we unveil the pronounced geographical variations of their energy proportion, ignored by current parameterisations of mixing in climate-scale models. Based on these results, we propose a physically consistent, observationally supported approach to accurately represent the dissipation of small-scale internal tides and their induced mixing in climate-scale models

Internal Tides Drive Nutrient Fluxes Into the Deep Chlorophyll Maximum Over Mid‐ocean Ridges

Diapycnal mixing of nutrients from the thermocline to the surface sunlit ocean is thought to be relatively weak in the world's subtropical gyres as energy inputs from winds are generally low. The interaction of internal tides with rough topography enhances diapycnal mixing, yet the role of tidally induced diapycnal mixing in sustaining nutrient supply to the surface subtropical ocean remains relatively unexplored. During a field campaign in the North Atlantic subtropical gyre, we tested whether tidal interactions with topography enhance diapycnal nitrate fluxes in the upper ocean. We measured an order of magnitude increase in diapycnal nitrate fluxes to the deep chlorophyll maximum (DCM) over the Mid‐Atlantic Ridge compared to the adjacent deep ocean. Internal tides drive this enhancement, with diapycnal nitrate supply to the DCM increasing by a factor of 8 between neap and spring tides. Using a global tidal dissipation database, we find that this spring‐neap enhancement in diapycnal nitrate fluxes is widespread over ridges and seamounts. Mid‐ocean ridges therefore play an important role in sustaining the nutrient supply to the DCM, and these findings may have important implications in a warming global ocean

Groupe de géographie sociale et d’études urbaines

Alain Musset, Marie-Vic Ozouf-Marignier et Marcel Roncayolo, directeurs d’étudesAlice Ingold, maitre de conférences avec Bernard André et Annie Sevin, ingénieurs d’étudesSophie Clément et Annick Tanter-Toubon, ingénieurs de recherche et Nicolas Verdier, chargé de recherche au CNRS Le territoire dans la pratique et les sciences sociales : moments, sources et méthodes La reconfiguration contemporaine des espaces du politique, qu’il s’agisse de la dimension intercommunale ou de l’Europe des régi..

Groupe de géographie sociale et d’études urbaines – GGSEU

Alain Musset, Marie-Vic Ozouf-Marignier, directeurs d’étudesAlice Ingold, maître de conférencesGeneviève Tranchand, maître de conférences des UniversitésBernard André, Annie Sevin, ingénieurs d’étudesSophie Clément, Annick Tanter-Toubon, ingénieurs de rechercheNicolas Verdier, chargé de recherche au CNRS Le territoire dans la pratique et les sciences sociales : moments, sources et méthodes Entre usages historiques et usages géographiques de la mémoire, qu’il met en parallèle, Nicolas Verdier ..

Groupe de géographie sociale et d’études urbaines

Alain Musset, Marie-Vic Ozouf-Marignier et Marcel Roncayolo, directeurs d’étudesAlice Ingold, maître de conférencesBernard André et Annie Sevin, ingénieurs d’étudesSophie Clément et Annick Tanter-Toubon, ingénieurs de rechercheNicolas Verdier, chargé de recherche au CNRS Le territoire dans la pratique et les sciences sociales : moments, sources et méthode Le séminaire s’est ouvert à l’exposé de nouveaux horizons de recherche qui enregistrent, d’une part, un élargissement du cadre géographique..

Groupe de géographie sociale et d’études urbaines – GGSEU

Alain Musset, Marie-Vic Ozouf-Marignier, directeurs d’étudesAlice Ingold, maître de conférencesGeneviève Tranchand, maître de conférences des UniversitésBernard André, Annie Sevin, ingénieurs d’étudesSophie Clément, Annick Tanter-Toubon, ingénieurs de rechercheNicolas Verdier, chargé de recherche au CNRS Le territoire dans la pratique et les sciences sociales : moments, sources et méthodes Entre usages historiques et usages géographiques de la mémoire, qu’il met en parallèle, Nicolas Verdier ..