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Clément Vic, Bruno Ferron, Virginie Thierry, Herlé Mercier, and Pascale Lherminier

Abstract

Internal waves in the semidiurnal and near-inertial bands are investigated using an array of seven moorings located over the Reykjanes Ridge in a cross-ridge direction (57.6°–59.1°N, 28.5°–33.3°W). Continuous measurements of horizontal velocity and temperature for more than 2 years allow us to estimate the kinetic energy density and the energy fluxes of the waves. We found that there is a remarkable phase locking and linear relationship between the semidiurnal energy density and the tidal energy conversion at the spring–neap cycle. The energy-to-conversion ratio gives replenishment time scales of 4–5 days on the ridge top versus 7–9 days on the flanks. Altogether, these results demonstrate that the bulk of the tidal energy on the ridge comes from near-local sources, with a redistribution of energy from the top to the flanks, which is endorsed by the energy fluxes oriented in the cross-ridge direction. Implications for tidally driven energy dissipation are discussed. The time-averaged near-inertial kinetic energy is smaller than the semidiurnal kinetic energy by a factor of 2–3 but is much more variable in time. It features a strong seasonal cycle with a winter intensification and subseasonal peaks associated with local wind bursts. The ratio of energy to wind work gives replenishment time scales of 13–15 days, which is consistent with the short time scales of observed variability of near-inertial energy. In the upper ocean (1 km), the highest levels of near-inertial energy are preferentially found in anticyclonic structures, with a twofold increase relative to cyclonic structures, illustrating the funneling effect of anticyclones.

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Clément Vic, Henrick Berger, Anne-Marie Tréguier, and Xavier Couvelard

Abstract

The Congo River has the second largest rate of flow in the world and is mainly responsible for the broad tongue of low-salinity water that is observed in the Gulf of Guinea. Despite their importance, near-equatorial river plumes have not been studied as thoroughly as midlatitude plumes and their dynamics remain unclear. Using both theory and idealized numerical experiments that reproduce the major characteristics of the region, the authors have investigated the dynamics of the Congo River plume and examine its sensitivity to different forcing mechanisms. It is found that near-equatorial plumes are more likely to be surface trapped than midlatitude plumes, and the importance of the β effect in describing the strong offshore extent of the low-salinity tongue during most of the year is demonstrated. It is shown that the buoyant plume constrained by the geomorphology is subject to the β pulling of nonlinear structures and wavelike equatorial dynamics. The wind is found to strengthen the intrinsic buoyancy-driven dynamics and impede the development of the coastal southward current, in coherence with observations.

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Bieito Fernández-Castro, Dafydd Gwyn Evans, Eleanor Frajka-Williams, Clément Vic, and Alberto C. Naveira-Garabato

Abstract

A 4-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 and 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 quasiperiodic 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 basin’s interior.

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Clément Vic, Alberto C. Naveira Garabato, J. A. Mattias Green, Carl Spingys, Alexander Forryan, Zhongxiang Zhao, and Jonathan Sharples

Abstract

The life cycle of semidiurnal internal tides over the Mid-Atlantic Ridge (MAR) sector south of the Azores is investigated using in situ, a high-resolution mooring and microstructure profiler, and satellite data, in combination with a theoretical model of barotropic-to-baroclinic tidal energy conversion. The mooring analysis reveals that the internal tide horizontal energy flux is dominated by mode 1 and that energy density is more distributed among modes 1–10. Most modes are compatible with an interpretation in terms of standing internal tides, suggesting that they result from interactions between waves generated over the MAR. Internal tide energy is thus concentrated above the ridge and is eventually available for local diapycnal mixing, as endorsed by the elevated rates of turbulent energy dissipation ε estimated from microstructure measurements. A spring–neap modulation of energy density on the MAR is found to originate from the remote generation and radiation of strong mode-1 internal tides from the Atlantis-Meteor Seamount Complex. Similar fortnightly variability of a factor of 2 is observed in ε, but this signal’s origin cannot be determined unambiguously. A regional tidal energy budget highlights the significance of high-mode generation, with 81% of the energy lost by the barotropic tide being converted into modes >1 and only 9% into mode 1. This has important implications for the fraction (q) of local dissipation to the total energy conversion, which is regionally estimated to be ~0.5. This result is in stark contrast with the Hawaiian Ridge system, where the radiation of mode-1 internal tides accounts for 30% of the regional energy conversion, and q < 0.25.

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