The processes leading to the depletion of oceanic mesoscale kinetic energy (KE) and the energization of near‐inertial internal waves are investigated using a suite of realistically forced regional ...ocean simulations. By carefully modifying the forcing fields we show that solutions where internal waves are forced have ∼25% less mesoscale KE compared with solutions where they are not. We apply a coarse‐graining method to quantify the KE fluxes across time scales and demonstrate that the decrease in mesoscale KE is associated with an internal wave‐induced reduction of the inverse energy cascade and an enhancement of the forward energy cascade from sub‐to super‐inertial frequencies. The integrated KE forward transfer rate in the upper ocean is equivalent to half and a quarter of the regionally averaged near‐inertial wind work in winter and summer, respectively, with the strongest fluxes localized at surface submesoscale fronts and filaments.
Plain Language Summary
Oceanic eddies contain most of the kinetic energy in the ocean and therefore play an important role in determining the ocean's response to future climate scenarios. Oceanic wind‐ and tidally forced internal waves are energetic fast motions that contribute substantially to the vertical mixing of water, thereby affecting biogeochemical and climate processes. This work shows for the first time in high‐resolution, realistically forced, numerical simulations that wave motions can drain a substantial amount of eddy energy by altering the way in which energy is transferred across scales. This has important implications to ocean energetics and to climate models that often lack the resolution and forcing components to represent these wave‐induced effects.
Key Points
Wind forced near‐inertial waves and internal tides can efficiently drain oceanic mesoscale eddy energy
Eddy energy “draining” is largely a result of an internal‐wave induced modifications to the turbulent energy cascades
The strongest forward energy transfers are found in submesoscale fronts and filaments that dynamically depart from geostrophic balance
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.
An asymmetry in the clustering of oceanic surface material has been observed at the submesoscales. Energetic and ephemeral submesoscale cyclonic fronts are associated with convergence zones, hence ...cluster surface material. Their anticyclonic counterparts do not feature such an effect. Yet, at the mesoscale, literature has been contradictory about such an asymmetry. Here, we combine surface drifter trajectories with an altimetry‐derived mesoscale eddy database in the North Atlantic to show that mesoscale cyclones contain 24% more drifters than anticyclones. A numerical Lagrangian experiment using a mesoscale‐resolving model quantitatively reproduces the observational results. It reveals that particles preferentially cluster in cyclonic regions, both in fronts and eddies. The model further suggests that ageostrophic cyclonic fronts concentrate particles a few days before the eddy formation and detection.
Plain Language Summary
Earth's oceans are filled with swirling coherent structures called eddies, whose dominant scales range from 10 to 100 km across. Clockwise‐rotating and counter‐clockwise‐rotating eddies, called anticyclones and cyclones in the northern hemisphere, coexist in the oceans with similar proportions and covered areas. Dominant theories for the life cycle of the largest eddies (mesoscale eddies) have concurred on their kinematic symmetry. However, this symmetry breaks down at the small scales (submesoscale eddies and fronts) and cyclonic structures have been shown to be associated with convergence zones. Here, we combined a surface drifter database with a satellite‐derived mesoscale eddy database to show that mesoscale cyclones contain significantly more drifters than anticyclones. The use of a numerical Lagrangian experiment, that is, flow‐following inert particles, unveiled that the clustering of particles occurs in the formation stage of the cyclones. This work has implications for our global understanding of the transport of surface material in the oceans, for example, debris and plastics.
Key Points
Surface Velocity Program drifters are preferentially trapped into mesoscale cyclones rather than in anticyclones
Lagrangian analysis of a mesoscale‐resolving simulation shows that particles cluster preferentially in cyclonic fronts and eddies
Particles cluster in cyclonic regions a few days before the formation and detection of mesoscale cyclones
Future changes in subduction are suspected to be critical for the ocean deoxygenation predicted by climate models over the 21st century. However, the drivers of global oxygen subduction have not been ...fully described or quantified. Here, we address the physical mechanisms responsible for the oxygen transport across the late‐winter mixed layer base and their relation with water mass formation. Up to 70% of the global oxygen uptake takes place during Mode Water subduction mostly in the Southern Ocean and the North Atlantic. Te driving mechanisms are (i) the combination of strong currents with large mixed‐layer‐depth gradients at localized hot spots and (ii) the wind‐driven vertical velocity within the subtropical gyres. Oxygen diffusion, despite being underestimated in this study, is likely to play an important role in the global ocean oxygenation. The physical mass flux dominates the total oxygen subduction while the oxygen solubility plays a minor role in its modulation.
Key Points
Up to 70% of the global oxygen uptake occurs during Mode Water subduction, driven by lateral induction and vertical velocity
Oxygen diffusion, despite large uncertainties, is likely to play an important role in the global oxygen uptake
Total oxygen subduction is driven by the mass flux, with little contribution of the latitudinal variability of oxygen concentration
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.
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.
Plain Language Summary
Oxygen is present in the ocean and is required by all marine plants and animals to breathe. In certain regions around the world, oxygen concentrations reach very low levels. These are known as “oxygen minimum zones”. When oxygen is absent, chemical cycling of nitrogen, a key nutrient for plant growth, changes dramatically. Computer simulations of ocean oxygen show a decrease in oxygen over the next century and growing oxygen minimum zones. However, these simulations have a difficult time representing small but very important features such as eddies, which impact how oxygen is transported. It is difficult to predict what will happen in the biggest of the world's oxygen minimum zones, the Arabian Sea, as piracy and geopolitical tensions have limited past opportunities for observing these processes. To remedy this, we deployed two remote‐controlled submarines, known as a Seagliders, in the Gulf of Oman. These instruments measured a strong decrease of oxygen in the oxygen minimum zone compared to pre‐1990 values. We then combined the Seaglider data with a very high resolution computer simulation to determine how oxygen is spread around the northwestern Arabian Sea throughout different seasons and the monsoons.
Key Points
Climatologies overestimate oxygen concentrations and underestimate denitrification in the Gulf of Oman because of insufficient sampling
Submesoscale and mesoscale processes regulate oxygen concentrations by stirring oxygenated Persian Gulf Water throughout the oxygen minimum zone
Spiciness serves as a physical proxy for determining low oxygen concentrations in the Gulf of Oman
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.
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.
Internal tides are key players in ocean dynamics above mid‐ocean ridges. The generation and propagation of internal tides over the Mid‐Atlantic Ridge (MAR) have been studied through theoretical and ...numerical models, as well as through moored, that is, one‐dimensional, observations. Yet, observations remain sparse and often restricted to the vertical direction. Here we report on the first two‐dimensional in situ observation of an internal tide beam sampled by a shipboard acoustic Doppler current profiler through a vertical section over the MAR. The beam is generated by the interaction of the barotropic tidal current with a supercritical abyssal hill that sits in the rift valley of the MAR. A vertical mode decomposition is carried out to characterize the spatio‐temporal variability of the beam. Although the modal content of the velocity field is dominated by modes 1 to 3, higher modes display localized and not persistent bursts of energy. The use of an analytical theory for linear internal waves allows us to rationalize the observed velocity field and interpret it as the superposition of modal waves generated on the hill and propagating in the same direction. The observed beam is qualitatively reconstructed as the superposition of waves of modes 2 to 6. The velocity field was sampled seven times across the same section and displayed qualitatively different patterns, unveiling the complexity of the dynamics above the MAR. A ray tracing of modal waves shows that the refraction by mesoscale currents could explain the observed variability of the tidal beam.
Plain Language Summary
In the stratified ocean, the interaction of tidal currents with the seafloor topography generates waves that propagate along and across density layers. Those waves, called internal tides, play important roles in the equilibrium of the ocean. Yet, their fine‐scale observations are sparse and uncertainties remain on their lifecycle. Using a shipboard current profiler through a section over the Mid‐Atlantic Ridge, we sampled a clear signal of an internal tide beam, a structure whose existence has been unveiled by theories and models but never observed on the Mid‐Atlantic Ridge. We use a simple linear theory to characterize the beam and rationalize the observed velocity field, which can be interpreted as the superposition of many waves with distinct spatial structures.
Key Points
An internal tide beam generated by an abyssal hill is observed by a Shipboard Acoustic Doppler Current Profiler through a section over the Mid‐Atlantic Ridge
A linear wave theory quantitatively reconstructs the beam structure, interpreted as the superposition of modal internal tides
Modes 1–3 are the most energetic but modes 2–6 are the ones that fit best the horizontal structure of modal internal tides
Deep-sea hydrothermal vents provide sources of geochemical materials that impact the global ocean heat and chemical budgets, and support complex biological communities. Vent effluents and larvae are ...dispersed and transported long distances by deep ocean currents, but these currents are largely undersampled and little is known about their variability. Submesoscale (0.1–10 km) currents are known to play an important role for the dispersion of biogeochemical materials in the ocean surface layer, but their impact for the dispersion in the deep ocean is unknown. Here, we use a series of nested regional oceanic numerical simulations with increasing resolution (from δx=6km to δx=0.75km) to investigate the structure and variability of highly-resolved deep currents over the Mid-Atlantic Ridge (MAR) and their role on the dispersion of the Lucky Strike hydrothermal vent effluents and larvae. We shed light on a submesoscale regime of oceanic turbulence over the MAR at 1500 m depth, contrasting with open-ocean – i.e., far from topographic features – regimes of turbulence, dominated by mesoscales.
Impacts of submesoscale and tidal currents on larval dispersion and connectivity among vent populations are investigated by releasing neutrally buoyant Lagrangian particles at the Lucky Strike hydrothermal vent. Although the absolute dispersion is overall not sensitive to the model resolution, submesoscale currents are found to significantly increase both the horizontal and vertical relative dispersion of particles at O(1–10) km and O(1–10) days, resulting in an increased mixing of the cloud of particles. A fraction of particles are trapped in submesoscale coherent vortices, which enable transport over long time and distances. Tidal currents and internal tides do not significantly impact the horizontal relative dispersion. However, they roughly double the vertical dispersion. Specifically, particles undergo strong tidally-induced mixing close to rough topographic features, which allows them to rise up in the water column and to cross topographic obstacles.
The mesoscale variability controls at first order the connectivity between hydrothermal sites and we do not have long enough simulations to conclude on the connectivity between the different MAR hydrothermal sites. However, our simulations suggest that the connectivity might be increased by submesoscale and tidal currents, which act to spread the cloud of particles and help them cross topographic barriers.
•High-resolution numerical simulations shed light on a submesoscale regime of turbulence at the depth of the Mid-Atlantic Ridge.•Submesoscales and tides enhance the relative dispersion of particles and may augment the connectivity between MAR vents.•Tidal currents and internal tides double the vertical dispersion of particles and help crossing topographic barriers.
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