scholarly journals Eddy-Modulated Internal Waves and Mixing on a Midocean Ridge

2012 ◽  
Vol 42 (7) ◽  
pp. 1242-1248 ◽  
Author(s):  
Xinfeng Liang ◽  
Andreas M. Thurnherr
Keyword(s):  
Kinetic Energy ◽  
Energy Content ◽  
World Ocean ◽  
Low Frequency ◽  
Mesoscale Eddies ◽  
Deep Ocean ◽  
Small Scale ◽  
Inertial Waves ◽  
Topographic Roughness ◽  
Geostrophic Flows

Abstract Mesoscale eddies are ubiquitous in the World Ocean and dominate the energy content on subinertial time scales. Recent theoretical and numerical studies suggest a connection between mesoscale eddies and diapycnal mixing in the deep ocean, especially near rough topography in regions of strong geostrophic flow. However, unambiguous observational evidence for such a connection has not yet been found, and it is still unclear what physical processes are responsible for transferring energy from mesoscale to small-scale processes. Here, the authors present observations demonstrating that finescale variability near the crest of the East Pacific Rise is strongly modulated by low-frequency geostrophic flows, including those due to mesoscale eddies. During times of strong subinertial flows, the authors observed elevated kinetic energy on vertical scales <50 m and in the near-inertial band, predominantly upward-propagating near-inertial waves, and increased incidence of layers with Richardson number . In contrast, during times of weak subinertial flows, kinetic energy in the finescale and near-inertial bands is lower, Ri values are higher, and near-inertial waves propagate predominantly downward through the water column. Diapycnal diffusivities estimated indirectly from a simple Ri-based parameterization are consistent with results from a tracer-release experiment and a microstructure survey bracketing the mooring measurements. These observations are consistent with energy transfer (a “cascade”) from subinertial flows, including mesoscale eddies, to near-inertial oscillations, turbulence, and mixing. This interpretation suggests that, in addition to topographic roughness and tidal forcing, parameterization of deep-ocean mixing should also take subinertial flows into account. The findings presented here are expected to be useful for validating and improving numerical-model parameterizations of turbulence and mixing in the ocean.

scholarly journals Surface kinetic energy transfer in surface quasi-geostrophic flows

2008 ◽  
Vol 604 ◽  
pp. 165-174 ◽  
Author(s):  
XAVIER CAPET ◽  
PATRICE KLEIN ◽  
BACH LIEN HUA ◽  
GUILLAUME LAPEYRE ◽  
JAMES C. MCWILLIAMS
Keyword(s):  
Kinetic Energy ◽  
Surface Velocity ◽  
Small Scale ◽  
Surface Dynamics ◽  
Spectral Flux ◽  
Advection Term ◽  
Wide Range ◽  
Geostrophic Flows ◽  
Scale Range ◽  
Density Variance

The relevance of surface quasi-geostrophic dynamics (SQG) to the upper ocean and the atmospheric tropopause has been recently demonstrated in a wide range of conditions. Within this context, the properties of SQG in terms of kinetic energy (KE) transfers at the surface are revisited and further explored. Two well-known and important properties of SQG characterize the surface dynamics: (i) the identity between surface velocity and density spectra (when appropriately scaled) and (ii) the existence of a forward cascade for surface density variance. Here we show numerically and analytically that (i) and (ii) do not imply a forward cascade of surface KE (through the advection term in the KE budget). On the contrary, advection by the geostrophic flow primarily induces an inverse cascade of surface KE on a large range of scales. This spectral flux is locally compensated by a KE source that is related to surface frontogenesis. The subsequent spectral budget resembles those exhibited by more complex systems (primitive equations or Boussinesq models) and observations, which strengthens the relevance of SQG for the description of ocean/atmosphere dynamics near vertical boundaries. The main weakness of SQG however is in the small-scale range (scales smaller than 20–30 km in the ocean) where it poorly represents the forward KE cascade observed in non-QG numerical simulations.

scholarly journals Emergence of Wind-Driven Near-Inertial Waves in the Deep Ocean Triggered by Small-Scale Eddy Vorticity Structures

2011 ◽  
Vol 41 (7) ◽  
pp. 1297-1307 ◽  
Author(s):  
Eric Danioux ◽  
Patrice Klein ◽  
Matthew W. Hecht ◽  
Nobumasa Komori ◽  
Guillaume Roullet ◽  
...  
Keyword(s):  
Mesoscale Eddy ◽  
Deep Ocean ◽  
Mean Amplitude ◽  
Relative Vorticity ◽  
Wind Forcing ◽  
Small Scale ◽  
Inertial Waves ◽  
Deep Interior ◽  
Eddy Field ◽  
Baroclinic Modes

Abstract Using numerical simulations forced by a uniform realistic wind time series, the authors show that the presence of a mesoscale eddy field at midlatitudes accelerates the vertical propagation of the wind-forced near-inertial waves (NIW) and produces the emergence of a maximum of vertical velocity into the deep ocean (around 2500 m) characterized by a mean amplitude of 25 m day−1, a dominant 2f frequency, and scales as small as O(30 km). These results differ from previous studies that reported a smaller depth and larger scales. The authors show that the larger depth observed in the present study (2500 m instead of 1700 m) is due to the wind forcing duration that allows the first five baroclinic modes to disperse and to impact the deep NIW maximum (instead of the first two modes as reported before). The smaller scales (30 km instead of 90 km) are explained by a resonance mechanism (described in previous studies) that affects the high NIW baroclinic modes, but only when small-scale relative vorticity structures (related to the mesoscale eddy field) have an amplitude that is large enough. These results, which point out the importance of the wind forcing duration and the resolution, indicate that the emergence of a deep NIW maximum with a 2f frequency reported before is a robust feature that is enhanced with more realistic conditions. Such 2f frequency in the deep interior raises the question of the mechanisms, still unresolved, that may ultimately transfer this superinertial energy into mixing at these depths.

Near-inertial waves modulated by background flow in realistic global ocean simulations

2021 ◽  
Author(s):  
Keshav Raja ◽  
Maarten Buijsman ◽  
Oladeji Siyanbola ◽  
Miguel Solano ◽  
Jay Shriver ◽  
...  
Keyword(s):  
Kinetic Energy ◽  
Normal Modes ◽  
Deep Ocean ◽  
Ocean Model ◽  
Global Ocean ◽  
Inertial Waves ◽  
Flux Divergence ◽  
Large Wave ◽  
Wind Input ◽  
Anticyclonic Eddies

<p>Wind generated near-inertial waves (NIWs) are a major source of energy for deep-ocean mixing by transmitting wind energy from the ocean surface into the interior. Recently, it has been established that the NIW energy transmission to ocean depths is significantly modulated by background mesoscale vorticity. Thus, understanding NIW energetics in the presence of mesoscale eddies on a global scale is crucial.</p><p>We study the generation, propagation and dissipation of NIWs in global 1/25<sup>o</sup> Hybrid Coordinate Ocean Model (HYCOM) simulations with realistic tidal forcing. The model has 41 layers with uniform vertical coordinates in the mixed layer and isopycnal coordinates in the ocean interior. The model is forced by 1/3hr wind from the NAVGEM atmospheric model. We analyze one month of model data for May-June 2019. The 3D HYCOM fields are projected on vertical normal modes to compute the wind input, wave kinetic energy (KE), flux divergence and dissipation per mode.</p><p>We find that the globally integrated wind input in surface near-inertial motions is 0.21 TW for the 30-day period and is consistent with previous studies. The sum of the wind input to the first 5 modes accounts to only 31% of the total wind input while the sum of the NIW kinetic energy in the first 5 modes adds up to 60% of the total NIW KE. The difference in the fraction of the total between the wind input and NIW KE (31% and 60%) suggests that a significant portion of wind-induced near-inertial motions is dissipated close to the surface without being projected onto modes. We also find that NIW horizontal fluxes diverge from areas with cyclonic vorticity and converge in areas with anticyclonic vorticity, i.e., anticyclonic eddies are a sink for NIW energy in the global ocean.</p><p>The residual NIW KE that does not project onto modes is found to be largely trapped in anticyclonic eddies. In a next step, we will study the fate of this energy, which most likely propagate downward as beam-like features with large wave numbers. We will compute the near-inertial wave energy balance for fixed subsurface layers and consider the energy exchange between these layers to understand the vertical structure of NIW energy dissipation. We find that the downward NIW radiation to the ocean interior at 500 m depth is 19% of the surface near-inertial wind input for the 30-day period.</p>

scholarly journals Observations of Small-Scale Secondary Instabilities during the Shoaling of Internal Bores on a Deep-Ocean Slope

2016 ◽  
Vol 46 (1) ◽  
pp. 219-231 ◽  
Author(s):  
Frédéric Cyr ◽  
Hans van Haren
Keyword(s):  
Kinetic Energy ◽  
Potential Energy ◽  
Dissipation Rate ◽  
Deep Ocean ◽  
Temperature Sensors ◽  
Small Scale ◽  
Cooling Phase ◽  
Northeast Atlantic Ocean ◽  
Internal Bores ◽  
Secondary Instabilities

AbstractThe Rockall Bank area, located in the northeast Atlantic Ocean, is a region dominated by topographically trapped diurnal tides. These tides generate up- and downslope displacements that can be locally described as swashing motions on the bank. Using high spatial and time resolution of moored temperature sensors, the transition toward the upslope flow (cooling phase) is described as a rapid upslope-propagating bore, likely generated by breaking trapped internal waves. Buoyant anomalies are found during the bore propagation, likely resulting from small-scale instabilities. The imbalance between the rate of disappearance of available potential energy and the dissipation rate of turbulent kinetic energy suggests that these instabilities are growing (i.e., young) and have high mixing potential.

Secondary instabilities in rapidly rotating fluids: inertial wave breakdown

1999 ◽  
Vol 382 ◽  
pp. 283-306 ◽  
Author(s):  
R. R. KERSWELL
Keyword(s):  
Three Dimensional ◽  
Finite Amplitude ◽  
Small Scale ◽  
Inertial Waves ◽  
Rotating Fluids ◽  
Initial Excitation ◽  
Geostrophic Flows ◽  
Inertial Wave ◽  
Secondary Instabilities ◽  
Wave Breakdown

Inertial waves are a ubiquitous feature of rapidly rotating fluids. Although much is known about their initial excitation, little is understood about their stability. Experiments indicate that they are generically unstable and in many cases catastrophically so, quickly causing the whole flow to collapse to small-scale disorder. The linear stability of two three-dimensional inertial waves observed to break down in the laboratory is considered here at experimentally small but finite Ekman numbers of [les ]10−4. Surprisingly small threshold amplitudes for instability are found. The results support the conjecture that triad resonances are the generic mechanism for secondary instability in rapidly rotating fluids but also highlight the ability of geostrophic flows to derive energy through a finite-amplitude inertial wave. This latter finding may go some way to explaining the significant mean circulations typically observed in inertial wave experiments.

scholarly journals Deep-sea turbulence evolution observed by multiple closely spaced instruments

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Chu-Fang Yang ◽  
Wu-Cheng Chi ◽  
Hans van Haren
Keyword(s):  
Kinetic Energy ◽  
Internal Waves ◽  
Deep Sea ◽  
Large Scale ◽  
Time Derivative ◽  
Deep Ocean ◽  
Energy Dissipation Rate ◽  
Small Scale ◽  
Ocean Bottom ◽  
Seafloor Topography

AbstractTurbulent mixing in the deep ocean is not well understood. The breaking of internal waves on sloped seafloor topography can generate deep-sea turbulence. However, it is difficult to measure turbulence comprehensively due to its multi-scale processes, in addition to flow–flow and flow–topography interactions. Dense, high-resolution spatiotemporal coverage of observations may help shed light on turbulence evolution. Here, we present turbulence observations from four broadband ocean bottom seismometers (OBSs) and a 200-m vertical thermistor string (T-string) in a footprint of 1 × 1 km to characterize turbulence induced by internal waves at a depth of 3000 m on a Pacific continental slope. Correlating the OBS-calculated time derivative of kinetic energy and the T-string-calculated turbulent kinetic energy dissipation rate, we propose that the OBS-detected signals were induced by near-seafloor turbulence. Strong disturbances were detected during a typhoon period, suggesting large-scale inertial waves breaking with upslope transport speeds of 0.2–0.5 m s−1. Disturbances were mostly excited on the downslope side of the array where the internal waves from the Pacific Ocean broke initially and the turbulence oscillated between < 1 km small-scale ridges. Such small-scale topography caused varying turbulence-induced signals due to localized waves breaking. Arrayed OBSs can provide complementary observations to characterize deep-sea turbulence.

Update on the Global Energy Dissipation Rate of Deep-Ocean Low-Frequency Flows by Bottom Boundary Layer

2018 ◽  
Vol 48 (6) ◽  
pp. 1243-1255 ◽  
Author(s):  
Chao Huang ◽  
Yongsheng Xu
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Dissipation Rate ◽  
Low Frequency ◽  
Deep Ocean ◽  
Power Input ◽  
Energy Dissipation Rate ◽  
Bottom Boundary Layer ◽  
Geostrophic Currents ◽  
Bottom Boundary

AbstractThe global dissipation caused by bottom boundary layer drag is one of the major pathways for the consumption of kinetic energy in the deep ocean. However, the spatial distribution and global integral of the drag dissipation are still debatable. This paper presents an updated estimate of the dissipation rate, using the barotropic component of surface geostrophic currents and 632 in situ velocity measurements. Also, the seafloor roughness is proposed as a parameter of drag efficiency in the parameterized method. The results provide a map of the drag dissipation rate with a global integral of ~0.26 TW. Approximately 66% of this dissipation occurs in the Southern Ocean, which is consistent with the proportion of wind power input into this region. Building upon the work in previous studies on the bottom boundary layer drag, more long-period observations are used, eliminating the influence of the baroclinic contribution to the surface geostrophic currents in the construction of the bottom velocity, and taking topographic roughness into account. The estimates have implications for the maintenance of density structure in the deep ocean and understanding of the kinetic energy budget.

scholarly journals Spatiotemporal Variability of Mesoscale Eddies in the Indonesian Seas

2021 ◽  
Vol 13 (5) ◽  
pp. 1017
Author(s):  
Zhanjiu Hao ◽  
Zhenhua Xu ◽  
Ming Feng ◽  
Qun Li ◽  
Baoshu Yin
Keyword(s):  
World Ocean ◽  
Mesoscale Eddies ◽  
Spatial Inhomogeneity ◽  
Spatiotemporal Variability ◽  
Identification Algorithm ◽  
Mean Flow ◽  
Sulu Sea ◽  
Indonesian Seas ◽  
Highly Nonlinear ◽  
Eddy Generation

Mesoscale eddies are ubiquitous in the world ocean and well researched both globally and regionally, while their properties and distributions across the whole Indonesian Seas are not yet fully understood. This study investigates for the first time the spatiotemporal variations and generation mechanisms of mesoscale eddies across the whole Indonesian Seas. Eddies are detected from altimetry sea level anomalies by an automatic identification algorithm. The Sulu Sea, Sulawesi Sea, Maluku Sea and Banda Sea are the main eddy generation regions. More than 80% of eddies are short-lived with a lifetime below 30 days. The properties of eddies exhibit high spatial inhomogeneity, with the typical amplitudes and radiuses of 2–6 cm and 50–160 km, respectively. The most energetic eddies are observed in the Sulawesi Sea and Seram Sea. Eddies feature different seasonal cycles between anticyclonic and cyclonic eddies in each basin, especially given that the average latitude of the eddy centroid has inverse seasonal variations. About 48% of eddies in the Sulawesi Sea are highly nonlinear, which is the case for less than 30% in the Sulu Sea and Banda Sea. Instability analysis is performed using high-resolution model outputs from Bluelink Reanalysis to assess mechanisms of eddy generation. Barotropic instability of the mean flow dominates eddy generation in the Sulu Sea and Sulawesi Sea, while baroclinic instability is slightly more in the Maluku Sea and Banda Sea.

Sign in / Sign up

Export Citation Format

Share Document