scholarly journals Eddy-Induced Modulation of Turbulent Dissipation over Rough Topography in the Southern Ocean

2013 ◽  
Vol 43 (11) ◽  
pp. 2288-2308 ◽  
Author(s):  
J. Alexander Brearley ◽  
Katy L. Sheen ◽  
Alberto C. Naveira Garabato ◽  
David A. Smeed ◽  
Stephanie Waterman
Keyword(s):  
Southern Ocean ◽  
Internal Wave ◽  
Ocean Circulation ◽  
Wave Energy ◽  
Bottom Topography ◽  
Temporal Correlation ◽  
Drake Passage ◽  
Wave Radiation ◽  
Bottom Current ◽  
Turbulent Dissipation

Abstract Mesoscale eddies are universal features of the ocean circulation, yet the processes by which their energy is dissipated remain poorly understood. One hypothesis argues that the interaction of strong geostrophic flows with rough bottom topography effects an energy transfer between eddies and internal waves, with the breaking of these waves causing locally elevated dissipation focused near the sea floor. This study uses hydrographic and velocity data from a 1-yr mooring cluster deployment in the Southern Ocean to test this hypothesis. The moorings were located over a small (~10 km) topographic obstacle to the east of Drake Passage in a region of high eddy kinetic energy, and one was equipped with an ADCP at 2800-m depth from which internal wave shear variance and dissipation rates were calculated. Examination of the ADCP time series revealed a predominance of upward-propagating internal wave energy and a significant correlation (r = 0.45) between shear variance levels and subinertial near-bottom current speeds. Periods of strong near-bottom flow coincided with increased convergence of eddy-induced interfacial form stress in the bottom 1500 m. Predictions of internal wave energy radiation were made from theory using measured near-bottom current speeds, and the mean value of wave radiation (5.3 mW m−2) was sufficient to support the dissipated power calculated from the ADCP. A significant temporal correlation was also observed between radiated and dissipated power. Given the ubiquity of strong eddy flows and rough topography in the Southern Ocean, the transfer from eddy to internal wave energy is likely to be an important term in closing the ocean energy budget.

Radiation and Dissipation of Internal Waves Generated by Geostrophic Motions Impinging on Small-Scale Topography: Application to the Southern Ocean

2010 ◽  
Vol 40 (9) ◽  
pp. 2025-2042 ◽  
Author(s):  
Maxim Nikurashin ◽  
Raffaele Ferrari
Keyword(s):  
Southern Ocean ◽  
Internal Wave ◽  
Internal Waves ◽  
Acoustic Doppler Current Profiler ◽  
Drake Passage ◽  
Internal Tides ◽  
Wave Radiation ◽  
Small Scale ◽  
Region Theory ◽  
Abyssal Mixing

Abstract Recent estimates from observations and inverse models indicate that turbulent mixing associated with internal wave breaking is enhanced above rough topography in the Southern Ocean. In most regions of the ocean, abyssal mixing has been primarily associated with radiation and breaking of internal tides. In this study, it is shown that abyssal mixing in the Southern Ocean can be sustained by internal waves generated by geostrophic motions that dominate abyssal flows in this region. Theory and fully nonlinear numerical simulations are used to estimate the internal wave radiation and dissipation from lowered acoustic Doppler current profiler (LADCP), CTD, and topography data from two regions in the Southern Ocean: Drake Passage and the southeast Pacific. The results show that radiation and dissipation of internal waves generated by geostrophic motions reproduce the magnitude and distribution of dissipation previously inferred from finescale measurements in the region, suggesting that it is one of the primary drivers of abyssal mixing in the Southern Ocean.

scholarly journals Early westward flow across the Tasman Gateway

2015 ◽  
Vol 11 (5) ◽  
pp. 5021-5048
Author(s):  
W. P. Sijp ◽  
A. S. von der Heydt ◽  
P. K. Bijl
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Surface Waters ◽  
Middle Eocene ◽  
Drake Passage ◽  
Easterly Wind ◽  
Wind Pattern ◽  
Sw Pacific ◽  
Field Evidence ◽  
Set Up

Abstract. The timing and role in ocean circulation and climate of the opening of Southern Ocean gateways is as yet elusive. Recent micropaleontological studies suggest the onset of throughflow of surface waters from the SW Pacific into the Australo-Antarctic Gulf through a southern shallow opening of the Tasman Gateway from 49–50 Ma onwards. Here, we present the first model results specific to the early-to-middle Eocene where, in agreement with the field evidence, southerly shallow opening of the Tasman Gateway indeed causes a westward flow across the Tasman Gateway. As a result, modelled estimates of dinoflagellate biogeography are in agreement with the recent findings. Crucially, in this situation where Australia is still situated far south and almost attached to Antarctica, the Drake Passage must be sufficiently restricted to allow the prevailing easterly wind pattern to set up this southerly restricted westward flow. In contrast, an open Drake Passage, to 517 m depth, leads to an eastward flow, even when the Tasman Gateway and the Australo-Antarctic gulf are entirely contained within the latitudes of easterly wind.

scholarly journals A Global Model for the Diapycnal Diffusivity Induced by Internal Gravity Waves

2013 ◽  
Vol 43 (8) ◽  
pp. 1759-1779 ◽  
Author(s):  
Dirk Olbers ◽  
Carsten Eden
Keyword(s):  
Energy Density ◽  
Internal Wave ◽  
Wave Field ◽  
Ocean Circulation ◽  
Gravity Waves ◽  
Wave Energy ◽  
Internal Gravity Waves ◽  
Radiation Balance ◽  
Dissipation Rates ◽  
Diapycnal Diffusivity

Abstract An energetically consistent model for the diapycnal diffusivity induced by breaking of internal gravity waves is proposed and tested in local and global settings. The model [Internal Wave Dissipation, Energy and Mixing (IDEMIX)] is based on the spectral radiation balance of the wave field, reduced by integration over the wavenumber space, which yields a set of balances for energy density variables in physical space. A further simplification results in a single partial differential equation for the total energy density of the wave field. The flux of energy to high vertical wavenumbers is parameterized by a functional derived from the wave–wave scattering integral of resonant wave triad interactions, which also forms the basis for estimates of dissipation rates and related diffusivities of ADCP and hydrography fine-structure data. In the current version of IDEMIX, the wave energy is forced by wind-driven near-inertial motions and baroclinic tides, radiating waves from the respective boundary layers at the surface and the bottom into the ocean interior. The model predicts plausible magnitudes and three-dimensional structures of internal wave energy, dissipation rates, and diapycnal diffusivities in rough agreement to observational estimates. IDEMIX is ready for use as a mixing module in ocean circulation models and can be extended with more spectral components.

scholarly journals The Impact of Finite-Amplitude Bottom Topography on Internal Wave Generation in the Southern Ocean

2014 ◽  
Vol 44 (11) ◽  
pp. 2938-2950 ◽  
Author(s):  
Maxim Nikurashin ◽  
Raffaele Ferrari ◽  
Nicolas Grisouard ◽  
Kurt Polzin
Keyword(s):  
Southern Ocean ◽  
Internal Wave ◽  
Linear Theory ◽  
Bottom Topography ◽  
Three Dimensional ◽  
Finite Amplitude ◽  
Wave Generation ◽  
Two Dimensional ◽  
Lee Wave ◽  
Internal Wave Generation

Abstract Direct observations in the Southern Ocean report enhanced internal wave activity and turbulence in a kilometer-thick layer above rough bottom topography collocated with the deep-reaching fronts of the Antarctic Circumpolar Current. Linear theory, corrected for finite-amplitude topography based on idealized, two-dimensional numerical simulations, has been recently used to estimate the global distribution of internal wave generation by oceanic currents and eddies. The global estimate shows that the topographic wave generation is a significant sink of energy for geostrophic flows and a source of energy for turbulent mixing in the deep ocean. However, comparison with recent observations from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean shows that the linear theory predictions and idealized two-dimensional simulations grossly overestimate the observed levels of turbulent energy dissipation. This study presents two- and three-dimensional, realistic topography simulations of internal lee-wave generation from a steady flow interacting with topography with parameters typical of Drake Passage. The results demonstrate that internal wave generation at three-dimensional, finite bottom topography is reduced compared to the two-dimensional case. The reduction is primarily associated with finite-amplitude bottom topography effects that suppress vertical motions and thus reduce the amplitude of the internal waves radiated from topography. The implication of these results for the global lee-wave generation is discussed.

scholarly journals Internal Waves and Turbulence in the Antarctic Circumpolar Current

2013 ◽  
Vol 43 (2) ◽  
pp. 259-282 ◽  
Author(s):  
Stephanie Waterman ◽  
Alberto C. Naveira Garabato ◽  
Kurt L. Polzin
Keyword(s):  
Internal Wave ◽  
Wave Field ◽  
Dissipation Rate ◽  
Antarctic Circumpolar Current ◽  
Wave Radiation ◽  
Turbulent Dissipation ◽  
Internal Wave Field ◽  
Diapycnal Diffusivity ◽  
Circumpolar Current ◽  
The Antarctic

Abstract This study reports on observations of turbulent dissipation and internal wave-scale flow properties in a standing meander of the Antarctic Circumpolar Current (ACC) north of the Kerguelen Plateau. The authors characterize the intensity and spatial distribution of the observed turbulent dissipation and the derived turbulent mixing, and consider underpinning mechanisms in the context of the internal wave field and the processes governing the waves’ generation and evolution. The turbulent dissipation rate and the derived diapycnal diffusivity are highly variable with systematic depth dependence. The dissipation rate is generally enhanced in the upper 1000–1500 m of the water column, and both the dissipation rate and diapycnal diffusivity are enhanced in some places near the seafloor, commonly in regions of rough topography and in the vicinity of strong bottom flows associated with the ACC jets. Turbulent dissipation is high in regions where internal wave energy is high, consistent with the idea that interior dissipation is related to a breaking internal wave field. Elevated turbulence occurs in association with downward-propagating near-inertial waves within 1–2 km of the surface, as well as with upward-propagating, relatively high-frequency waves within 1–2 km of the seafloor. While an interpretation of these near-bottom waves as lee waves generated by ACC jets flowing over small-scale topographic roughness is supported by the qualitative match between the spatial patterns in predicted lee wave radiation and observed near-bottom dissipation, the observed dissipation is found to be only a small percentage of the energy flux predicted by theory. The mismatch suggests an alternative fate to local dissipation for a significant fraction of the radiated energy.

scholarly journals The Low-Frequency Variability of the Southern Ocean Circulation

2013 ◽  
Vol 26 (16) ◽  
pp. 6081-6091 ◽  
Author(s):  
Emanuel Giarolla ◽  
Ricardo P. Matano
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Average Rate ◽  
Bottom Topography ◽  
Low Frequency ◽  
Dominant Mode ◽  
Sea Surface ◽  
Wind Stress Curl ◽  
Low Frequency Variability ◽  
Long Time

Abstract Long time series of sea surface height (SSH), sea surface temperature, and wind stress curl are used to determine the main modes of low-frequency variability of the Southern Ocean (SO) circulation. The dominant mode is a trend of increasing SSH at an average rate of 3.3 mm yr−1. Similar trends have been reported in previous studies and the analysis indicates that the tendency of sea level increase over the SO has become more spatially homogeneous during the last decade, with changes in the increasing rate in 2000 and 2006. The other modes consist of stationary, basin-type modes, and an eastward-propagating wave. The stationary modes are particularly dominant in the Indian and Atlantic Ocean basins, where their spatial structure appears to be shaped by the basin geometry and the bottom topography. The wavelike patterns travel eastward with a period of approximately 10 years. Two waves were identified in the analysis: a complete cycle between 1997 and 2007 and a second cycle starting in 2000. Such waves have rarely been mentioned or identified in studies using recent satellite-derived SSH products.

scholarly journals Downstream Propagation and Remote Dissipation of Internal Waves in the Southern Ocean

2019 ◽  
Vol 49 (7) ◽  
pp. 1873-1887 ◽  
Author(s):  
Kaiwen Zheng ◽  
Maxim Nikurashin
Keyword(s):  
Energy Dissipation ◽  
Southern Ocean ◽  
Internal Wave ◽  
Linear Theory ◽  
Internal Waves ◽  
Turbulent Energy ◽  
Wave Radiation ◽  
Small Scale ◽  
Length Scale ◽  
Geostrophic Flows

AbstractRecent microstructure observations in the Southern Ocean report enhanced internal gravity waves and turbulence in the frontal regions of the Antarctic Circumpolar Current extending a kilometer above rough bottom topography. Idealized numerical simulations and linear theory show that geostrophic flows impinging on rough small-scale topography are very effective generators of internal waves and estimate vigorous wave radiation, breaking, and turbulence within a kilometer above bottom. However, both idealized simulations and linear theory assume periodic and spatially uniform topography and tend to overestimate the observed levels of turbulent energy dissipation locally at the generation sites. In this study, we explore the downstream evolution and remote dissipation of internal waves generated by geostrophic flows using a series of numerical, realistic topography simulations and parameters typical of Drake Passage. The results show that significant levels of internal wave kinetic energy and energy dissipation are present downstream of the rough topography, internal wave generation site. About 30%–40% of the energy dissipation occurs locally over the rough topography region, where internal waves are generated. The rest of the energy dissipation takes place remotely and decays downstream of the generation site with an e-folding length scale of up to 20–30 km. The model we use is two-dimensional with enhanced viscosity coefficients, and hence it can result in the underestimation of the remote wave dissipation and its decay length scale. The implications of our results for turbulent energy dissipation observations and mixing parameterizations are discussed.

scholarly journals Jet–Topography Interactions Affect Energy Pathways to the Deep Southern Ocean

2017 ◽  
Vol 47 (7) ◽  
pp. 1799-1816 ◽  
Author(s):  
Alice Barthel ◽  
Andrew McC. Hogg ◽  
Stephanie Waterman ◽  
Shane Keating
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Baroclinic Instability ◽  
Deep Ocean ◽  
Ocean Model ◽  
Shear Instability ◽  
Horizontal Shear ◽  
Mean Flow ◽  
Flat Bottom ◽  
Turbulent Dissipation

AbstractIn the Southern Ocean, strong eastward ocean jets interact with large topographic features, generating eddies that feed back onto the mean flow. Deep-reaching eddies interact with topography, where turbulent dissipation and generation of internal lee waves play an important role in the ocean’s energy budget. However, eddy effects in the deep ocean are difficult to observe and poorly characterized. This study investigates the energy contained in eddies at depth, when an ocean jet encounters topography. This study uses a two-layer ocean model in which an imposed unstable jet encounters a topographic obstacle (either a seamount or a meridional ridge) in a configuration relevant to an Antarctic Circumpolar Current frontal jet. The authors find that the presence of topography increases the eddy kinetic energy (EKE) at depth but that the dominant processes generating this deep EKE depend on the shape and height of the obstacle as well as on the baroclinicity of the jet before it encounters topography. In cases with high topography, horizontal shear instability is the dominant source of deep EKE, while a flat bottom or a strongly sheared inflow leads to deep EKE being generated primarily through baroclinic instability. These results suggest that the deep EKE is set by an interplay between the inflowing jet properties and topography and imply that the response of deep EKE to changes in the Southern Ocean circulation is likely to vary across locations depending on the topography characteristics.

scholarly journals Energy Sinks for Lee Waves in Shear Flow

2019 ◽  
Vol 49 (11) ◽  
pp. 2851-2865 ◽  
Author(s):  
Eric Kunze ◽  
Ren-Chieh Lien
Keyword(s):  
Wave Generation ◽  
Flow Speed ◽  
Wave Action ◽  
Wave Radiation ◽  
Bottom Current ◽  
Mean Flow ◽  
Turbulent Dissipation ◽  
Lee Waves ◽  
Lee Wave ◽  
The Mean

AbstractMicrostructure measurements in Drake Passage and on the flanks of Kerguelen Plateau find turbulent dissipation rates ε on average factors of 2–3 smaller than linear lee-wave generation predictions, as well as a factor of 3 smaller than the predictions of a well-established parameterization based on finescale shear and strain. Here, the possibility that these discrepancies are a result of conservation of wave action E/ωL = E/|kU| is explored. Conservation of wave action will transfer a fraction of the lee-wave radiation back to the mean flow if the waves encounter weakening currents U, where the intrinsic or Lagrangian frequency ωL = |kU| ↓ |f| and k the along-stream horizontal wavenumber, where kU ≡ k ⋅ V. The dissipative fraction of power that is lost to turbulence depends on the Doppler shift of the intrinsic frequency between generation and breaking, hence on the topographic height spectrum and bandwidth N/f. The partition between dissipation and loss to the mean flow is quantified for typical topographic height spectral shapes and N/f ratios found in the abyssal ocean under the assumption that blocking is local in wavenumber. Although some fraction of lee-wave generation is always dissipated in a rotating fluid, lee waves are not as large a sink for balanced energy or as large a source for turbulence as previously suggested. The dissipative fraction is 0.44–0.56 for topographic spectral slopes and buoyancy frequencies typical of the deep Southern Ocean, insensitive to flow speed U and topographic splitting. Lee waves are also an important mechanism for redistributing balanced energy within their generating bottom current.

scholarly journals Radiation and Dissipation of Internal Waves Generated by Geostrophic Motions Impinging on Small-Scale Topography: Theory

2010 ◽  
Vol 40 (5) ◽  
pp. 1055-1074 ◽  
Author(s):  
Maxim Nikurashin ◽  
Raffaele Ferrari
Keyword(s):  
Internal Waves ◽  
Bottom Topography ◽  
Drake Passage ◽  
Wave Radiation ◽  
Small Scale ◽  
Ocean Bottom ◽  
Inertial Oscillations ◽  
Weakly Nonlinear ◽  
Weakly Nonlinear Theory ◽  
Internal Wave Generation

Abstract Observations and inverse models suggest that small-scale turbulent mixing is enhanced in the Southern Ocean in regions above rough topography. The enhancement extends O(1) km above the topography, suggesting that mixing is supported by the breaking of gravity waves radiated from the ocean bottom. In this study, it is shown that the observed mixing rates can be sustained by internal waves generated by geostrophic motions flowing over bottom topography. Weakly nonlinear theory is used to describe the internal wave generation and the feedback of the waves on the zonally averaged flow. Vigorous inertial oscillations are driven at the ocean bottom by waves generated at steep topography. The wave radiation and dissipation at equilibrium is therefore the result of both geostrophic flow and inertial oscillations differing substantially from the classical lee-wave problem. The theoretical predictions are tested versus two-dimensional high-resolution numerical simulations with parameters representative of Drake Passage. This work suggests that mixing in Drake Passage can be supported by geostrophic motions impinging on rough topography rather than by barotropic tidal motions, as is commonly assumed.

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