Impacts of Mesoscale Currents on the Diurnal Critical Latitude Dependence of Internal Tides: A Numerical Experiment Based on Barcoo Seamount

The disintegration of the equatorward-propagating K1 internal tide in the South China Sea (SCS) by parametric subharmonic instability (PSI) at its critical latitude of 14.52&#186;N is investigated numerically. The multiple-source generation and long-range propagation of K1 internal tides are successfully reproduced. Using equilibrium analysis, the internal wave field near the critical latitude is found to experience two quasi-steady states, between which the subharmonic waves develop constantly. The simulated subharmonic waves agree well with classic PSI theoretical prediction. The PSI-induced near-inertial waves are of half the K1 frequency and dominantly high modes, the vertical scales ranging from 50 to 180 m in the upper ocean. From an energy perspective, PSI mainly occurs in the critical latitudinal zone from 13&#8211;15&#186;N. In this zone, the incident internal tide loses ~14% energy in the mature state of PSI. PSI triggers a mixing elevation of O(10-5&#8211;10-4 m2/s) in the upper ocean at the critical latitude, which is several times larger than the background value. The contribution of PSI to the internal tide energy loss and associated enhanced mixing may differ regionally and is closely dependent on the intensity and duration of background internal tide. The results elucidate the far-field dissipation mechanism by PSI in connecting interior mixing with remotely generated K1 internal tides in the Luzon Strait.

Download Full-text

Latitudinal Variation and Nonlinear Behavior of Internal Tides in the East China Sea

10.5194/egusphere-egu2020-18704 ◽

2020 ◽

Author(s):

Weidong Wang ◽

Robin Robertson

Keyword(s):

East China Sea ◽

Wave Spectrum ◽

Nonlinear Behavior ◽

Tidal Energy ◽

Internal Tides ◽

East China ◽

The East China Sea ◽

Spectra Analysis ◽

China Sea ◽

Critical Latitude

We present four sets of concurrent ADCP data obtained from the East China Sea shelf, and it suggests that near-inertial waves induced by parametric subharmonic instability (PSI) associated with harmonic transfer beyond diurnal critical latitude (O1:27.6&#176;, K1:30&#176;). Two type different nonlinear behavior (harmonic transfer and subharmonic transfer) occur varying with the latitude on different location. The velocity data indicated a transfer of diurnal internal tidal energy poleward of the diurnal critical latitude. Kinetic energy and shear spectra analysis at these moorings reveals that the prominent peaks enhance and appear at not only at the even order of diurnal tide such as semidiurnal band, 4cpd, 6cpd and even 8cpd, but also some unfamiliar odd harmonics 3cpd and 5cpd. Furthermore, additional energy is converted to higher mode locally through continuum internal wave spectrum. Besides the harmonic transfer, on the critical latitude for D2/2 wave(28.9&#176;), D1 wave is extracted from a D2 tidal driven model output current. PSI conversion of semidiurnal internal tidal energy was confirmed by spectra analysis and bi-spectra, because of the distinguish of M2/2 separated from the diurnal tidal (O1, K1).

Download Full-text

Global Patterns of Low-Mode Internal-Wave Propagation. Part I: Energy and Energy Flux

Journal of Physical Oceanography ◽

10.1175/jpo3085.1 ◽

2007 ◽

Vol 37 (7) ◽

pp. 1829-1848 ◽

Cited By ~ 66

Author(s):

Matthew H. Alford ◽

Zhongxiang Zhao

Keyword(s):

Internal Wave ◽

Time Scales ◽

Energy Flux ◽

Internal Tide ◽

Companion Paper ◽

Internal Tides ◽

Frequency Bands ◽

Order Of Magnitude ◽

Hawaiian Ridge ◽

Mesoscale Currents

Abstract Extending an earlier attempt to understand long-range propagation of the global internal-wave field, the energy E and horizontal energy flux F are computed for the two gravest baroclinic modes at 80 historical moorings around the globe. With bandpass filtering, the calculation is performed for the semidiurnal band (emphasizing M2 internal tides, generated by flow over sloping topography) and for the near-inertial band (emphasizing wind-generated waves near the Coriolis frequency). The time dependence of semidiurnal E and F is first examined at six locations north of the Hawaiian Ridge; E and F typically rise and fall together and can vary by over an order of magnitude at each site. This variability typically has a strong spring–neap component, in addition to longer time scales. The observed spring tides at sites northwest of the Hawaiian Ridge are coherent with barotropic forcing at the ridge, but lagged by times consistent with travel at the theoretical mode-1 group speed from the ridge. Phase computed from 14-day windows varies by approximately ±45° on monthly time scales, implying refraction by mesoscale currents and stratification. This refraction also causes the bulk of internal-tide energy flux to be undetectable by altimetry and other long-term harmonic-analysis techniques. As found previously, the mean flux in both frequency bands is O(1 kW m−1), sufficient to radiate a substantial fraction of energy far from each source. Tidal flux is generally away from regions of strong topography. Near-inertial flux is overwhelmingly equatorward, as required for waves generated at the inertial frequency on a β plane, and is winter-enhanced, consistent with storm generation. In a companion paper, the group velocity, ĉg ≡ FE−1, is examined for both frequency bands.

Download Full-text

The Surface Expression of Semidiurnal Internal Tides near a Strong Source at Hawaii. Part II: Interactions with Mesoscale Currents*

Journal of Physical Oceanography ◽

10.1175/2010jpo4223.1 ◽

2010 ◽

Vol 40 (6) ◽

pp. 1180-1200 ◽

Cited By ~ 37

Author(s):

C. Chavanne ◽

P. Flament ◽

D. Luther ◽

K-W. Gurgel

Keyword(s):

Internal Wave ◽

Ray Tracing ◽

Internal Tide ◽

Wave Dispersion ◽

Internal Tides ◽

Vertical Shear ◽

Buoyancy Frequency ◽

Phase Lag ◽

Vertical Wavelength ◽

Mesoscale Currents

Abstract Observations of semidiurnal surface currents in the Kauai Channel, Hawaii, are interpreted in the light of the interaction of internal tides with energetic surface-intensified mesoscale currents. The impacts on internal tide propagation of a cyclone of 55-km diameter and ∼100-m vertical decay scale, as well as of vorticity waves of ∼100-km wavelength and 100–200-m vertical decay scales, are investigated using 3D ray tracing. The Doppler-shifted intrinsic frequency is assumed to satisfy the classic hydrostatic internal wave dispersion relation, using the local buoyancy frequency associated with the background currents through thermal-wind or gradient-wind balance. The M2 internal tide rays with initial horizontal wavelength of 50 km and vertical wavelength of O(1000 m) are propagated from possible generation locations at critical topographic slopes through idealized mesoscale currents approximating the observed currents. Despite the lack of scale separation between the internal waves and background state, which is required by the ray-tracing approximation, the results are qualitatively consistent with observations: the cyclone causes the energy of internal tide rays propagating through its core to increase near the surface (up to a factor of 15), with surfacing time delayed by up to 5 h (∼150° phase lag), and the vorticity waves enhance or reduce the energy near the surface, depending on their phase. These examples illustrate the fact that, even close to their generation location, semidiurnal internal tides can become incoherent with astronomical forcing because of the presence of mesoscale variability. Internal tide energy is mainly affected by refraction through the inhomogeneous buoyancy frequency field, with Doppler shifting playing a secondary but not negligible role, inducing energy transfers between the internal tides and background currents. Furthermore, the vertical wavelength can be reduced by a factor of 6 near the surface in the presence of the cyclone, which, combined with the energy amplification, leads to increased vertical shear within the internal tide rays, with implications for internal wave-induced mixing in the ocean.

Download Full-text

Impact of a Mean Current on the Internal Tide Energy Dissipation at the Critical Latitude

Journal of Physical Oceanography ◽

10.1175/jpo-d-16-0197.1 ◽

2017 ◽

Vol 47 (6) ◽

pp. 1457-1472 ◽

Cited By ~ 13

Author(s):

O. Richet ◽

C. Muller ◽

J.-M. Chomaz

Keyword(s):

Large Scale ◽

Internal Tide ◽

Internal Tides ◽

Small Scale ◽

Latitudinal Dependence ◽

Propagating Waves ◽

Parametric Subharmonic Instability ◽

The Impact ◽

Mean Current ◽

Critical Latitude

AbstractPrevious numerical studies of the dissipation of internal tides in idealized settings suggest the existence of a critical latitude (~29°) where dissipation is enhanced. But observations only indicate a modest enhancement at this latitude. To resolve this difference between observational and numerical results, the authors study the latitudinal dependence of internal tides’ dissipation in more realistic conditions. In particular, the ocean is not a quiescent medium; the presence of large-scale currents or mesoscale eddies can impact the propagation and dissipation of internal tides. This paper investigates the impact of a weak background mean current in numerical simulations. The authors focus on the local dissipation of high spatial mode internal waves near their generation site. The vertical profile of dissipation and its variation with latitude without the mean current are consistent with earlier studies. But adding a weak mean current has a major impact on the latitudinal distribution of dissipation. The peak at the critical latitude disappears, and the dissipation is closer to a constant, albeit with two weak peaks at ~25° and ~35° latitude. This disappearance results from the Doppler shift of the internal tides’ frequency, which hinders the nonlinear transfer of energy to small-scale secondary waves via the parametric subharmonic instability (PSI). The new two weak peaks correspond to the Doppler-shifted critical latitudes of the left- and right-propagating waves. The results are confirmed in simulations with simple sinusoidal topography. Thus, although nonlinear transfers via PSI are efficient at dissipating internal tides, the exact location of the dissipation is sensitive to large-scale oceanic conditions.

Download Full-text

Internal tides over abrupt topography in the Southern California Bight: Observations of diurnal waves poleward of the critical latitude

Journal of Geophysical Research Atmospheres ◽

10.1029/2006jc003905 ◽

2008 ◽

Vol 113 (C2) ◽

Cited By ~ 11

Author(s):

Edwin Beckenbach ◽

Eric Terrill

Keyword(s):

Southern California ◽

Southern California Bight ◽

Internal Tides ◽

Critical Latitude

Download Full-text

Tidal Conversion and Dissipation at Steep Topography in a Channel Poleward of the Critical Latitude

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0132.1 ◽

2019 ◽

Vol 49 (5) ◽

pp. 1269-1291 ◽

Cited By ~ 3

Author(s):

Kenneth G. Hughes ◽

Jody M. Klymak

Keyword(s):

Three Dimensional ◽

Tidal Energy ◽

Kelvin Waves ◽

Internal Tides ◽

The Arctic ◽

Energy Budgets ◽

Dimensional System ◽

Deep Layers ◽

The Stability ◽

Critical Latitude

Abstract In high-latitude fjords and channels in the Canadian Arctic Archipelago, walls support radiating internal tides as Kelvin waves. Such waves allow for significant barotropic to baroclinic tidal energy conversion, which is otherwise small or negligible when poleward of the critical latitude. This fundamentally three-dimensional system of a subinertial channel is investigated with a suite of numerical simulations in rectangular channels of varying width featuring idealized, isolated ridges. Even in channels as wide as 5 times the internal Rossby radius, tidal conversion can remain as high as predicted by an equivalent two-dimensional, nonrotating system. Curves of tidal conversion as a function of channel width, however, do not vary monotonically. Instead, they display peaks and nulls owing to interference between the Kelvin waves along the wall and similar waves that propagate along the ridge flanks, the wavelengths of which can be estimated from linear theory to guide prediction. Because the wavelengths are comparable to width scales of Arctic channels and fjords, the interference will play a first-order role in tidal energy budgets and may consequently influence the stability of glaciers, the ventilation of deep layers, the locations of sediment deposition, and the fate of freshwater exiting the Arctic Ocean.

Download Full-text

Disintegration of the K1 Internal Tide in the South China Sea due to Parametric Subharmonic Instability

Journal of Physical Oceanography ◽

10.1175/jpo-d-19-0320.1 ◽

2020 ◽

Vol 50 (12) ◽

pp. 3605-3622

Author(s):

Kun Liu ◽

Zhongxiang Zhao

Keyword(s):

South China Sea ◽

South China ◽

Internal Tide ◽

The South China Sea ◽

Internal Tides ◽

Upper Ocean ◽

The South ◽

China Sea ◽

Parametric Subharmonic Instability ◽

Critical Latitude

AbstractThe disintegration of the equatorward-propagating K1 internal tide in the South China Sea (SCS) by parametric subharmonic instability (PSI) at its critical latitude of 14.52°N is investigated numerically. The multiple-source generation and long-range propagation of K1 internal tides are successfully reproduced. Using equilibrium analysis, the internal wave field near the critical latitude is found to experience two quasi-steady states, between which the subharmonic waves develop constantly. The simulated subharmonic waves agree well with classic PSI theoretical prediction. The PSI-induced near-inertial waves are of half the K1 frequency and dominantly high modes, the vertical scales ranging from 50 to 180 m in the upper ocean. From an energy perspective, PSI mainly occurs in the critical latitudinal zone from 13° to 15°N. In this zone, the incident internal tide loses ~14% energy in the mature state of PSI. PSI triggers a mixing elevation of O(10−5–10−4) m2 s−1 in the upper ocean at the critical latitude, which is several times larger than the background value. The contribution of PSI to the internal tide energy loss and associated enhanced mixing may differ regionally and is closely dependent on the intensity and duration of background internal tide. The results elucidate the far-field dissipation mechanism by PSI in connecting interior mixing with remotely generated K1 internal tides in the Luzon Strait.

Download Full-text

Internal tides off the Amazon shelf during two contrasted seasons: Interactions with background circulation and SSH imprints

10.5194/os-2021-114 ◽

2022 ◽

Author(s):

Michel Tchilibou ◽

Ariane Koch-Larrouy ◽

Simon Barbot ◽

Florent Lyard ◽

Yves Morel ◽

...

Keyword(s):

Regional Model ◽

Horizontal Resolution ◽

Shelf Break ◽

Internal Tides ◽

Sea Surface ◽

Baroclinic Tide ◽

Amazon Shelf ◽

Ocean Topography ◽

Mesoscale Currents ◽

Propagation Properties

Abstract. The Amazon shelf break is a key region for internal tides (IT) generation. The region also shows a large seasonal variation of circulation and associated stratification. The objective of this study is to document how these variations will impact IT generation and propagation properties. A high-resolution regional model (1/36° horizontal resolution), explicitly resolving IT is analyzed to investigate their interactions with the background circulation and stratification, over two seasons: first MAMJJ (March to July), with weaker mesoscale currents, shallower and stronger pycnocline, and second ASOND (August to December) with stronger mesoscale currents, deeper and weaker pycnocline. IT are generated on the shelf break between the 100 and 1800 m isobaths, with a maximum on average at about 10 km offshore. South of 2° N, the conversion from barotropic to baroclinic tide is more efficient in MAMJJ than in ASOND. At the eight main IT generations sites, the local dissipation is higher in MAMJJ (30 %) than in ASOND (22 %). The remaining fraction propagates away from the generation sites and mainly dissipates locally every 90–120 km. The remote dissipation increases slightly during ASOND and the coherent M2 fluxes seem blocked between 4°–6° N west of 47° W. Further analysis of 25 hours mean snapshots of the baroclinic flux shows deviation and branching of the IT when interacting with strong mesoscale and stratification. We evaluated sea surface height (SSH) frequency and wavenumber spectra for subtidal (f < 1/28h−1), tidal (1/28h−1 < f < 1/11h−1) and super tidal (f > 1/11h−1) frequencies. Tidal frequencies explain most of the SSH variability for wavelengths between 300 km and 70 km. Below 70 km, the SSH is mainly incoherent and supertidal. The length scale at which the SSH becomes dominated by unbalanced IT was estimated to be around 250 km. Our results highlight the complexity of correctly predicting IT SSH in order to better observe mesoscale and submesoscale from existing and upcoming altmetrics missions, notably the Surface Water Ocean Topography (SWOT) mission.

Download Full-text