Effects of an along-shelf current on the generation of internal tides near the critical latitude

Internal Wave Generation ◽

Spatially Varying ◽

Beam Patterns ◽

The effects of along-shelf barotropic geostrophic currents on internal wave generation by the $K_1$ tide interacting with a shelf at near-critical latitudes are investigated. The horizontal shear of the background current results in a spatially varying effective Coriolis frequency which modifies the slope criticality and potentially creates blocking regions where freely propagating internal tides cannot exist. This paper is focused on the barotropic to baroclinic energy conversion rate, which is affected by a combination of three factors: slope criticality, size and location of the blocking region where the conversion rate is extremely small and the internal tide (IT) beam patterns. All of these are sensitive to the current parameters. In our parameter space, the current can increase the conversion rate up to 10 times.

Disintegration of the K1 internal tide in the South China Sea due to parametric subharmonic instability

10.5194/egusphere-egu21-2695 ◽

2021 ◽

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 ◽

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.

Investigation of the Internal Tides in the Northwest Pacific Ocean Considering the Background Circulation and Stratification

10.1175/jpo-d-19-0177.1 ◽

2020 ◽

Vol 50 (11) ◽

pp. 3165-3188

Author(s):

Pengyang Song ◽

Xueen Chen

Keyword(s):

Pacific Ocean ◽

Ocean Circulation ◽

Conversion Rate ◽

Internal Tide ◽

Internal Tides ◽

Global Ocean ◽

Northwest Pacific ◽

Northwest Pacific Ocean ◽

Baroclinic Tide ◽

The Northwest Pacific Ocean

AbstractA global ocean circulation and tide model with nonuniform resolution is used in this work to resolve the ocean circulation globally as well as mesoscale eddies and internal tides regionally. Focusing on the northwest Pacific Ocean (NWP, 0°–35°N, 105°–150°E), a realistic experiment is conducted to simulate internal tides considering the background circulation and stratification. To investigate the influence of a background field on the generation and propagation of internal tides, idealized cases with horizontally homogeneous stratification and zero surface fluxes are also implemented for comparison. By comparing the realistic cases with idealized ones, the astronomical tidal forcing is found to be the dominant factor influencing the internal tide conversion rate magnitude, whereas the stratification acts as a secondary factor. However, stratification deviations in different areas can lead to an error exceeding 30% in the local internal tide energy conversion rate, indicating the necessity of a realistic stratification setting for simulating the entire NWP. The background shear is found to refract propagating diurnal internal tides by changing the effective Coriolis frequencies and phase speeds, while the Doppler-shifting effect is remarkable for introducing biases to semidiurnal results. In addition, nonlinear baroclinic tide energy equations considering the background circulation and stratification are derived and diagnosed in this work. The mean flow–baroclinic tide interaction and nonlinear energy flux are the most significant nonlinear terms in the derived equations, and nonlinearity is estimated to contribute approximately 5% of the total internal tide energy in the greater Luzon Strait area.

Generation of Internal Tides by Tide-Topography Resonance over Weak Topography: Analytical Calculation

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.588-589.1964 ◽

2012 ◽

Vol 588-589 ◽

pp. 1964-1971

Author(s):

Li Dan Wu ◽

Chun Bao Miao

Keyword(s):

Conversion Rate ◽

Mode Conversion ◽

Bottom Topography ◽

Analytical Calculation ◽

Internal Tide ◽

Resonant Mode ◽

Internal Tides ◽

Resonant Modes ◽

Minimum Amplitude ◽

Baroclinic Modes

Internal tides generated by the interaction of the barotropic tide with bottom topography are studied by using an analytical solution. Tide-topography resonance takes place when the wavenumber of the truncated sinusoidal topography is equal to that of one baroclinic mode. The minimum amplitude of the resonant mode increases from the center of the domain to both sides of the topography; while the maximum keeps the same. Amplitudes of the internal tides and mode conversion rate are analyzed as a function of the length and wavenumber of the topography. For non-resonant modes, the amplitudes are weak and vary periodically with the extending of the topography, and are exactly zero when the length of topography is integral times of the mode-1 wavelength. For resonant modes, the amplitudes increase with the length of the topography. For each internal tide mode, there is a response zone, where the amplitude for one mode is obviously larger than other baroclinic modes. The response zones for high modes are wider than those for low modes. Mode conversion rate is obviously high when the wavenumber of the topography is equal to that of the baroclinic modes; otherwise it is almost zero. Furthermore, mode conversion rate for small topography wavenumber is more than that for large topography wavenumber with the same number of the sinusoidal topography, and is less with the same topography length.

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

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 ◽

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.

Internal Tides in Monterey Submarine Canyon

10.1175/2010jpo4471.1 ◽

2011 ◽

Vol 41 (1) ◽

pp. 186-204 ◽

Cited By ~ 43

Author(s):

Rob A. Hall ◽

Glenn S. Carter

Keyword(s):

Energy Conversion ◽

Energy Flux ◽

Internal Tide ◽

Submarine Canyon ◽

Ocean Model ◽

Internal Tides ◽

Remote Areas ◽

Order Of Magnitude ◽

Baroclinic Energy Conversion

Abstract The M2 internal tide in Monterey Submarine Canyon is simulated using a modified version of the Princeton Ocean Model. Most of the internal tide energy entering the canyon is generated to the south, on Sur Slope and at the head of Carmel Canyon. The internal tide is topographically steered around the large canyon meanders. Depth-integrated baroclinic energy fluxes are up canyon and largest near the canyon axis, up to 1.5 kW m−1 at the mouth of the upper canyon and increasing to over 4 kW m−1 around Monterey and San Gregorio Meanders. The up-canyon energy flux is bottom intensified, suggesting that topographic focusing occurs. Net along-canyon energy flux decreases almost monotonically from 9 MW at the canyon mouth to 1 MW at Gooseneck Meander, implying that high levels of internal tide dissipation occur. The depth-integrated energy flux across the 200-m isobath is order 10 W m−1 along the majority of the canyon rim but increases by over an order of magnitude near the canyon head, where internal tide energy escapes onto the shelf. Reducing the size of the model domain to exclude remote areas of high barotropic-to-baroclinic energy conversion decreases the depth-integrated energy flux in the upper canyon by 20%. However, quantifying the role of remote internal tide generation sites is complicated by a pressure perturbation feedback between baroclinic energy flux and barotropic-to-baroclinic energy conversion.

Low-mode internal tide generation by topography: an experimental and numerical investigation

Journal of Fluid Mechanics ◽

10.1017/s0022112009007654 ◽

2009 ◽

Vol 636 ◽

pp. 91-108 ◽

Cited By ~ 41

Author(s):

PAULA ECHEVERRI ◽

M. R. FLYNN ◽

KRAIG B. WINTERS ◽

THOMAS PEACOCK

Keyword(s):

Large Scale ◽

Internal Tide ◽

Finite Depth ◽

Internal Tides ◽

Nonlinear Phenomena ◽

Nonlinear Behaviour ◽

Mode Structure ◽

Channel Depth ◽

Ocean Ridges ◽

Baroclinic Energy Conversion

We analyse the low-mode structure of internal tides generated in laboratory experiments and numerical simulations by a two-dimensional ridge in a channel of finite depth. The height of the ridge is approximately half of the channel depth and the regimes considered span sub- to supercritical topography. For small tidal excursions, of the order of 1% of the topographic width, our results agree well with linear theory. For larger tidal excursions, up to 15% of the topographic width, we find that the scaled mode 1 conversion rate decreases by less than 15%, in spite of nonlinear phenomena that break down the familiar wave-beam structure and generate harmonics and inter-harmonics. Modes two and three, however, are more strongly affected. For this topographic configuration, most of the linear baroclinic energy flux is associated with the mode 1 tide, so our experiments reveal that nonlinear behaviour does not significantly affect the barotropic to baroclinic energy conversion in this regime, which is relevant to large-scale ocean ridges. This may not be the case, however, for smaller scale ridges that generate a response dominated by higher modes.

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

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 ◽

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.

The Mechanism of Internal Tide Generation over Weak Topography

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.588-589.1972 ◽

2012 ◽

Vol 588-589 ◽

pp. 1972-1978

Author(s):

Li Dan Wu ◽

Chun Bao Miao

Keyword(s):

Energy Conversion ◽

Conversion Rate ◽

Internal Tide ◽

Baroclinic Mode ◽

Resonance Case ◽

Numerical Techniques ◽

Barotropic Tide ◽

Baroclinic Modes ◽

Perturbation Pressure

Here the process of internal tide generation over idealized (sinusoidal) topography was investigated using numerical techniques, in which the barotropic-to-baroclinic energy conversion was discussed. The result shows that when the wavenumber of the sinusoidal topography, ktopo, is equal to the horizontal wavenumber of the m-th baroclinic mode km, the conversion from the barotropic tide to the m-th baroclinic mode is enhanced with the increase of topography length; When the wavenumber of the sinusoidal topography is not equal to horizontal wavenumbers of any baroclinic modes, ktopo≠km(m=1,2,...), conversion from the barotropic energy to baroclinic modes is decreased with the increase of topography length. Furthermore, it shows that in resonance case, the phase of the perturbation pressure gradually agrees with the phase of the truncated sinusoidal topography, and the conversion rate is always positive over the topography, thus the baroclinic mode which matches with the wavenumber of the sinusoidal topography persists in absorbing energy from the barotropic tide, and the conversion rate is increased.

The influence of a geostrophic current on the internal tide generation

10.5194/egusphere-egu21-1002 ◽

2021 ◽

Author(s):

Yangxin He ◽

Kevin Lamb

Keyword(s):

Conversion Rate ◽

Gaussian Function ◽

Internal Tide ◽

Maximum Velocity ◽

Propagation Path ◽

Geostrophic Current ◽

Total Conversion ◽

Shelf Slope ◽

Linear Slope

We investigate the influence of a barotropic geostrophic current on internal tide (IT) generation over a shelf slope. The current $V_g(x)$ is modeled as an idealized Gaussian function centered at $x_0$ with width $x_r$ and maximum velocity $V_{max}$. The bathymetry is modelled as a linear slope with smoothed corners. We calculate the total barotropic-to-baroclinic energy conversion $C = \int \overbar{C} \,dx = \int \int \rho' g W \,dx\, dz$.&#160; $\overbar{C}(x,t)$ can be either positive or negative. Positive (negative) conversion means energy is converted from barotropic to baroclinic (baroclinic to barotropic) waves.&#160; The main conclusions are: 1) $V_g(x)$ changes the effective frequency $f_{eff}$. This has a direct impact on the slope of the IT characteristics and the slope criticality, which affects the total conversion rate; 2) Since $(V_g)_x$ is not a constant value, $f_{eff}$ varies along the slope. This has a significant effect on the IT beam generation location and its propagation path. If the current is strong enough so that $f_{eff}$ is greater than the barotropic tidal frequency $\sigma_T$, a blocking region is formed where the conversion vanishes and IT propagation is blocked; 3) Changes of sign in $\bar{C}(x,t)$ correspond to the locations where IT beams reflect from the boundaries. As a result, the total conversion rate $C$ is also strongly affected by the IT beam pattern. In conclusion, the total conversion rate $C$ is affected by a combination of three factors: slope criticality, size and location of the blocking region and the IT beam patterm, all of which can be varied by changing the strength, width and location of the geostrophic current $V_g(x)$.

Modeling Semidiurnal Internal Tide Variability in the Southern California Bight

10.1175/2011jpo4597.1 ◽

2012 ◽

Vol 42 (1) ◽

pp. 62-77 ◽

Cited By ~ 40

Author(s):

M. C. Buijsman ◽

Y. Uchiyama ◽

J. C. McWilliams ◽

C. R. Hill-Lindsay

Keyword(s):

High Resolution ◽

Internal Wave ◽

Internal Waves ◽

Southern California ◽

Internal Tide ◽

Southern California Bight ◽

Complex Topography ◽

Santa Cruz ◽

Internal Wave Generation

Abstract The Regional Oceanic Modeling System (ROMS) is applied in a nested configuration with realistic forcing to the Southern California Bight (SCB) to analyze the variability in semidiurnal internal wave generation and propagation. The SCB has a complex topography with supercritical slopes that generate linear internal waves at the forcing frequency. The model predicts the observed barotropic and baroclinic tides reasonably well, although the observed baroclinic tides feature slightly larger amplitudes. The strongest semidiurnal barotropic to baroclinic energy conversion occurs on a steep sill slope of the 1900-m-deep Santa Cruz Basin. This causes a forced, near-resonant, semidiurnal Poincaré wave that rotates clockwise in the basin and is of the first mode along the radial, azimuthal, and vertical directions. The associated tidal-mean, depth-integrated energy fluxes and isotherm oscillation amplitudes in the basin reach maximum values of about 5 kW m−1 and 100 m and are strongly modulated by the spring–neap cycle. Most energy is locally dissipated, and only 10% escapes the basin. The baroclinic energy in the remaining basins is orders of magnitudes smaller. High-resolution coastal models are important in locating overlooked mixing hotspots such as the Santa Cruz Basin. These mixing hotspots may be important for ocean mixing and the overturning circulation.