NP7.2

Extreme Internal Wave Events: Generation, Transformation, Breaking and Interaction with the Bottom Topography

This session welcomes contributions presenting advances in, and approaches to, the modelling, monitoring, and forecasting of internal waves in stratified estuaries, lakes and the coastal oсean.
Internal solitary waves (ISWs) and large-amplitude internal soliton packets are a commonly observed event in oceans and lakes. In the oceans ISWs are mainly generated by the interaction of the barotropic tides with bottom topography. Large amplitude solitary waves are energetic events that generate strong currents. They can also trap fluid with larvae and sediments in the cores of waves and transport it a considerable distance. ISWs can cause hazards to marine engineering and submarine navigation, and significantly impact marine ecosystems and particle transport in the bottom layer of the ocean and stratified lakes. Contributions studying flows due to internal waves, their origin, propagation and influence on the surrounding environment are thus of broad scientific importance.
The scope of the session involves all aspects of ISWs generation, propagation, transformation and the interaction of internal waves with bottom topography and shelf zones, as well as an evaluation of the role of internal waves in sediment resuspension and transport. Breaking of internal-waves also drives turbulent mixing in the ocean interior that is important for climate ocean models. Discussion of parameterizations for internal-wave driven turbulent mixing in global ocean models is also invited.

Co-organized by OS4
Convener: Marek Stastna | Co-conveners: Tatiana Talipova, Kateryna Terletska, Zhenhua Xu
vPICO presentations
| Mon, 26 Apr, 15:30–17:00 (CEST)

vPICO presentations: Mon, 26 Apr

Chairperson: Marek Stastna
15:30–15:32
|
EGU21-228
Marek Stastna and Kevin Lamb

In the theory of internal waves in the coastal ocean, linear stratification plays an exceptional role. This is because the nonlinearity coefficient in KdV theory vanishes, and in the case of large amplitude waves, the DJL theory linearizes and fails to give solitary wave solutions. We consider small, physically consistent perturbations of a linearly stratified fluid that would result from a localized mixing near a particular depth. We demonstrate that the DJL equation does yield exact internal solitary waves in this case. These waves are long due to the weak nonlinearity, and we explore how this weak nonlinearity manifests during shoaling.

How to cite: Stastna, M. and Lamb, K.: Internal wave shoaling in nearly linear stratifications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-228, https://doi.org/10.5194/egusphere-egu21-228, 2020.

15:32–15:34
|
EGU21-408
Kateryna Terletska, Vladіmir Maderich, and Tatiana Talipova

The shoaling mechanisms of internal solitary waves that propagate horizontally are an important source of mixing and transport in the coastal zones. Numerical modelling, llaboratory experiments and observations are needed for understanding wave energetics, especially energy transformation during waves interaction with the slopes. Two shoaling mechanisms are important during interaction with the slope: (i) wave breaking that results in mixing and dissipation, (ii) changing of the polarity of the initial wave of depression on the slope. Classification based on regimes of interaction with the slope was presented in [1]. Four zones were separated in αβγ (γ - is slope angle, α-  is the non-dimensional wave amplitude (wave amplitude normalized on the thermocline thickness) and β – is the blocking parameter that is the ratio of the height of the bottom layer on the shelf to the incident wave amplitude) classification diagram: (I) without changing polarity and wave breaking, (II) changing polarity without breaking; (III) wave breaking without changing polarity; (IV) wave breaking with changing polarity. It was shown that results of field, laboratory and numerical experiments are in good agreement with proposed classification.  In the present study we estimate energy dissipation for all the types of interaction and present the algorithm for building a zone map with a ‘hot spot’ of energy dissipation for real slopes in the ocean.

 

[1] K Terletska, BH Choi, V Maderich, T Talipova  Classification of internal waves shoaling over slope-shelf topography RUSSIAN JOURNAL OF EARTH SCIENCES vol. 20, 4, 2020, doi: 10.2205/2020ES000730

How to cite: Terletska, K., Maderich, V., and Talipova, T.: Estimating energy dissipation in internal solitary waves breaking on slopes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-408, https://doi.org/10.5194/egusphere-egu21-408, 2021.

15:34–15:36
|
EGU21-1830
Tatiana Talipova, Ekaterina Didenkulova, Anna Kokorina, and Efim Pelinovsky

Internal solitary wave ensembles are often observed on the ocean shelves. The long internal baroclinic tide is generated by a barotropic tide on the shelf edges, and then transforms into the soliton-like wave packets during the nonlinear propagation to the beach. The tide is a periodic process and the solitary wave ensemble appears on the shelf usually each semi-diurnal period of 12.4 hours. This process is very sensitive to the variation of the tide characteristics and the hydrology.

We study the propagation of the soliton ensembles numerically in the framework of the spatial form of the Gardner equation (i.e., the Korteweg-de Vries equation with both, quadratic and cubic nonlinearities) assuming horizontally uniform background and applying periodic conditions in time. The water stratification and the local depth are taken similar to the conditions of the north-western Australian shelf, where the stratification admits the existence of solitons but not breathers. The numerical simulation is performed using the Gardner equation with the negative sign of the cubic nonlinearity. For the study of the statistic properties of the solitary waves we use the ensemble of 50 realizations with the same set of 13 solitary waves which are located randomly. The histograms of the wave amplitudes change as the waves travel. The histogram variations become significant after 50 km of the wave propagation. The third (skewness) and the fourth (kurtosis) statistical moments are computed versus the travel distance. It is shown that the both moments decrease by 20% when the solitary wave groups travel for about 150 km.

A similar simulation is conducted for a variable background within the framework of the variable-coefficient Gardner equation. At some location the water stratification corresponds to the positive sign of the local coefficient of the cubic nonlinearity, and then internal breathers may exist. The wave propagation in horizontally inhomogeneous hydrology leads to the occurrence of complicated patterns of solitons and breathers; in the course of the transformation they can disintegrate or form internal rogue waves. Under these conditions the statistical moments of the wave field are essentially different from case when the breather-like waves cannot occur.

The research was supported by the RFBR grants No 19-05-00161 (TT and EP) and 19-35-60022 (ED). The Foundation for the Advancement of Theoretical Physics and Mathematics “BASIS” (№ 20-1-3-3-1) is also acknowledged by ED

How to cite: Talipova, T., Didenkulova, E., Kokorina, A., and Pelinovsky, E.: Statistical properties of the internal solitary wave ensemble, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1830, https://doi.org/10.5194/egusphere-egu21-1830, 2021.

15:36–15:38
|
EGU21-2737
Nikolay Makarenko and Danila Denisenko

In present paper we consider the problem on solitary waves forced by a chain of gently sloped obstacles of small height. Steady two-dimensional free-surface flows over a complex topography are studied analytically in the case when the far upstream flow is slightly supercritical. Small height- and steepness restrictions are important here since these circumstances provide the balance between nonlinear dispersion and hydraulic effects both affecting nearly hydrostatic non-uniform flow. Fully non-linear irrotational Euler equations are formulated via the von Mises transformation that parametrizes the family of streamlines in a curvilinear flow domain. It is well known that the critical value of the Froude number is the bifurcation point providing non-uniqueness of stationary flow. In present work, we construct and analyze approximate solitary-wave solutions by using long-wave expansion procedure with two small parameters.  In addition, we apply the Lyapunov - Schmidt method which ensures an analytical condition of the wave-trapping formulated in terms of the Melnikov function. A specific class of multi-bumped topographies is considered in order to demonstrate multiplicity of forced waves. The amount of different wave regimes depends on the number of bumps and pits, as well as on their location and size in relation to each other.

How to cite: Makarenko, N. and Denisenko, D.: Forced solitary waves over complex topography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2737, https://doi.org/10.5194/egusphere-egu21-2737, 2021.

15:38–15:40
|
EGU21-3045
Kevin Lamb

Previous studies have suggested that fully nonlinear internal solitary waves (ISWs) are very soliton-like as the interaction of two ISWs results in only very small changes in amplitude of the interacting ISWs and in the production of a very small amplitude wave train. Previous studies have, however, considered ISWs with the polarity predicted by the sign of the quadratic nonlinear coefficient of the KdV equation. The Gardner equation, which is an extension of the KdV equation that includes a cubic nonlinear term, has ISWs of two polarities (i.e., waves of depression and elevation) when the cubic coefficient of the Gardner equation is positive. These waves are soliton solutions of the Gardner equations.  In this talk I will discuss the interaction of ISWs of opposite polarity in continuous asymmetric three layer stratifications. Regions in parameter space where ISWs of opposite polarity exist will be discussed and I will demonstrate via fully nonlinear numerical simulations that the interaction of ISWs of opposite polarity waves are far from soliton-like: their interaction can result in very large changes in wave amplitude and may produce a very complicated wave field with multiple large ISWs, a large linear wave field and breather-like waves. 

How to cite: Lamb, K.: Interaction of Fully-Nonlinear Internal Solitary Waves of Opposite Polarity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3045, https://doi.org/10.5194/egusphere-egu21-3045, 2021.

15:40–15:42
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EGU21-3823
|
ECS
Gong Yi, Song Haibin, Zhao Zhongxiang, Guan Yongxian, and Kuang Yunyan

Internal solitary waves (ISWs) make important contributions to energy cascade, ocean mixing and material transport in the ocean. However, there are few observational studies on the vertical structure of ISWs. The high-spatial resolution of seismic data enables us to obtain clear internal structure image of ISWs, so we can conduct a detailed research on their vertical structure. In this article, we report 11 ISWs near Dongsha Atoll in the South China Sea using two-dimensional seismic data.

We first extracted the amplitudes of ISW from seismic section, and obtained a series of discrete amplitude points. Then, the least-squares spline fitting was used to fit these amplitude points into a vertical structure curve. We calculated vertical structures by linear theory and first-order nonlinear theory, respectively, and compared the observed vertical structure with the two theories. We found that three ISWs conform to the linear vertical structure function, four ISWs conform to the first-order nonlinear vertical structure function, and four ISWs do not conform to the two theories. In order to figure out the reason why the observation did not conform to the theories, we decomposed the fitted vertical structures of these four ISWs by the empirical mode decomposition (EMD) algorithm, and compare the residuals of decomposition with the two theories. The results showed that the residuals of two ISWs are in agreement with the linear vertical structure function, the residual of one ISW conforms to the first-order nonlinear vertical structure function, and one residual of ISW still cannot conform to the two theories. We calculated key parameters of these ISWs to analyze the reasons for difference between observation and theory.

In summary, we found that the shape of vertical structure is mainly determined by nonlinearity. The vertical structure with low degree nonlinearity can be described by linear theory, while ISW with high degree nonlinearity conform to the first-order nonlinear theory. Besides, for an ISW with large amplitude propagating in shallow water, its vertical structure is more susceptible to be affected by the topography. Moreover, the background flow can also affect the vertical structure. We found an ISW was passing through an eddy which was trapped near seafloor, and resulted in the bottom of vertical structure decayed rapidly.

How to cite: Yi, G., Haibin, S., Zhongxiang, Z., Yongxian, G., and Yunyan, K.: On the vertical structure of internal solitary waves in the northeastern South China Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3823, https://doi.org/10.5194/egusphere-egu21-3823, 2021.

15:42–15:44
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EGU21-3925
|
ECS
Seung-Woo Lee and SungHyun Nam

Oceanic nonlinear internal waves (NLIWs) play an important role in regional circulation, biogeochemistry, energetics, vertical mixing, underwater acoustics, marine engineering, and submarine navigation, most commonly generated by the interaction between barotropic tides and bathymetry. Here, we present characteristics of first mode NLIWs observed using high-resolution in-situ data collected using moored and underway temperature sensors in a relatively flat bottom in the northeastern East China Sea during May 15-28, 2015. During the experiment, totally 34 events of first mode NLIWs were identified and characterized with amplitude of 4–16 m, characteristic width of 310–610 m, propagation speed of 0.53–0.56 m s-1, and propagation direction (mainly southwestward propagation), respectively. Most NLIWs were observed during period of spring tide with phases locked to semidiurnal barotropic tides. Generation and propagation of the first mode NLIWs observed in the region are discussed in relation to satellite images and historical hydrographic data collected in the region. Our results support significance of first mode NLIWs and their interactions on turbulent mixing and regional circulation particularly in a broad and shallow continental shelves where the NLIWs generated from multiple sources propagate into multi-directions experiencing wave-wave interactions.

How to cite: Lee, S.-W. and Nam, S.: Observations of nonlinear internal waves of tidal origin in the northeastern East China Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3925, https://doi.org/10.5194/egusphere-egu21-3925, 2021.

15:44–15:46
|
EGU21-5404
|
ECS
|
Highlight
Gabin Urbancic, Kevin Lamb, Ilker Fer, and Laurie Padman

North of the critical latitude (78.4), internal waves of the M2 tidal frequency can no longer freely propagate, and the energy conversion from the barotropic to the internal tides vanishes. Near the continental slopes around the Arctic Ocean, internal wave energy is enhanced and comparable to values at mid-latitudes (Rippeth et al. 2015, Levine et al. 1985). Observations on the northern flank of the Yermak Plateau (YP) has characterized the region as one of enhanced internal wave activity and nonlinear internal waves have been observed (Czipott et al. 1991, Padman and Dillon 1991).

The YP is a bathymetry feature stretching out into the Fram Strait north-west of Svalbard. The YP plays a prominent role in the Arctic’s heat balance due to its interaction with the West-Spitsbergen current which is a main contributor to the heat transport into the Arctic Ocean. Nonlinear waves generated over the YP are a significant energy source for mixing and can therefore modulate and force exchange processes.

To study the nonlinear internal wave generation mechanisms over the YP, we used a high resolution, nonlinear, non-hydrostatic model. We found that nonlinear internal waves are forced not by the M2 but the K1 tide which has been observed to have significant variability over the YP (Padman et al. 1992). Barotropic, diurnal shelf waves generated on the eastern side of the YP propagates counter-clockwise, amplifying the cross-slope currents. This amplification is the necessary condition for nonlinear internal wave generation over the YP.

How to cite: Urbancic, G., Lamb, K., Fer, I., and Padman, L.: Nonlinear internal wave generation over the Yermak Plateau , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5404, https://doi.org/10.5194/egusphere-egu21-5404, 2021.

15:46–15:48
|
EGU21-5910
|
ECS
Egor Svergun and Alexey Zimin

In the Bering Sea, as in the Kuril-Kamchatka region, which includes the waters near the Kuril Islands and the Pacific waters of the Kamchatka Peninsula, previously performed satellite radar observations show numerous scattered surface manifestations of short-period internal waves (SIW’s). However, the summative study of the characteristics of surface manifestations of SIW’s are currently not available in this region.

In this study, radar images from the Sentinel-1A and B satellites from July 1, 2019 to September 30, 2019 were used to record the surface manifestations of SIW’s. For each surface manifestation of SIW’s, such characteristics as the position of the manifestation, the wavelength, the arc length of the leading ridge in the packet, the direction of propagation, and the number of waves in the packet are determined. Wave detection on the radar images is performed using ESA SNAP and Matlab software.

In the study region, 1,540 SIW’s. manifestations with a wavelength of 80 to 1,900 meters and a leading crest length of 1 to 70 km were registered on 772 radar images. The ranges of variability of the main geometric characteristics of the manifestations in the Kuril-Kamchatka region and in the Bering Sea are very similar. The maximum number of manifestations in the Kuril-Kamchatka region was recorded in the first half of September, and in the Bering Sea – in the second half of July. This difference seems to be related to regional features of pycnocline formation.

Manifestations of internal waves are mainly recorded in the shelf zone. The constant occurrence of manifestations of internal waves located in the southern part of the Kuril Islands, around the Pacific coast of the Kamchatka Peninsula, East of the Litke Strait, and the Straits of the Aleutian chain. It was found that the areas of constant occurrence of the manifestations of SIW’s coincide with the areas of intense dissipation of the internal tide. In the Kuril-Kamchatka region, in contrast to the Bering Sea, manifestations of internal waves are recorded over significant depths at a great distance from the shelf zone, which is associated with the collapse of the internal tide reflected from the edge of the continental slope.

The information obtained in this study will allow us to improve our understanding of the field of short-period internal waves of the north-eastern Pacific Ocean.

The study of surface manifestations of internal waves in the Kuril-Kamchatka region was supported by RFBR grant No. 20-35-90054. The study of surface manifestations of internal waves in the Bering Sea was supported by RFBR grant No. 18-35-20078.

How to cite: Svergun, E. and Zimin, A.: Surface manifestations of short-period internal waves of the Kuril-Kamchatka region and the Bering Sea according to satellite observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5910, https://doi.org/10.5194/egusphere-egu21-5910, 2021.

15:48–15:50
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EGU21-6862
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ECS
Yujun Yu, Shuya Wang, and Xu Chen

Internal Solitary Waves (ISW) are ubiquitous in the Andaman Sea as revealed by Synthetic Aperture Radar (SAR) images, but their generation mechanism and corresponding influence factors remain unknown. Based on a non-hydrostatic two-dimensional model, the generation of ISW across the channel between the Batti Malv Island and the Car Nicobar Island is investigated. Influences of the topography characteristics, seasonal stratification and tidal forcing are analyzed with a series of sensitivity runs. The simulated results indicate that no apparent ISW appear near the ridge because of small tidal excursion and low Froude number. Instead, they are evolved from the disintegrated internal tides which gradually steepen due to nonlinearity during propagation. East-west asymmetry of ISWs is revealed, which can be attributed to different topographic features on the two sides of the ridge. Two sills on the east side of the ridge further complicate the generation of eastward-propagating internal tides, resulting in the enhancement of ISWs in the Andaman Sea. Seasonally varying stratification has minor effect on the generation and evolution of ISWs. In addition, generation of ISW is mainly contributed by semidiurnal tidal forcing, while diurnal forcing only generates linear internal tides.

How to cite: Yu, Y., Wang, S., and Chen, X.: On the Generation and Evolution of Internal Solitary Waves in the Andaman Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6862, https://doi.org/10.5194/egusphere-egu21-6862, 2021.

15:50–15:52
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EGU21-7300
|
ECS
Elizaveta Khimchenko, Alexander Ostrovskii, and Alexey Klyuvitkin

The Black Sea is practically tideless basin where inertial variability dominates the energy spectra at the high-frequency band f > 1 day-1.  The near-inertial internal waves are easier to infer from the observational data in the absence of the tidal motions. Modern observing tools e.g., the temperature sensor strings, the ADCPs, and the profiler moorings allow for continuous measurements at fixed locations with high temporal resolution sufficient to resolve the inertial time scale.

Here we present an analysis of the time series of hydrophysical measurements both at the continental slope and in the deep central part of the Black Sea. The measurements over the continental slope were carried out using the Aqualog moored profiler with the CTD probe and acoustic Doppler current meter [1] in different seasons during 2015–2019. The time series of vertical profiles of temperature, salinity, density, dissolved oxygen, and current velocity were obtained for the water column from 20–30 m to 200–230 m depths. As for the deep basin measurements, these were done by using the moorings equipped with the temperature sensors and acoustic Doppler current meters at fixed depths of 100 m and 1700 m. The data included the year-long time series of temperature and current velocity from December 2016 to October 2017.

The vertical oscillations with a period close to the local inertial were clear cut in the multiparameter data vertical profiles in the main pycnocline at the continental slope. The examples of the near-inertial wave packs trapped in the pycnocline are shown. The maximum heights of the observed internal waves reached 30 m. During the passage of the near-inertial internal wave, the direction of the current changes to the opposite, which is typical for the first mode wave.

The seasonal variability of the near-inertial internal motions was studied by applying conventional statistical tools including spectral analysis to the mooring data in the Black Sea central part. It was found that intensification of inertial oscillations occurs from September to February. At the frequency close to the local inertial, the velocity rotation vector (hodograph) rotates clockwise, which is typical for inertial internal waves. The radius of the circle described by the vectors of the inertial currents varies within 0.5–1.5 km. The seasonal change of the cross-correlations between inertial motions in the upper and near-bottom layers was also revealed.

The research was conducted by the assignment of the Ministry of Science and Higher Education of Russian Federation No. 0149-2019-0011 and partly supported by RFBR grant No. 19-05-00459.      

How to cite: Khimchenko, E., Ostrovskii, A., and Klyuvitkin, A.: Near-inertial internal waves in the Black Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7300, https://doi.org/10.5194/egusphere-egu21-7300, 2021.

15:52–15:54
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EGU21-7301
|
ECS
Johan Fourdrinoy, Julien Dambrine, Madalina Petcu, Morgan Pierre, and Germain Rousseaux

While seeking to revisit an old experiment of John Scott Russell, we discovered a new mechanism for generating a non-shoaling bolus (an ovoid coherent mass of recirculating mixed fluids immerged in a surrounding medium/a of different density/ies) propagating along a pycnocline. In a study about dead-water (Fourdrinoy et al. 2020), a wave resistance phenomenon induced by internal waves formation at the interface between waters of different densities, we modified the setup used by Scott Russell. The Scottish engineer studied the formation and propagation of dispersive waves when an object is removed from a laterally confined open channel with a shallow layer of water. The “vacuum” created by the mass removal generates a linear dispersive free surface deformation with a front of negative polarity followed by a wave train. If we extend this configuration to a two-layers stratification, we can observe a linear dispersive wave with negative polarity à la Scott Russell, propagating along the interface. In addition, the removal of the object generates under certain conditions a bolus which induces a mixing zone and a gradient transition layer. We will present this new method of boluses creation, as well as an experimental characterization with space-time diagrams thanks to a subpixel detection procedure.

The dual nature of the dead-water phenomenology: Nansen versus Ekman wave-making drags.
Johan Fourdrinoy, Julien Dambrine, Madalina Petcu, Morgan Pierre and Germain Rousseaux.
Proceedings of the National Academy of Sciences, Volume 117, Issue 29, p. 16739-16742, July 2020.

How to cite: Fourdrinoy, J., Dambrine, J., Petcu, M., Pierre, M., and Rousseaux, G.: A new mechanism for generating a bolus within a double layer following Scott Russell, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7301, https://doi.org/10.5194/egusphere-egu21-7301, 2021.

15:54–15:56
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EGU21-8160
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ECS
Yunyan Kuang, Haibin Song, Yongxian Guan, Wenhao Fan, and Yi Gong

The nonlinear internal solitary waves (ISWs) are ubiquitous and recently many mode-1 ISWs have been reported to be detected in the northeast South China Sea by using the seismic oceanography method. However, few mode-2 ISWs are discovered in seismic data in the South China Sea. Thus, waveform characteristics and kinematics parameters of the mode-2 ISWs in this region need further study.

In this paper, one convex mode-2 ISW is presented near Dongsha Plateau on September 20th, 2009, and is analyzed by the combination of reprocessed seismic section and reanalysis hydrographic data. The seismic events of the multi-channel seismic section are extracted to obtain the vertical amplitude distribution and water depth of the mode-2 ISW. The seismic events can be used to analyze the structural characteristics in a snapshot, while different pre-stack common-offset gathers (COGs) can observe the seismic fine structures of the mode-2 ISW in chronological order. Furthermore, we use COGs method to calculate the apparent phase velocities of the peak and trough part of the mode-2 ISW on the seismic section and then correct the phase velocities according to the seismic measurement direction and ISWs propagation direction derived from satellite data. Theoretically, the reanalysis hydrographic data can be used to calculate the vertical structure and propagation speed of ISW based on the KdV model, and the theoretical results can be compared with those from seismic observations.

In total, 10 seismic events are extracted to obtain wave amplitudes and corresponding water depth distribution. Among the seismic events, only 2 events are elevation wave types and the rest 8 events are depression wave types. The maximum amplitude is about 25.5m of a depression wave event at 200m water depth. The dimensionless amplitude is 2.56, this number shows that the mode-2 ISW is of large amplitude. Moreover, the pycnocline is displaced over 20% from the mid-depth of the total seawater depth, illustrating the mode-2 ISW is of asymmetry. The fine structures of the mode-2 ISW observed on COGs also show the asymmetric and complex wave disturbance in different acquisition times. The apparent phase velocity of the crest is 1.59m/s, while the apparent phase velocity of the trough (the maximum amplitude) is 0.8065, the results indicate that the elevation waves of the mode-2 ISW may move faster than the underlying depression waves. Finally, the corrected phase speed of the mode-2 ISW is consistent with the propagation speed calculated by the KdV equation. More pieces of evidence are needed to explain the generation and to predict further evolution of the asymmetric mode-2 ISW, and seismic oceanography may be one of the key techniques to answer these questions.

How to cite: Kuang, Y., Song, H., Guan, Y., Fan, W., and Gong, Y.: Seismic oceanography study of a mode-2 internal solitary wave in the northeast South China Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8160, https://doi.org/10.5194/egusphere-egu21-8160, 2021.

15:56–15:58
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EGU21-8477
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ECS
Xiaolin Bai, Kevin Lamb, and José da Silva

In the presence of topography, two main contributors for internal wave energy are tide-topography interaction transferring energy from the barotropic tide to internal tides, and lee wave generation when geostrophic currents or eddying abyssal flows interact with topography. In the past few decades, many studies considered the respective contribution of the oscillating flows or steady background flows, but few investigations have considered both.  

In this talk, we consider the joint effects of tidal and steady currents to investigate internal wave generation and propagation on the Amazon shelf, a hotspot for internal solitary wave (ISW) generation. The Amazon Shelf is off the mouth of the Amazon River in the southwest tropical Atlantic Ocean, affected by strong tidal constituents over complex bottom bathymetry and a strong western boundary current, the North Brazilian Current (NBC). Both satellite observations and numerical modelling are used in this study. Satellite observations provide a clear visualization of the wave characteristics, such as temporal and spatial distributions, propagating direction and its relation to background currents. Based on parameters from satellite observations and reanalysis dataset, we set up a model to numerically investigate the dynamics of the ISW generation. We demonstrate that the small-scale topography contributes to a rich generation of along-shelf propagating ISW, which significantly contribute to the ocean mixing and potentially cause sediment resuspension. Moreover, the ISW-induced currents also contribute to the sea surface wave breaking as observed by satellite measurements. In addition, statistics based on a decade of satellite images and numerical investigations on seasonal variations of the ISWs and the NBC improve our understanding of the generation and evolution of these nonlinear internal waves in the presence of background currents.

How to cite: Bai, X., Lamb, K., and da Silva, J.: Internal Wave Generation by a Combination of Tidal and Steady Currents, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8477, https://doi.org/10.5194/egusphere-egu21-8477, 2021.

15:58–16:00
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EGU21-8556
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ECS
Jia You, Zhenhua Xu, Qun Li, and Peiwen Zhang

Turbulent mixing in the ocean interior is mainly contributed by internal wave breaking; however, the mixing properties and the modulation effects of mesoscale environmental factors are not well-known. Here, the spatially inhomogeneous and seasonally variable diapycnal diffusivities in the upper Philippine Sea were estimated from ARGO float data using a strain-based finescale parameterization. Based on a coordinated analysis of multi-source data, we found that the driving processes for diapycnal diffusivities mainly included the near-inertial waves and internal tides. Mesoscale features were important in intensifying the mixing and modulating its spatial pattern. One interesting finding was that, besides near-inertial waves, internal tides also contributed significant diapycnal mixing for the upper Philippine Sea. The seasonal cycles of diapycnal diffusivities and their contributors differed zonally. In the mid-latitudes, wind-mixing dominated and was strongest in winter and weakest in summer. In contrast, tidal-mixing was more predominant in the lower-latitudes and had no apparent seasonal variability. Furthermore, we provide evidence that the mesoscale environment in the Philippine Sea played a significant role in regulating the intensity and shaping the spatial inhomogeneity of the internal tidal mixing. The magnitudes of internal tidal mixing was greatly elevated in regions of energetic mesoscale processes. The anticyclonic mesoscale features were found to enhance diapycnal mixing more significantly than did cyclonic ones.

How to cite: You, J., Xu, Z., Li, Q., and Zhang, P.: Enhanced internal tidal mixing in the Philippine Sea mesoscale environment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8556, https://doi.org/10.5194/egusphere-egu21-8556, 2021.

16:00–16:02
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EGU21-8560
Bo Zhao, Zhenhua Xu, Qun Li, Yang Wang, and Baoshu Yin

An exact geostrophic vortex generate spontaneously inertia-gravity waves (IGWs) with spiral patterns via singularity instability mechanism. In the vertical direction, the energy of the IGWs is dominated by mode-1 in the generation and propagation processes, leading to weak dissipation and long-distance propagation. The amplitude of the IGWs increases linearly with the Rossby number in the range 0.04–0.1. Additionally, the IGWs emitted from an anticyclonic vortex are stronger than those radiated from the cyclonic vortex. Anticyclonic and cyclonic geostrophic vortices transfer roughly 0.54% and 0.41% of their kinetic energy to IGWs in this transient generation process, respectively. However, quasi-geostrophic mesoscale eddies are decomposed to balanced geostrophic component and unbalanced near-inertial oscillations with different timescales. Near-inertial waves (NIWs) also can be generated as a forced response to the nonlinear coupling of the geostrophic component and high-frequency oscillations of the quasi-geostrophic eddies. Afterwards, the NIWs resonate with the near-inertial oscillations and share the same horizontal wavenumbers with the eddy. Generally, an anticyclonic mesoscale eddy can emit much stronger NIWs than does a cyclonic eddy. The NIW intensity strengthens exponentially with the Rossby number. The spontaneous generated NIWs represent an effective pathway for mesoscale eddy energy skin and non-negligible contribution to the global NIW energy.

How to cite: Zhao, B., Xu, Z., Li, Q., Wang, Y., and Yin, B.: Spontaneous inertia-gravity wave generation from mesoscale eddies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8560, https://doi.org/10.5194/egusphere-egu21-8560, 2021.

16:02–16:04
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EGU21-8563
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ECS
Wenjia Min, Zhenhua Xu, Qun Li, Peiwen Zhang, and Baoshu Yin

The slope area northeast of Taiwan was known as a hotspot for internal tides and internal solitary waves (ISWs), while their specific sources and generation mechanism of ISWs remain unclear. We investigate the generation and evolution processes of internal tides and ISWs with realistic configuration based on the high resolution non-hydrostatic numerical simulations. The ISWs northeastern Taiwan show a complex pattern according to the satellite image and our numerical results. ISWs propagate to various direction, and both shoreward and seaward propagating ISWs are generated on the continental slope. The ISWs observed on the continental slope-shelf region northeastern Taiwan can be generated by two ways. One is the local tide-topography interaction, and the other is the disintegration of remote internal tides generated over the I-Lan Ridge. The generated internal tides propagate northward to the Okinawa Trough, and can reach the continental slope-shelf region. During the propagation of the internal tides, the internal tides start to steepen and internal solitary waves are formed about 80 km north of I-Lan Ridge. The amplitude of the generated internal solitary waves is about 30 m. Furthermore, the Kuroshio is important to modulate the propagation and evolution of internal tides and ISWs, especially to the complexity of the ISW spatial pattern. We revealed most of the generated internal wave energy is dissipated locally over the double-canyon region, and strong mixing occur over the canyons.

How to cite: Min, W., Xu, Z., Li, Q., Zhang, P., and Yin, B.: Three dimensional simulation on the generation and propagation of internal tides and solitary waves northeast of Taiwan Island, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8563, https://doi.org/10.5194/egusphere-egu21-8563, 2021.

16:04–16:06
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EGU21-10176
Eugene Morozov, Dmitry Frey, and Elizaveta Khimchenko

Observations of tidal internal waves in the Bransfield Strait, Antarctica, are analyzed. The measurements were carried out for 14 days on a moored station equipped with five autonomous temperature and pressure sensors. The mooring was deployed on the slope of Nelson Island (South Shetland Islands archipelago) over a depth of 70 m at point 62°21ꞌ S, 58°49ꞌ W. Analysis is based on the fluctuations of isotherms.  Vertical displacements of temperature revealed that strong internal vertical oscillations up to 30–40 m are caused by the diurnal internal tide. Spectral analysis of vertical displacements of the 0.9°C isotherm showed a clear peak at a period of 24 h. It is known that the tides in the Bransfield Strait are mostly mixed diurnal and semidiurnal, but during the Antarctic summer, diurnal tide component may intensify. The velocity ellipses of the barotropic tidal currents were estimated using the global tidal model TPXO9.0. It was found that tidal ellipses rotate clockwise with a period of 24 h and anticlockwise with a period of 12 h. The waves are forced due to the interaction of the barotropic tide with the bottom topography. Diurnal internal tides do not develop at latitudes higher than 30º over flat bottom. The research was supported by RFBR grant 20-08-00246.

How to cite: Morozov, E., Frey, D., and Khimchenko, E.: Tidal Internal Waves in the Bransfield Strait, Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10176, https://doi.org/10.5194/egusphere-egu21-10176, 2021.

16:06–16:08
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EGU21-10519
Haibin Song, Yi Gong, Yongxian Guan, Wenhao Fan, and Yunyan Kuang

In the study of shoaling internal solitary waves, the observation and research on the internal fine structure and the effect of the topography are still insufficient. We try to make up for such insufficiency by seismic oceanography method. A first-mode depression internal solitary wave was observed propagating on the continental slope in the northeast South China Sea near Dongsha Atoll. We used common offset gathers (COGs) to obtain a series of images of this internal solitary wave that evolved over time, and studied the changes in internal fine structure by analyzing the seismic events in COG migrated sections. We found that the seismic events were broken during the shoaling, which was caused by the instability induced by internal solitary wave. We picked six events which represent six waveform and analyzed their evolution. It was found that the change in shape of waveform at different depths is different. The waveform in deep water deforms before that in shallow water, and the waveform in shallow water deforms to a greater degree. In addition, we also counted four parameters of phase velocity, amplitude, wavelength, and slopes of front and rear during the shoaling. The results show that the phase velocity and amplitude of waveform in shallow water increases, the wavelength decreases, and the slope of rear gradually becomes larger than that of the front. We have compared the observed changes with previous study made by numerical simulation.

How to cite: Song, H., Gong, Y., Guan, Y., Fan, W., and Kuang, Y.: Observations of fine structure changes in shoaling internal solitary waves based on seismic oceanography method, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10519, https://doi.org/10.5194/egusphere-egu21-10519, 2021.

16:08–16:10
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EGU21-12316
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ECS
Nicolas Castro-Folker, Christopher Subich, and Marek Stastna

We report on numerical simulations of stratified adjustment that yield radially propagating mode-2 waves. The initial inward propagating mode-2 wave increases in amplitude, but it does not lead to significant overturning even during the period of self-interaction near the origin. However, post-focusing, the pycnocline thins and secondary waves propagate into an environment that is very different from the undisturbed stratification. These resulting waves break, and create intrusions above and below the thinned pycnocline. While most experimental realizations of extreme internal solitary-like waves use a rectangular geometry, it should be possible to realize this situation experimentally. We discuss the resolution requirements of this situation, as well as irreversible mixing.

How to cite: Castro-Folker, N., Subich, C., and Stastna, M.: Focusing of mode-2 internal solitary-like waves: an unexpected extreme internal wave, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12316, https://doi.org/10.5194/egusphere-egu21-12316, 2021.

16:10–16:12
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EGU21-14737
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ECS
Sam Hartharn-Evans, Magda Carr, Marek Stastna, and Peter Davies

Shoaling is a key mechanism by which Internal Solitary Waves (ISWs) dissipate energy, induce mixing, and transport sediment. Past studies of shoaling ISWs in a three-layer stratification (with homogeneous upper and lower layers separated by a thin pycnocline layer) have identified a classification system where waves over the shallowest slopes undergo fission, whilst over steeper slopes, the breaking type changes from surging, through collapsing to plunging as a function of increasing internal Irribaren number (Ir) defined with the topographic slope, s, and the incident wave’s amplitude and wavelength, Aw and Lw respectively, as . Here, a combined numerical and laboratory study extends this prior work into new stratifications, representing the diversity of ocean structures across the world. Numerical results were able to successfully reproduce past studies in the three-layer stratification, and those in the two-layer stratification in the laboratory. Where a linear stratified layer overlays a homogeneous lower layer (two-layer stratification), it is found that plunging dynamics are inhibited by the density gradient throughout the upper layer, instead forming collapsing-type breakers. In numerical experiments, where the density gradient is continuous throughout the full water column (linear stratification), not only are the plunging dynamics inhibited, but the density gradient at the bottom boundary also prevents the formation of collapsing dynamics, instead all waves in this stratification either fission, or form surging breakers. Where the wave steepness is particularly high in the linear stratification, the upslope bolus formed by surging was unstable, and Kelvin-Helmholtz instabilities were observed on the upper boundary of the bolus, dynamics not previously observed in the literature. These results indicate the importance of using representative stratifications in laboratory and numerical studies of ISW behaviours.

How to cite: Hartharn-Evans, S., Carr, M., Stastna, M., and Davies, P.: Stratification effects on shoaling Internal Solitary Waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14737, https://doi.org/10.5194/egusphere-egu21-14737, 2021.

16:12–16:14
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EGU21-14991
Magda Carr, Peter Sutherland, Andrea Haase, Karl-Ulrich Evers, Ilker Fer, Atle Jensen, Henrik Kalisch, Jarle Berntsen, Emilian Parau, Oyvind Thiem, and Peter Davies

Oceanic internal waves (IWs) propagate along density interfaces and are ubiquitous in stratified water. Their properties are influenced strongly by the nature and form of the upper and lower bounding surfaces of the containing basin(s) in which they propagate.  As the Arctic Ocean evolves to a seasonally more ice-free state, the IW field will be affected by the change. The relationship between IW dynamics and ice is important in understanding (i) the general circulation and thermodynamics in the Arctic Ocean and (ii) local mixing processes that supply heat and nutrients from depth into upper layers, especially the photic zone. This, in turn, has important ramifications for sea ice formation processes and the state of local and regional ecosystems.  Despite this, the effect of diminishing sea ice cover on the IW field (and vice versa) is not well established. A better understanding of IW dynamics in the Arctic Ocean and, in particular, how the IW field is affected by changes in both ice cover and stratification, is central in understanding how the rapidly changing Arctic will adapt to climate change.

 

An experimental study of internal solitary waves (ISWs) propagating in a stably stratified two-layer fluid in which the upper boundary condition changes from open water to ice are studied for grease, level, and nilas ice. The experiments show that the internal wave-induced flow at the surface is capable of transporting sea-ice in the horizontal direction. In the level ice case, the transport speed of, relatively long ice floes, nondimensionalized by the wave speed is linearly dependent on the length of the ice floe nondimensionalized by the wave length. It will also be shown that bottom roughness associated with different ice types can cause varying degrees of vorticity and small-scale turbulence in the wave-induced boundary layer beneath the ice. Measures of turbulent kinetic energy dissipation under the ice are shown to be comparable to those at the wave density interface. Moreover, in cases where the ice floe protrudes into the pycnocline, interaction with the ice edge can cause the ISW to break or even be destroyed by the process. The results suggest that interaction between ISWs and sea ice may be an important mechanism for dissipation of ISW energy in the Arctic Ocean.

 

Acknowledgements

This work was funded through the EU Horizon 2020 Research and Innovation Programme Hydralab+.

How to cite: Carr, M., Sutherland, P., Haase, A., Evers, K.-U., Fer, I., Jensen, A., Kalisch, H., Berntsen, J., Parau, E., Thiem, O., and Davies, P.: Laboratory experiments on Internal Solitary Waves in ice covered waters , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14991, https://doi.org/10.5194/egusphere-egu21-14991, 2021.

16:14–16:16
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EGU21-16458
Peter Diamessis, Takahiro Sakai, and Gustaaf Jacobs

The development of the separated bottom boundary layer (BBL) in the footprint of a large-amplitude ISW of depression is examined using high-accuracy/resolution implicit Large Eddy Simulation. The talk will focus on a single relatively idealized case of a large-amplitude ISW propagating against an oncoming barotropic current with its own, initially laminar, BBL under the inevitable restriction of laboratory-scale Reynolds number. Significant discussion will be dedicated to the non-trivial computational cost of setting up and conducting the above simulation, within long domains and over long-integration times, in a high-performance-computing environment. Results will focus on documenting the full downstream evolution of the structure of the separated BBL development. Particular emphasis will be placed on the existence of a three-dimensional global instability mode, at the core of the separation bubble where typically one might assume two-dimensional dynamics. The particular instability mode is spontaneously excited and is considered responsible for the self-sustained nature of the resulting near-bed turbulent wake in the lee of the ISW. Fundamental mean BBL flow metrics will then be presented along with a short discussion for potential for particulate resuspension. The talk will close with a discussion of the relevance of the existing flow configuration to both the laboratory and ocean, in light of recent measurements in the NW Australian Shelf.

How to cite: Diamessis, P., Sakai, T., and Jacobs, G.: The structure of self-sustained instability, transition and turbulence in the separating boundary layer under an internal solitary wave of depression, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16458, https://doi.org/10.5194/egusphere-egu21-16458, 2021.

16:16–17:00