Observations Of Internal Waves Research Articles

Marine profiling lidars and their application for oceanological problems

The review focuses on research conducted using profiling (radiometric) lidars. The paper presents the current state of lidar surveying equipment, methods for processing lidar data, and describes the problems of scientific and practical interest in oceanology that can be solved using lidar sensing. The review does not cover issues related to laser bathymetry, spectral (Raman) and spaceborne lidars, as they are separate specific fields. The main focus is on recent research in profiling lidar field. Summary tables of the technical characteristics of several of the most interesting airborne and shipborne lidars are provided. Their design features are discussed. Results from using lidars to determine near-surface hydrooptical characteristics, including employing polarization lidars and recently developed high-resolution spectral lidars, are presented. Findings from observing thin scattering layers across various aquatic regions are shown. The paper explores theoretical studies on lidar images of internal waves and experimental observations of internal waves in waters with different hydrooptical stratification. Lidars' application in addressing fisheries-related issues is examined. An overview of current development trends and future research directions is provided.

Observations of Near-Inertial Internal Waves over the Continental Slope in the Northeastern Black Sea

The article presents observations of near-inertial internal waves (NIWs) in the slope waters of the Black Sea in winter and summer. Rotary spectral analysis of a time series of sea current velocity measurements revealed the prevailing anticyclonic component of the motions near the local inertial frequency f. The clockwise rotation of the velocity vector with depth implies that the NIWs propagate downwards. The amplitude of NIWs usually was 0.1–0.2 m s−1. NIWs were observed in the layer of the permanent pycnocline and the seasonal pycnocline, which attenuate below depths of 160 m and 80 m in winter and summer, respectively. The amplitude of the near-inertial kinetic energy (NIKE) showed a close relationship with vertical stratification. During winter, NIKE exhibited maximum values in the layer of the permanent pycnocline, whereas, in summer, it was primarily observed in the seasonal pycnocline layer. The near-inertial oscillations were generally more energetic in winter.

Internal Solitary Waves within the Cold Tongue of the Equatorial Pacific Generated by Buoyant Gravity Currents

Abstract The equatorial cold tongue in the Pacific Ocean has been intensely studied during the last decades as it plays an important role in air–sea interactions and climate issues. Recently, Warner et al. revealed gravity currents apparently originating in tropical instability waves. Both phenomena have strong dissipation rates and were considered to play a significant role in cascading energy from the mesoscale to smaller horizontal scales, as well as to vertical scales less than 1 m. Here, we present Sentinel-3 satellite observations of internal solitary waves (ISWs) in the Pacific cold tongue near the equator, in a zonal band stretching from 210° to 265°E, away from any steep bottom topography. Within this band these waves propagate in multiple directions. Some of the waves’ characteristics, such as the distance between wave crests, crest lengths, and time scales, are estimated from satellite observations. In total we identify 116 ISW trains during one full year (2020), with typical distances between crests of 1500 m and crest lengths of hundreds of kilometers. These ISW trains appear to be generated by buoyant gravity currents having sharp fronts detectable in thermal infrared satellite images. A 2D numerical model confirms that resonantly generated nonlinear internal waves with amplitudes of O(10) m may be continuously initiated at the fronts of advancing gravity currents. Significance Statement Satellite imagery reveals the repeated occurrence of internal solitary waves in the near-equatorial region of the east Pacific, despite the absence of topography. These waves appear to be resonantly generated over the sheared Equatorial Undercurrent by gravity currents that propagate as frontal zones of 1000-km scale tropical instability waves, providing a physical link with viscous mixing scales.

Fiber‐Optic Observations of Internal Waves and Tides

AbstractAlthough typically used to measure dynamic strain from seismic and acoustic waves, Rayleigh‐based distributed acoustic sensing (DAS) is also sensitive to temperature, offering longer range and higher sensitivity to small temperature perturbations than conventional Raman‐based distributed temperature sensing. Here, we demonstrate that ocean‐bottom DAS can be employed to study internal wave and tide dynamics in the bottom boundary layer, a region of enhanced ocean mixing but scarce observations. First, we show temperature transients up to about 4 K from a power cable in the Strait of Gibraltar south of Spain, associated with passing trains of internal solitary waves in water depth <200 m. Second, we show the propagation of thermal fronts associated with the nonlinear internal tide on the near‐critical slope of the island of Gran Canaria, off the coast of West Africa, with perturbations up to about 2 K at 1‐km depth and 0.2 K at 2.5‐km depth. With spatial averaging, we also recover a signal proportional to the barotropic tidal pressure, including the lunar fortnightly variation. In addition to applications in observational physical oceanography, our results suggest that contemporary chirped‐pulse DAS possesses sufficient long‐period sensitivity for seafloor geodesy and tsunami monitoring if ocean temperature variations can be separated.

Modulation Effects of Internal-Wave Evolution on Acoustic Modal Intensity Fluctuations in a Shallow-Water Waveguide

Internal solitary waves evolving with time in shallow water are known to affect sound propagation significantly. Unlike prior work studying the acoustic effects of individual internal-wave properties separately, this paper elucidates and evaluates the influence of a complete evolution process of internal waves on acoustic fields both theoretically and by the coupled ocean-acoustic simulation. Two evolving wave properties considered here are shape deformations including the variations of wave amplitudes and widths and packet dispersion manifested as the increasing wavelength (i.e., the distance between successive solitons). The acoustic modal intensity expressed by the Dyson series solution is reformulated to explicitly reveal the modulation effects induced by the deformation and dispersion of internal waves. Dispersion leads to modal interference and causes the intensity envelope to oscillate with the varying wavelength. Deformation modulates intensity in a non-oscillatory manner that is less predictable due to the complexity of amplitude and width variations. In the environment reconstructed from the field observations of internal waves in the South China Sea, the modal intensity simulated by the parabolic-equation model exhibits pronounced modulation effects, where the modal interference due to dispersion dominates the intensity-envelope shape, and deformation affects the extremum positions of envelopes.

The role of seasonal circulation in the variability of dynamic parameters of internal solitary waves in the Sulu Sea

In-situ observations of the Sulu Sea internal solitary waves (ISWs) are too limited to reveal their variability. In this study, based on verified model reanalysis data, we study the variability of three basic dynamic parameters of ISWs, i.e., linear phase speed, nonlinear and dispersive coefficients, affected by the Sulu Sea circulations. Overall, the linear speed and dispersive coefficient of ISWs modulated by the annual-mean Sulu Sea circulation decrease, while the nonlinear coefficient increases, and these variations are largely attributed to the variable stratification rather than current. Particularly, at the deep basin and near the western boundary of the Sulu Sea, the modulated climatological annual-mean linear phase speed reduces at least ∼10% of that resulted from only topography, while at the deep basin the modulated dispersive (nonlinear) coefficient has a significant decrease (increase) up to approximately 25–30% (20–30%) relative to that resulted from only topography. Moreover, the first EOF mode of the three ISW dynamic parameters, dominant in winter and summer, reflects the modulation of the Sulu Sea basin-scale circulation, while the second EOF mode, dominant in spring and autumn, illustrates the modulation associated with the meander flow along the western boundary. Specifically, it is found that the seasonal cycles of linear phase speed (nonlinear and dispersive coefficients) are largely attributed to the variable current (stratification) induced by the seasonal circulations. The circulation-induced peak-to-peak fluctuations of linear speed, nonlinear and dispersive coefficients reach up to ∼15%, ∼30% and ∼20%, respectively, of those resulted from only topography.

Observation of Near-Inertial Internal Gravity Waves in the Southern South China Sea

Two sets of more than 850 days of mooring records and satellite altimeter data are used to explore the features and spatiotemporal evolution of near-inertial waves (NIWs) near Nansha Island in the southern South China Sea (SCS). The observed NIWs are dominated by clockwise (downward energy propagation) motions and show a clear blue shift with a distinct peak frequency of 1.09 f during two large NIW events. The near-inertial kinetic energy (NIKE) is primarily concentrated in the upper layer and radiated downward. The largest value of depth-integrated NIKE reaches 3.5 KJ/m2. Besides, the NIWs are dominated by the first three modes, which account for 80% of the total NIKE. Moreover, the depth-integrated NIKE exhibits an apparent seasonal variation, with the largest NIKE in winter, which is almost three times larger than that in other seasons. Every large NIKE event is attributed to the passage of storms and is dominated by mode-2 NIWs. The dominance of the mode-2 NIWs is likely caused by the interaction between NIWs and mesoscale eddies.

Internal Wave and Turbulence Observations with Very High-Resolution Temperature Sensors along the Cabauw Mast

Abstract Knowledge about the characteristics of the atmospheric boundary layer is vital for the understanding of redistribution of air and suspended contents that are particularly driven by turbulent motions. Despite many modeling studies, detailed observations are still demanded of the development of turbulent exchange under stable and unstable conditions. In this paper, we present an attempt to observationally describe atmospheric internal waves and their associated turbulent eddies in detail, under varying stable conditions. Therefore, we mounted 198 high-resolution temperature (T) sensors with 1-m spacing on a 200-m-long cable. The instrumented cable was attached along the 213-m-tall meteorological mast of Cabauw, Netherlands, during late summer 2017. The mast has standard meteorological equipment at extendable booms at six levels in height. A sonic anemometer is at 60 m above ground. The T sensors have a time constant in air of τa ≈ 3 s and an apparent drift about 0.1°C month−1. Also due to radiation effects, short-term measurement instability is 0.05°C h−1 during nighttime and 0.5°C h−1 during daytime. These T-sensor characteristics hamper quantitative atmospheric turbulence research, due to a relatively narrow inertial subrange of maximum one order of magnitude. Nevertheless, height–time images from two contrasting nights show internal waves up to the buoyancy period of about 300 s, and shear and convective deformation of the stratification over the entire 197-m range of observations, supported by nocturnal marginally stable stratification. Moderate winds lead to 20-m-tall convection across weaker stratification, weak winds to episodic <10-m-tall shear instability across larger stratification.

Non-tidal superinertial internal waves in a short microtidal submarine canyon

This study describes observations of non-tidal Superinertial Internal Waves (SIWs) for the first time in Jolúcar Canyon, which is a short prototype canyon in the Alborán Sea (western Mediterranean). The analysis, based on current velocities and acoustic echo-amplitude signals, identified non-tidal SIWs and their vertical and temporal variability. Spectral analysis of the echo anomaly and the vertical component of velocity disclosed internal oscillations corresponding to a period of about 24–36 minutes (frequency band of 40–60 cpd). Power spectra Hovmöller diagrams derived from the wavelet amplitude of the echo anomaly and its vertical gradient at this frequency band showed that non-tidal SIWs were mainly found at 50 and 100 m depth, where stratification was strongest. This type of waves could be generated by the canyon walls, in the along-canyon direction. Remarkably, these submesoscale internal waves were not permanent and only observed during daytime. Based on similar observations in the Alborán Sea, it is hypothesized that this behavior is induced by planktonic organisms, which may disrupt the water column structure during nighttime when they migrate to feed.

Короткопериодные внутренние волны на крутом шельфе Черного моря летом 2018 года

The results of the analysis of data on observation of short-period internal waves on the northeastern shelf of the Black Sea in June 2018 are presented. The study area is distinguished by a narrow shelf with a sharp drop in depths, the bottom slope is 23°. The measurements were carried out from the stationary marine platform of The Institute of Ecology of the Academy of Sciences of the Abkhazian Republic, where the depth of the site reaches 13 m. An anchored station was installed, equipped with autonomous point temperature sensors at a depth of 52 m. At the platform, the variability of the sea temperature structure was monitored using point temperature sensors, an autonomous line temperature sensor (LTS), and vertical hourly profiling using a probe. In addition, coastal currents were measured by the acoustic Doppler current profiler ADCP equipped near the platform. As a result of the analysis, a frequent approach of near-inertial internal waves close to the coast was revealed. Trains of short-period internal waves and solitary waves related to the first mode, as well as individual manifestations of internal waves of the second mode, were recorded. Examples of observed waves and their parameters are given. During the entire observation period, internal waves with a period of 5–10 min and a height of 1–2 m were most often recorded. Solitary waves were distinguished by a large period and height. It was found that short-period internal waves at the measurement site are predominantly tied to near-inertial internal waves which approach the coast.

Long lived second mode internal solitary waves in the Andaman Sea

Internal waves are density oscillations propagating along the ocean’s inner stratification, which are now acknowledged as a key constituent of the ocean’s dynamics. They usually result from barotropic tides, which flow over bottom topography, causing density oscillations to propagate along the pycnocline with a tidal frequency (i.e. internal tides). These large-scale waves propagate away from their forcing bathymetry and frequently disintegrate into nonlinear short-scale (higher-frequency) internal wave packets. Typically, short-scale internal wave observations in the ocean are associated with vertical structures (in the water column) of the lowest fundamental mode. Higher vertical modes have recently been documented as well, but these are commonly short-lived (up to a few hours). However, unprecedented satellite images showing long-lived short-scale mode-2 internal waves have now been documented in the Andaman Sea, and we report here the companion results of a non-hydrostatic and fully nonlinear numerical model. The simulations reproduce the waves’ main characteristics as observed in satellite imagery, and the results suggest a resonant coupling with a larger-scale mode-4 internal tide as an explanation for their long-lived character.

Characteristics of Short-Period Internal Waves in the Avacha Bay Based on the In Situ and Satellite Observations in August-September, 2018

Purpose. The paper is aimed at evaluating the characteristics of short-period internal waves in different regions of the Avacha Bay in the Рacific Ocean concerning their hydrological and morphometric conditions. Methods and Results. The characteristics of internal waves are assessed through synthesizing the results of the in situ studies in the Avacha Bay in August-September, 2018, the high-resolution remote sensing data and the results of tidal modeling. The data of the in situ and satellite observations of internal waves were also directly compared. The results show that in the shallow part of the Avacha Bay, the waves, whose heights are from 10 to 15 m were observed. They constitute 10 % of the total number of cases. In the deep-water part of the bay, the internal waves are also often observed, but their maximum height does not exceed 10 m. The satellite images show 72 manifestations of short-period internal waves. Some of them spread to the coast with a tidal frequency from the generation source located around the 500 m isobath where the bottom abruptly slows down. Conclusions. The results of the investigation revealed a pronounced relationship between the wave trains recorded in the shallow-water area and the semidiurnal tidal dynamics. Analysis of the hydrological situation and the satellite images permitted to assume that the internal waves could be generated as a result of not only a barotropic tide collapse, but also due to the inertial oscillations of the frontal zone formed by the Kamchatka current meanders in the presence of a shallow sharpened pycnocline. Having been analyzed, the synchronous satellite and in situ observations made it possible to find out that the internal waves of the 5–8 m height were distinctly manifested on the sea surface in case the pycnocline depth was 10–20 m.

Observation of internal waves with OLCI and SRAL on board Sentinel-3

The ocean and land color instrument (OLCI) and synthetic aperture radar altimeter (SRAL) installed aboard the Sentinel-3 satellite have been in orbit for operational uses. In this study, data collected from Sentinel-3 are used to investigate internal waves in the South China Sea. An internal wave is detected using an OLCI image with a resolution of 300 m, and an analysis was performed with a quasi-synchronous moderate-resolution imaging spectroradiometer (MODIS) image. The opposite characteristics of OLCI and MODIS images of the same internal wave are explained by the critical angle in brightness reversals. The unique observational geometry of the OLCI image and its influence on observations of internal waves are discussed. The distribution of σ0 and sea surface height anomalies (SSHAs) induced by internal waves are studied using SRAL records. The σ0 records of SRAL occasionally show less sensitivity to the modulation of internal waves, which may be attributed to the observational geometry, while SSHAs show obvious variations. The synchronous pairing of OLCI images and SRAL records are analyzed to extract the three-dimensional sea surface signatures induced by internal waves. The analysis demonstrates that the profile of SSHAs in the surface shows an opposite phase to the profiles of internal waves in the ocean. The opposite phase relationship, observed in the remote sensing view, is also confirmed with a laboratory experiment.

Synthetic observations of internal gravity waves in the solar atmosphere

Aims. We study the properties of internal gravity waves (IGWs) detected in synthetic observations that are obtained from realistic numerical simulation of the solar atmosphere. Methods. We used four different simulations of the solar magneto-convection performed using the CO5BOLD code. A magnetic-field-free model and three magnetic models were simulated. The latter three models start with an initial vertical, homogeneous field of 10, 50, and 100 G magnetic flux density, representing different regions of the quiet solar surface. We used the NICOLE code to compute synthetic spectral maps from all the simulated models for the two magnetically insensitive neutral iron lines Fe I λλ 5434 Å and 5576 Å. We carried out Fourier analyses of the intensity and Doppler velocities to derive the power, phase, and coherence in the kh − ω diagnostic diagram to study the properties of internal gravity waves. Results. We find the signatures of the internal gravity waves in the synthetic spectra to be consistent with observations of the real Sun. The effect of magnetic field on the wave spectra is not as clearly discernible in synthetic observations as in the case of numerical simulations. The phase differences obtained using the spectral lines are significantly different from the phase differences in the simulation. The phase coherency between two atmospheric layers in the gravity wave regime is height dependent and is seen to decrease with the travel distance between the observed layers. In the studied models, the lower atmosphere shows a phase coherency above the significance level for a height separation of ∼400 km, while in the chromospheric layers it reduces to ∼100–200 km depending on the average magnetic flux density. Conclusion. We conclude that the energy flux of IGWs determined from the phase difference analysis may be overestimated by an order of magnitude. Spectral lines that are weak and less temperature sensitive may be better suited to detecting internal waves and accurately determining their energy flux in the solar atmosphere.

Internal solitary wave observations in the Flores Sea using the Himawari-8 geostationary satellite

ABSTRACT The Japanese Meteorological Agency’s geostationary satellite Himawari-8/Advanced Himawari Imager (AHI) images, acquired over the Flores Sea in Indonesia, observed that long internal solitary waves (ISWs) seem to be generated from the Sape Strait, which then propagates through the Java Sea, the Sumba Strait, and the Savu Sea. Shallow water wave refraction behaviour is present during the observations. These important parameters related to the ISW propagation (wavelength, soliton number, first crest length, propagation direction, and phase speed) could also be estimated using Himawari-8 images. These observational results were also compared with Moderate Resolution Imaging Spectroradiometer (MODIS) images, Sentinel-1A Synthetic Aperture Radar (SAR) images, and the Korteweg-de Vries (KdV) equation. The geostationary satellite Himawari-8 showed great potential and was very helpful in observing the ISW propagation parameters, spatially and temporally, in the Flores Sea in detail.

Internal tsunami waves transport sediment released by underwater landslides

Accelerated by gravity, submarine landslides transfer energy to the marine environment, most notably leading to catastrophic tsunamis. While tsunamis are thought to use less than 15% of the total energy released by landslides, little is known about subsurface processes comprising the rest of their energy budgets. Here, we analyze the first set of observations depicting a lake’s interior response to underwater landslides and find that sediment transport is modulated by baroclinic waves that propagate along vertical gradients in temperature and sediment concentration. When traveling along a shallow thermocline, these waves can reach past topographic features that bound turbidity currents and thus expand the influence area of underwater landslides. With order of magnitude calculations, we estimate that observed thermocline internal waves received roughly 0.7% of available landslide energy and infer their contribution to homogenize the lake’s thermodynamical properties by means of turbulent mixing. Lastly, we show that landslides in our data set modified the lake’s intrinsic dynamical modes and thus had a permanent impact on its circulation. This suggests that measurements of subsurface wave propagation are sufficient to diagnose bathymetric transformations. Our experiment constitutes the first direct observation of both internal tsunami waves and turbidity current reflection. Moreover, it demonstrates that background density stratification has a significant effect on the transport of sediment after submarine landslides and provides a valuable reference for numerical models that simulate submarine mass failures.

Baza dannyh nablyudeniy vnutrennih voln v Mirovom okeane

Purpose. The purpose of the paper is to describe the permanently updated public database containing the information on observations of the internal waves, as well as the scientific sources on the corresponding themes. The observations are based on the data obtained by remote sensing and direct contact measurements in various areas of the World Ocean and in the inland basins (lakes, reservoirs). Methods and Results. The data from 503 literature sources on the observations of internal waves are structured. The structure, format, volume and current content of the database are considered; the stored information is analyzed. Described is the process of adding and viewing the records using the IGWAtlas web application (the online project for working with the database of the observations of internal waves in the oceans and for public access to observations and sources that has an intuitive user interface). Geographical distribution of the recorded observations and their dependence on a season are illustrated. The examples of images of various types of records contained in the database, as well as their distribution according to the types of observations are given. The considered in the paper database is integrated into the IGWResearch software package intended for numerical simulation of propagation and transformation of the internal waves in the World Ocean. Conclusions. The database contains the materials for 1972–2018 on 2296 recorded manifestations of the internal waves, which correspond to 2465 images, namely device records, satellite images, graphs, maps and tables. The database scope includes geographic information systems, statistical analysis, knowledge bases, web-services for the tasks of the World Ocean research.

The vertical heat transport of internal solitary waves over the continental slope in the northern South China Sea

An integrated analysis of internal solitary wave (ISW) observations obtained from two moorings over the continental slope in the northern South China Sea (SCS) leads to an assessment of the vertical heat transport of the ISWs. The clusters of ISW packets are phase-locked to the fortnightly cycle of the semidiurnal tide. The ISWs appear during large semidiurnal tides, and there is a period of 5–6 d when no ISWs are observed. The effect of the ISWs on the continental slope heat budget is observed. The ISWs can modify a local temperature field in which the temperature in the upper layer can be changed by O (100) °C after the ISWs passed the mooring. Both ISWinduced diffusion and ISW-induced advection contribute to the temperature variation. The estimates imply an average vertical heat flux of 0.01 to 0.1 MW/m2 in the ISWs in the upper 500 m of the water column. The vertical heat transport ranges from 0.56 to 2.83 GJ/m2 with a mean value of 1.63 GJ/m2. The observations suggest that the vertical heat transport is proportional to the maximum vertical displacement.

Year-round observations of sea-ice drift and near-inertial internal waves in the Northwind Abyssal Plain, Arctic Ocean

In this study, intra-annual variation of near-inertial internal wave (NIW) in the Arctic Ocean is examined using year-round mooring in the Northwind Abyssal Plain. Our emphasis is on dynamical responses of NIW to local sea-ice variables such as concentration, draft, and drift. We obtained those using a coupling system of ice profiling sonar (IPS) and an acoustic Doppler current profiler (ADCP) deployed at the top of the mooring. According to the wavelet spectrum, the inertial oscillation of ice drift becomes considerably strong during periods of ice formation and decay. Results show that the NIW amplitude in the upper part of the water column responds more sensitively to the sea-ice inertial oscillation than to the mean component of ice drift heading to the northwest. We also conducted an experiment with a mixed-layer slab model using the IPS-ADCP measured ice speed to examine the NIW generation responding to the ice-to-ocean stress. Experiment results suggest that the mixed-layer inertial oscillation is amplified in the early time of ice formation, through the ice-water resonance process. It is then concluded that the mixed-layer inertial current driven by ice drift is the primary driver of the enhanced NIW generation.

Observations of internal waves generated by an anticyclonic eddy: a case study in the ice edge region of the Greenland Sea

Internal waves in the ocean play an important role in turbulence generation due to wave-breaking processes and mixing of the ocean. Airborne radar images of internal waves and ocean eddies north of Svalbard suggested that ocean eddies could generate internal waves. Here, we test this hypothesis using data from a dedicated internal wave experiment in the Greenland Sea. Internal waves with dominant frequencies of 1–3 cycles per hour and amplitudes up to 15 m were observed using three thermistor chains suspended from a drifting array conveniently placed on the ice in a triangle with sides of several km. Analysis shows that internal waves propagated westwards with a speed of about 0.2 m/s and wavelength of 0.4–1.0 km, away from an anticyclonic ocean eddy located just east of the array. This was consistent with the remote-sensing observations of internal waves whose surface signature was imaged by an airborne radar in the western part of this eddy, and with theories that eddies and vortexes can directly generate internal waves. This case study supports our hypothesis that ocean eddies can be the direct sources of internal waves reported here for the first time and not only enhancing the local internal wave field by draining energy from the eddies, as studied previously. The present challenge is to explore the role of eddies as a new source in generating internal waves in the global ocean.