Observation Of Internal Solitary Waves Research Articles

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.

Numerical study on the surface signature of internal solitary waves

The propagation of internal solitary waves (ISWs) underwater and their interactions with surface waves can affect the sea surface state and generate distinct surface signatures of bright–dark striped patterns in remote sensing imagery. These patterns have been one of the most important methods for wide coverage detection and observation of ISWs. This study establishes a fully nonlinear numerical model for ISW–surface wave interaction simulation based on the multi-domain boundary element method. By numerical simulation, the formation mechanism, characteristics, and influencing factors of ISW surface signatures are revealed. The physical essence of the ISW surface signature is the spatially inhomogeneous wavelength and roughness changes in the surface waves due to the convergence and divergence effects of the ISW. Based on the surface wave wavelength changes, the surface area affected by an ISW is divided into three regions from front to back—the converging, diverging, and restoring regions—corresponding to the regions where the surface waves are compressed, stretched, and recover, respectively. By comparing different cases, it is found that the ISW amplitude, surface wave amplitude, and surface wave wavelength are the main factors influencing the ISW surface signature. ISWs with larger amplitudes have stronger convergence and divergence effects on surface waves, making the overall scale of the ISW surface signature larger. Larger surface wave amplitudes provide stronger resistance to the ISW divergence effect and lead to smaller diverging regions, and larger surface wave wavelengths make the converging region larger and the diverging and restoring regions smaller.

Study of internal wave characteristics off northwest Bay of Bengal using synthetic aperture radar

Internal solitary waves (IWs) in the northwest Bay of Bengal were studied using ENVISAT ASAR imageries. Simultaneous in situ observations were also carried out to study the characteristics of the internal waves. Internal wave features have often been observed over the ocean area between 18.5°–19.5° N and 84.5°–85.5° E. The direction of propagation of IWs is generally toward the coast with wavelengths ranging from 0.4 to 1.1 km. The phase speeds of the IWs vary from 0.12 to 1.3 m/s. Internal wave amplitudes computed using Korteweg and de Vries model show good agreement with in situ data. Based on their direction of propagation and time series analysis, IWs generated were found to be of tidal forcing origin over local topographic features. IWs of different types like wave packets or wave trains and single solitons of weak manifestations were observed depending on the phase and the tidal amplitude as well as the stratification of the ocean, which in turn vary due to the dynamic and complex nature of Bay of Bengal, as it experiences strong monsoon winds and low pressure systems with calm weather in between them. The internal waves are often observed during non-monsoon period over this region. During monsoon period, continuous vertical mixing takes place due to strong winds causing unfavorable conditions for IW formation and so the observation of IWs during monsoons is rather rare.

SAR-Mode Altimetry Observations of Internal Solitary Waves in the Tropical Ocean Part 2: A Method of Detection

It is demonstrated that the synthetic aperture radar altimeter (SRAL) on board of the Sentinel-3A can detect short-period internal solitary waves (ISWs) with scales of the order of a kilometer. A variety of signatures owing to the surface manifestations of the ISWs are apparent in the SRAL Level-2 products over the ocean. These signatures are identified in several geophysical parameters, such as radar backscatter ( σ 0 ) and sea level anomaly (SLA). Radar backscatter is the primary parameter in which ISWs can be identified owing to the measurable sea surface roughness perturbations in the along-track direction resulting from the sharpened SRAL footprint. The SRAL footprint is sufficiently small (300 m in the along-track direction) to capture radar power fluctuations over successive wave crests and troughs, which produce rough and slick surface patterns arrayed in parallel bands with scales of a few kilometers along-track. Furthermore, it was possible to calculate the mean square slope ( s 2 ¯ ) for the dual-band (Ku and C bands) altimeter of Sentinel-3, which made the ISW signatures unambiguously identified because of the large s 2 ¯ variations in exact synergy with ocean and land color instrument (OLCI) images. Hence, the detection method is validated in cloud-free sun glint OLCI images. It is shown that both σ 0 and SLA yield realistic estimates for routine observation of ISWs with the SRAL. The detection method that is used relies on the parameter s 2 ¯ which is calculated from σ 0 . This is a significant improvement from previous observations recently reported for conventional pulse-limited altimeters (Jason-2). An algorithm is developed to be used in any ocean region. Wavelets were applied for a first analysis of the s 2 ¯ variations because ISWs can be readily identified in high-frequency signals. Other geophysical parameters such as SLA were used to exclude phenomena that are unlikely to be ISWs.

Observation of Internal Solitary Waves Using an Underwater Glider in the Northern South China Sea

ABSTRACT Ma, W.; Wang, Y.; Yang, S.; Wang, S., and Xue, Z., 2018. Observation of internal solitary waves using an underwater glider in the northern South China Sea. Internal solitary waves (ISWs) are frequently observed in the ocean, in particular in the South China Sea. Petrel-II, the autonomous underwater glider developed by Tianjin University, conducted ISW observations in the northern South China Sea in August 2017. With the payload conductivity, temperature, and depth, Petrel-II can obtain the repeating thermohaline profiles from the surface to nearly 1500-m depth. Combined with the analysis of a moderate-resolution imaging spectroradiometer true-color image, it is verified that the thermohaline fluctuation is caused by the ISWs. The features of the ISW are estimated by calculation of vertical velocity using a glider flight model. The ISW captured by Profile-46 of the glider was a depression wave, with a period of about 24 minutes and an amplitude of about 127 m. The application of underwater gliders...

SAR Mode Altimetry Observations of Internal Solitary Waves in the Tropical Ocean Part 1: Case Studies

It is well known that internal waves (IWs) of tidal frequency (i.e., internal tides) are successfully detected in sea surface height (SSH) by satellite altimetry. Shorter period internal solitary waves (ISWs), whose periods (and spatial scales) are an order of magnitude smaller than tidal internal waves, have been generally assumed too small to be detected with conventional altimeters. This is because conventional (pulse-limited) radar altimeter footprints are somewhat larger than or of similar size, at best, as the typical wavelengths of the ISWs. Here we demonstrate that the synthetic aperture radar altimeter (SRAL) on board the Sentinel-3A can detect short-period ISWs. A variety of signatures owing to the surface manifestations of the ISWs are apparent in the SRAL Level-2 products over the ocean. These signatures are identified in several geophysical parameters, such as radar backscatter (sigma0), sea level anomaly (SLA), and significant wave height (SWH). Radar backscatter is the primary parameter in which ISWs can be identified owing to the measurable sea surface roughness perturbations in the along-track sharpened SRAL footprint. The SRAL footprint is sufficiently small to capture radar power fluctuations over successive wave crests and troughs, which produce rough and slick surface patterns arrayed in parallel bands with scales of a few kilometers. The ISW signatures are unambiguously identified in the SRAL because of the exact synergy with OLCI (Ocean Land Colour Imager) images, which in cloud-free conditions allow clear identification of the ISWs in the sunglint OLCI images. We show that both sigma0 and SLA yield realistic estimates for routine observation of ISWs with the SRAL, which is a significant improvement from previous observations recently reported for conventional pulse-limited altimeters (Jason-2). Several case studies of ISW signatures are interpreted in light of our knowledge of radar backscatter in the internal wave field. An analysis is presented for the tropical Atlantic Ocean off the Amazon shelf to infer the frequency of the phenomena, being consistent with previous satellite observations in the study region.

Modelling of Polarity Change in a Nonlinear Internal Wave Train in Laoshan Bay

AbstractThere are now several observations of internal solitary waves passing through a critical point where the coefficient of the quadratic nonlinear term in the variable coefficient Korteweg–de Vries equation changes sign, typically from negative to positive as the wave propagates shoreward. This causes a solitary wave of depression to transform into a train of solitary waves of elevation riding on a negative pedestal. However, recently a polarity change of a different kind was observed in Laoshan Bay, China, where a periodic wave train of elevation waves converted to a periodic wave train of depression waves as the thermocline rose on a rising tide. This paper describes the application of a newly developed theory for this phenomenon. The theory is based on the variable coefficient Korteweg–de Vries equation for the case when the coefficient of the quadratic nonlinear term undergoes a change of sign and predicts that a periodic wave train will pass through this critical point as a linear wave, where a phase change occurs that induces a change in the polarity of the wave, as observed. A two-layer model of the density stratification and background current shear is developed to make the theoretical predictions specific and quantitative. Some numerical simulations of the variable coefficient Korteweg–de Vries equation, and also the extended variable coefficient Korteweg–de Vries equation, are reported that confirm the theoretical predictions and are in good agreement with the observations.

Seismic, satellite, and site observations of internal solitary waves in the NE South China Sea.

Internal solitary waves (ISWs) in the NE South China Sea (SCS) are tidally generated at the Luzon Strait. Their propagation, evolution, and dissipation processes involve numerous issues still poorly understood. Here, a novel method of seismic oceanography capable of capturing oceanic finescale structures is used to study ISWs in the slope region of the NE SCS. Near-simultaneous observations of two ISWs were acquired using seismic and satellite imaging, and water column measurements. The vertical and horizontal length scales of the seismic observed ISWs are around 50 m and 1–2 km, respectively. Wave phase speeds calculated from seismic observations, satellite images, and water column data are consistent with each other. Observed waveforms and vertical velocities also correspond well with those estimated using KdV theory. These results suggest that the seismic method, a new option to oceanographers, can be further applied to resolve other important issues related to ISWs.

Mooring observations of internal solitary waves in the deep basin west of Luzon Strait

A total of 137 internal solitary waves (ISWs) are captured during a field experiment conducted in the deep basin west of the Luzon Strait (LS) from March to August, 2010. Mooring observations reveal that a fully developed ISW owns a maximum westward velocity of more than 1.8 m/s and an amplitude of about 200 m. The ISWs in the South China Sea (SCS) are most active in July, which may be due to the strong stratification in summer. Most of the ISW episodes are detected around and after the 1st or 15th lunar day, indicating that the ISW in the SCS is triggered by astronomic tides. Half part of the observed ISWs were detected around 19:00 local time, which can be explained by the fact that type-a ISWs emerged in the evening at roughly the same time each day. The propagation direction of the ISWs and the astronomic tides in the LS show that the area south of the Batan Island is probably the main source region of the type-a ISWs, while the area south of Itbayat Island and south of the Batan Island is likely the main source region of the type-b ISWs observed at the mooring. Moreover, for the resonance of semidiurnal internal tides emitting from the double ridges in the LS, the underwater ridge south of the Itbayat Island and south of the Batan Island is believed to favor the generation of the energetic ISWs.

Soliton generation by internal tidal beams impinging on a pycnocline: laboratory experiments

AbstractIn this paper, we present the first laboratory experiments that show the generation of internal solitary waves by the impingement of a quasi-two-dimensional internal wave beam on a pycnocline. These experiments were inspired by observations of internal solitary waves in the deep ocean from synthetic aperture radar (SAR) imagery, where this so-called mechanism of ‘local generation’ was argued to be at work, here in the form of internal tidal beams hitting the thermocline. Nonlinear processes involved here are found to be of two kinds. First, we observe the generation of a mean flow and higher harmonics at the location where the principal beam reflects from the surface and pycnocline; their characteristics are examined using particle image velocimetry (PIV) measurements. Second, we observe internal solitary waves that appear in the pycnocline, detected with ultrasonic probes; they are further characterized by a bulge in the frequency spectrum, distinct from the higher harmonics. Finally, the relevance of our results for understanding ocean observations is discussed.

SAR observations of internal solitary waves generated at the Estremadura Promontory off the west Iberian coast

A comprehensive dataset of Synthetic Aperture Radar (SAR) images from the west Iberian coast is used to identify the Estremadura Promontory as an oceanic hotspot for internal solitary waves. The full two-dimensional structure and the main physical properties of these waves are revealed for the first time. Composite maps and a statistical analysis show that these waves have two main propagation paths coming from the northern and southern slopes of the promontory, and that on average they have horizontal scales of the order of 400m and crestlengths around 9km. The overall propagation of the internal solitary wave field observed in the SAR images is evaluated, using a proxy measure for energy based on observed wavelengths, crestlengths and radar backscatter intensities. A refraction pattern is found to divert the meridionally propagating waves to the east, where they eventually dissipate in the inner-shelf regions close to shore. The barotropic body force is calculated and the most likely generation locations are identified. It is found that there is more energy emanating from the south while the tidal forcing seems to be equally divided between both flanks of the promontory. Calculations using the vertical excursion inverse Froude number (defined by Legg and Klymak, 2008) indicate that this is probably related to the steeper slopes located in the northern side. This pronounced bathymetry is more likely to generate internal hydraulic jumps and overturning, which may inhibit the disintegration of the internal tide into solitary waves.

Internal solitary waves in the Coastal Mixing and Optics 1996 experiment: Multimodal structure and resuspension

Observations of internal solitary waves (ISWs) and of their role in sediment resuspension during the Coastal Mixing and Optics 1996 (CMO 96) experiment are reported. The largest resuspension events observed in the experiment can be related to retarded flow under the wave footprint. Two distinctly different periods of resuspension events could be distinguished based on the prevailing atmospheric conditions: a calm period, which we consider characteristic of nominal conditions, and an energetic period, during which two major storms occurred. ISWs arrived at the mooring site from a variety of sources, though a common intermodal dynamics seemed to occur repeatedly. Both mode 1 and mode 2 ISWs have been observed. The present analysis of the month‐long portion of the CMO 96 data set constitutes the first reported observations, insofar as we are aware, of mode 2 waves on the continental shelf. Both mode 1 and mode 2 ISWs were found to stimulate resuspension. The mode 2 waves seem to be generated locally by resonant or topographic interactions with mode 1 ISWs. Internal wave fields of the type described here are expected to exist in a variety of other shallow seas as well.

Long solitary internal waves in stable stratifications

Abstract. Observations of internal solitary waves over an antarctic ice shelf (Rees and Rottman, 1994) demonstrate that even large amplitude disturbances have wavelengths that are bounded by simple heuristic arguments following from the Scorer parameter based on linear theory for wave trapping. Classical weak nonlinear theories that have been applied to stable stratifications all begin with perturbations of simple long waves, with corrections for weak nonlinearity and dispersion resulting in nonlinear wave equations (Korteweg-deVries (KdV) or Benjamin-Davis-Ono) that admit localized propagating solutions. It is shown that these theories are apparently inappropriate when the Scorer parameter, which gives the lowest wavenumber that does not radiate vertically, is positive. In this paper, a new nonlinear evolution equation is derived for an arbitrary wave packet thus including one bounded below by the Scorer parameter. The new theory shows that solitary internal waves excited in high Richardson number waveguides are predicted to have a halfwidth inversely proportional to the Scorer parameter, in agreement with atmospheric observations. A localized analytic solution for the new wave equation is demonstrated, and its soliton-like properties are demonstrated by numerical simulation.

Satellite observation of internal solitary waves converting polarity

On a SPOT‐3 visible image over the South China Sea continental shelf, two contrasting signatures of internal solitary waves (ISWs) are observed: darker stripes leading brighter stripes in the lower part, and brighter stripes leading darker stripes in the upper part. The authors suggest that the contrasting patterns are caused by ISWs of different polarities. A packet of ISWs is observed in the process of converting polarity due to bottom shoaling, both in the propagation direction and in the transect direction. The evolution process can be obtained by transferring information from the spatial domain to the temporal domain. The conversion process starts with the original depression ISWs passing the 160 m isobath. The evolution process can be divided into two phases: the first is the broadening of the original depression waves and the second is the appearance of the new‐born elevation waves. These two phases are verified using numerical simulation.

Observations of internal solitary waves near the Oregon coast with an inverted echo sounder

Observations of internal solitary waves (ISWs) were made near the Oregon coast with a moored, 200‐kHz inverted echo sounder (IES) and a 12‐element vertical array of thermistors over a 14‐day period during September 1995. Downward pycnocline displacements conforming to the classical Kortweg‐deVries, hyperbolic secant squared profile, with typical amplitudes of 5 to 14 m, propagation speeds of 0.4 to 0.6 m s−1, and duration of 100 to 200 s were observed with both the IES and thermistor array. Pycnocline displacements from 5 to 22 m depth were observed acoustically exploiting the backscatter from Zooplankton layers, which acted as tracers of water motions. The typical volumetric scattering strength of these Zooplankton layers was near −75 dB (referenced to 1 m−1). Within the top 10 m, plumes of microscopic air bubbles injected by breaking surface waves were advected downward by the ISWs, with volumetric scattering strengths in excess of −36 dB. Finally, near‐surface acoustic scattering zones suspected to exhibit backscatter from turbulent microstructure were observed, with typical volumetric scattering strength near −68 dB. These near‐surface zones were coincident with model predictions of large shear instabilities and local gradient Richardson numbers below the critical value for generation of turbulence.

Sediment Resuspension and Mixing by Resonantly Generated Internal Solitary Waves

The observation of internal solitary waves (ISWs) propagating upstream along a strongly stratified bottom layer on the California shelf is reported. An increased concentration of particulates in the water column accompanies the passage of these ISW packets. The estimated local Richardson number in the bottom vicinity is around 1/4, and a vertical coefficient of eddy diffusivity of order 10−2 m2 s−1 is associated with the upstream propagating leading ISW. The leading ISW gave rise to reversed flow in an 8-m layer above the bottom. It is argued that the upstream propagating ISWs were generated by resonant flow over bottom topography. Internal waves generated in this way seem to be frequent in the record of a month-long experiment. Model results suggest that the ISWs can carry up to 73% of such generated long wave energy. The ocean conditions at the site are similar to those of other coastal sites, which suggests that the phenomenon described here may be common.

Comment on ’’Experimental observation of internal solitary waves in a stratified fluid’’

Comment on ’’Experimental observation of internal solitary waves in a stratified fluid’’

Experimental observation of internal solitary waves in a stratified fluid

Propagation of internal solitary waves has been experimentally observed within a region of sudden vertical density variation. The propagation speeds compare well with the theoretical predictions of Benjamin and Davis.