Impact of Internal Solitary Waves on Marine Suspended Particulate Matter: A Review

Abstract

Suspended particulate matter (SPM) plays a pivotal role in marine source-to-sink sedimentary systems. Internal solitary waves (ISWs), a prevalent hydrodynamic phenomenon, significantly influence vertical mixing, cross-shelf material transport, and sediment resuspension. Acting as energetic nonlinear waves, ISWs can disrupt the settling trajectories of suspended particles, enhance lateral transport above the pycnocline, and generate nepheloid layers nearshore. Meanwhile, intense turbulent mixing induced by ISWs accumulates large quantities of SPM at both the leading surface and trailing bottom of the waves, thereby altering the structure and dynamics of the intermediate nepheloid layers. This review synthesizes recent advances in the in situ observational techniques for SPM under the influence of ISWs and highlights the key mechanisms governing their interactions. Particular attention is given to representative field cases in the SCS, where topographic complexity and strong stratification amplify ISWs–sediment coupling. Finally, current limitations in observational and modeling approaches are discussed, with suggestions for future interdisciplinary research directions that better integrate hydrodynamic and sediment transport processes.

1. Introduction

Internal waves are a common phenomenon in the world’s oceans and have significant impacts on marine environments. In recent research, internal waves on continental shelves and slopes have become a hot topic [1]. The propagation of internal tides from the deep sea to continental margin regions gradually evolves into internal waves through interactions with the topography. Internal waves found on continental shelves and slopes are often in the form of ISWs [2].

The composition, morphological characteristics, and distribution dynamics of SPM in seawater are intricately linked to various oceanic processes such as sediment transport, atmospheric deposition, hydrodynamics, marine biological activity, and geological phenomena [3]. Of particular importance are the constituent composition and size distribution of SPM, which serve as fundamental parameters for SPM research. Hydrodynamic conditions within marine environments play a pivotal role in dictating the size distribution of suspended particles. In weakly dynamic conditions, the low-flow marine environment promotes the aggregation of suspended particulate matter, thereby forming larger, settleable aggregates. Conversely, during periods of elevated flow rates, which characterize peak flow periods, the turbulent nature of high-velocity marine environments impedes the aggregation of SPM, leading to the disintegration of aggregates into smaller, less readily settleable entities or primary mineral particles [4].

ISWs, as a significant hydrodynamic phenomenon in the South China Sea, exert a profound influence on the sedimentation patterns of SPM. ISWs induce lateral displacement of downward-sinking SPM and promote horizontal transport of suspended particles towards the coastline above the thermocline [5]. Concurrently, owing to the vigorous turbulence associated with ISWs, SPM predominantly accumulates at the leading surface and trailing bottom of ISWs [6]. Empirical evidence suggests that a substantial proportion of fine suspended sediment in seawater persists in suspension over prolonged periods, forming a mid-water nepheloid layer [7]. During the disintegration phase of ISWs, boundary layer instability and vortex shedding trigger sediment resuspension, contributing to the diffusion of the nepheloid layer [8,9]. Consequently, the nepheloid layer emerges as a principal determinant of SPM diffusion dynamics (Figure 1).

2. Observation Techniques for ISWs and Suspended Particulate Matter

The research methods for ISWs primarily include in situ observations, remote sensing techniques, numerical simulations, and experimental studies. In the early stages of ISW research, significant progress was made with the advent of rapid sampling devices such as the Conductivity–Temperature–Depth (CTD) profiler. Perry and Schimke identified the presence of ISWs in the Andaman Sea during deep-sea flow observations using a CTD profiler [12]. The principle behind this method involves detecting changes in water density structure by measuring variations in physical parameters such as conductivity, temperature, and depth at specific depths within a water column. In 2001, the Asian Seas International Acoustics Experiment conducted observations of ISWs in the northern South China Sea using multiple temperature chains, which provided insights into the propagation and evolution of nonlinear ISWs [13,14]. Temperature chains are used to identify the propagation of ISWs by observing temperature variations at different depths in the ocean. These temperature variations reflect the vertical displacement of ISWs within the water column.

Synthetic Aperture Radar (SAR) and other remote sensing techniques can capture large-scale ocean remote sensing images [15]. Compared to traditional observation methods, this technology offers significant advantages such as all-weather, all-time, all-coverage, and high-resolution capabilities [16]. The principle behind SAR is that when ISWs with large amplitudes pass through a marine area, they create localized areas of intense wave activity on the ocean surface, leading to significant changes in the surface reflection characteristics (Figure 2). Researchers utilize remote sensing images to analyze these differences in reflection characteristics to determine the spatial location of ISWs. Furthermore, it provides a pathway for parameter inversion of ISWs [17,18]. Specifically, researchers extract key parameters of ISWs such as wave speed, wavelength, and amplitude through the hydrodynamic parameters of surface waves and other hydrological data [19,20,21].

In recent years, significant progress has been made in both theoretical and computational research on ISWs. Nonlinear mathematical models have been constructed and utilized to investigate the physical characteristics and propagation mechanisms of ISWs [22,23]. Ning et al. conducted a study on the spatial distribution characteristics of ISWs in the Malacca Strait based on Sentinel-1 and GF-3 remote sensing data [24]. They employed the weakly nonlinear Korteweg−deVries equation to invert and calculate the physical parameters of ISWs. The study revealed that ISWs in the Malacca Strait typically exist in the forms of wave packets and single ISWs, with a maximum crest length of 39 km, an amplitude range of 4.7 m to 23.9 m, and a phase velocity range of 0.26 m/s to 0.60 m/s. Tang et al., combining remote sensing imagery with thermal resistance chain mooring observations, proposed a method to invert the amplitude of internal waves based on the Modified Nonlinear Schrödinger (NLS) equation [25]. They established the relationship between the peak-to-peak distance and characteristic wavelength of internal waves under different nonlinear and dispersive coefficients.

Due to limitations in existing oceanographic survey techniques for tracking the propagation of ISWs, researchers often simulate and investigate the generation and propagation of ISWs in laboratory conditions. One specific method involves constructing a two-layer fluid system consisting of a freshwater layer and a saltwater layer within a wave flume [26,27]. In these experiments, a certain mass of saltwater is slowly moved to one side of a gate, creating a high-water interface. The gate is used to maintain hydrostatic equilibrium on both sides of the fluid. Due to the pressure gradient resulting from the difference in water levels on either side of the gate, moving the gate causes fluid from the higher water level side to flow towards the lower water level side, triggering the generation of ISWs and causing them to propagate within the wave flume.

Currently, significant progress has been made in the development of equipment for observing marine SPM. This equipment not only allows for the measurement of the particle size and concentration distribution of SPM in water but also enables the tracing and tracing of SPM sources [28]. However, to achieve a deeper understanding of the temporal and spatial variations of SPM composition in dynamic marine environments, efficient, real-time, and high-precision in situ observations of SPM are needed.

In recent years, as research on marine SPM has continued to deepen, various measurement methods and instruments suitable for different observation environments have emerged. Instruments for observing SPM mainly include inductive detection, acoustic detection, and optical detection (Figure 3). The Coulter counter uses resistive sensing, which can rapidly detect the particle size distribution and number of SPM in water and is widely used in particle counting [29]. The Acoustic Doppler Current Profiler (ADCP) utilizes acoustic detection, wherein it emits sound waves to detect the composition and distribution of SPM in the water based on their physical characteristics. It further inversely calculates the particle size and concentration values from the echo signals [30]. Optical microscopes utilize optical detection methods and serve as standard instruments for observing the characteristics and composition of SPM [3]. In addition, there are in situ observation instruments based on light scattering and transmission measurements. For example, the AC-S instrument is used to measure the attenuation coefficient and absorption coefficient of water, which characterize the spatial distribution of SPM [31]. Turbidity meters continuously and automatically measure the turbidity of water, which can be used to assess the trend of turbidity over time [32].

In the ocean, SPM primarily consists of coarse particles known as marine snow, which are highly susceptible to fragmentation under external stress [33]. Traditionally, the collection and determination of SPM in the ocean involves collecting on-site water samples for laboratory analysis. However, this approach tends to disrupt the structure of SPM, thus failing to accurately reflect its characteristics. In recent years, the Laser In Situ Scattering and Transmissometry (LISST) instrument has been widely used in research by scholars worldwide due to its ability to determine the concentration, particle size, and other characteristics of SPM without disrupting their structure [34]. However, its measurement capabilities are limited to a finite water layer, and it may provide less detailed characterization of the vertical distribution of SPM concentration. The application of LISST is predominantly concentrated in estuaries, coastal zones, and nearshore areas, with relatively fewer studies conducted in open ocean environments. SPM in water plays a significant role in material transformation and migration within seawater [35].

3. The Interaction Between SPM and ISWs

3.1. The Dynamic Characteristics of SPM in Fluids

The settling velocity of SPM is a critical parameter determining its migration and transformation in water bodies. For suspended particles with smaller diameters (<10 μm), their settling velocity can be calculated using Stokes’ Law [36]. Stokes’ Law is commonly employed to describe the free settling of particles in the absence of external forces. Typically, the horizontal velocity exceeds the settling velocity, indicating that fine suspended particles should deposit far from the river mouth. However, observational data suggest that they tend to deposit near river mouths. Nowacki et al., through field surveys, found that the settling velocity of SPM in river mouths and coastal areas was significantly higher than the Stokes settling velocity calculated based on single-particle calculations [37]. Apart from particle aggregation leading to an increase in particle density and accelerating settling, there may be other mechanisms involved in accelerating particle settling.

Double-diffusive convection and sediment-driven convection are potential mechanisms governing the dynamic processes of SPM in ocean and lake systems. When a layer of higher salinity water is situated beneath a layer of lower salinity water containing SPM, the difference between the rapid diffusion of salt and the relatively slow Brownian diffusion of SPM leads to instability in the density interface, resulting in the phenomenon known as the “salt finger” mechanism [38]. The salt finger mechanism involves continuous diffusion and exchange at the interface due to the density difference between the two layers of fluid, facilitating the downward transport of SPM and the upward transport of salt. This process leads to an increase in net density flux in the upper layer, further enhancing the density gradient, and reducing the buoyancy of SPM below the density interface, thereby accelerating its settling process. The formation of finger-like structures of SPM is correlated with the intensity of convection, which, in turn, is a crucial factor influencing the concentration of SPM and density anomalies. This research has found that when the densities of the upper and lower layers of water are close, double-diffusive convection can generate intense convective phenomena [39]. When the density ratio of the two layers of water approaches 1, the settling velocity of SPM significantly increases, even surpassing the Stokes velocity by an order of magnitude [40].

In a fluid medium, the secondary process occurring beneath a layer of water containing SPM is referred to as sediment-driven convection. Under the pattern of sediment-driven convection, the settling rate of SPM is also greater than the Stokes velocity, especially when SPM is in a state of dense fluid plumes, exhibiting higher settling velocities [41]. Additionally, Meiburg et al. indicate that when a fluid rich in SPM overlays a layer of high-density saline solution, the settling of SPM at the density interface forms a gravitationally unstable accumulation density distribution, triggering Rayleigh–Taylor instability at the leading edge of the SPM [42]. Figure 4 illustrates this process, where sediment-driven convection enhances the settling velocity of SPM plumes beneath the density interface. This process only occurs when SPM settles steadily and slowly downward at its terminal velocity, and there should be no convection above the density interface, although weak backflow of interstitial water with low velocity cannot be ruled out [43]. Furthermore, apart from a minor diffusive flux of salt, there is no significant double-diffusive convection leading to substantial vertical transport of salt to the upper layer of water. Since the convection in the lower layer of water is driven by the flux of SPM at the density interface, the intensity of convection depends only on the difference in settling velocities and SPM concentrations between the two layers. Importantly, sediment-driven convection is independent of salinity gradients.

The above studies mainly investigated the dynamic characteristics of suspended particles in fluids through experimental methods, without considering the dynamic changes in natural fluid systems such as oceans and lakes. The conclusions drawn from these studies are only applicable under static conditions. However, it is possible to enhance simulations of hydrodynamic processes in experiments to measure the turbulent characteristics of layers containing suspended particles and saline layers in two-layer fluid systems. From these measurements, one can determine the influence of hydrodynamics-induced turbulence on double-diffusive convection or settling-driven convection. Qualitative and quantitative analyses of these dynamic processes’ impacts on suspended particles are crucial for accurately predicting their dynamic changes in water bodies. This understanding aids in improving our knowledge of the transport patterns and differential distributions of suspended particles in natural environments.

3.2. Distribution and Transport of SPM in the Ocean

In the marine environment, SPM exhibits a stratified distribution in the vertical dimension, and this stratification significantly influences the physicochemical properties of seawater. Eittreim et al. observed a stratification effect in light attenuation within seawater while investigating SPM in the Atlantic Ocean [44]. Subsequently, Biscaye and Eittreim et al. noted a correlation between the stratification of seawater density and the distribution of SPM [45]. Presently, the phenomenon of stratified SPM in the ocean has been recognized and confirmed by numerous experts and researchers [46].

Storms, tides, ISWs, and the Kuroshio current are key hydrodynamic factors driving the transport of SPM. Storms enhance the flux of nutrients from the ocean floor to the surface waters, resulting in elevated concentrations of SPM in surface layers [47]. Wilson et al. observed underwater mist layers at a depth of 2500 m in a canyon, which may be related to the semidiurnal tides, with weak seasonal stratification allowing storms to induce mixing at greater depths than expected, altering the distribution of SPM within the mist layer [48]. Internal solitary waves found on the continental shelf can transport sediment, with the distribution and settling of suspended particles related to the properties of these waves, and changes in the geomorphology of the continental slope correlated with the angle of incidence of internal waves [2,49]. According to turbulent diffusion theory, the volume of suspended particles transported upwards per unit time on a unit horizontal cross-section is proportional to the gradient of suspended particle concentration at the bottom, meaning that the higher the concentration of suspended particles at the bottom, the greater the volume of suspended particles transported upwards into the upper layer, and the greater the velocity gradient, the higher the distance of upward transport. However, suspended particles in the upper seawater also continuously settle under the influence of gravity (Figure 5). Tidal currents generate vertical eddies, another reason for the upward transport of suspended particles. Suspended particles pass through areas of strong tidal currents through horizontal transport, accumulating in deep-water areas with weaker dynamics to form high-concentration zones [50,51,52].

The studies outlined above underscore the significance of stratification as a pivotal feature governing the vertical distribution of SPM in marine environments. This distribution is subject to the broad regulatory influence of various oceanic dynamic processes, including storms, tides, ISWs, and ocean currents like the Kuroshio. These hydrodynamic drivers exert a determining influence on both the vertical movement and horizontal dispersion of suspended particles. The findings emphasize that the distribution pattern of SPM in the ocean is dynamic rather than static, intricately shaped and controlled by a multitude of variables within oceanic physical dynamics. Consequently, elucidating the interplay between hydrodynamic phenomena and SPM is instrumental in advancing our comprehension of the dynamic mechanisms governing particle transport, settling dynamics, and material cycling within marine ecosystems.

3.3. The Process of SPM Variation Under the Influence of ISWs

Internal solitary waves are nonlinear internal waves propagating within continuous fluid media, characterized by fast propagation speed, large amplitude, and short period. They exert significant influences on vertical mixing, material transport, and sediment resuspension in the ocean [53] (Figure 6). As a crucial hydrodynamic process in the South China Sea, ISWs play a pivotal role in the transport and distribution of suspended particles [54]. Currently, preliminary investigations have been conducted on the impact of ISWs on suspended particles. Masunaga et al. examined the transport dynamics of suspended particles under the influence of ISWs, revealing that near the coast, suspended particles migrate vertically toward the sloping bottom and horizontally toward the nearshore direction above the thermocline [55]. Lamb et al. explored the scale of suspended particle transport induced by ISWs, indicating that vertical transport primarily occurs at depths exceeding 100 m, with horizontal transport extending up to 1.6–2.5 km [5]. Johnson et al. observed that ISWs are typically associated with intense turbulent mixing, resulting in suspended particles being predominantly distributed on the surface ahead of the wave and at the bottom behind the wave [6] (Figure 5). Additionally, Durrieu de Madron et al. demonstrated that a considerable portion of suspended fine-grained sediments in seawater remains in a state of high concentration, forming an internal nepheloid layer within the ocean rather than undergoing sedimentation [56].

The current research on ISWs and suspended particles reveals a notable disparity in spatial scales. While ISWs exhibit wavelengths spanning tens to hundreds of kilometers, investigations into suspended particles are conducted at smaller scales, typically focusing on particle distribution at specific locations. For instance, Johnson et al. utilized surface velocity measurements and Acoustic Doppler Current Profilers to examine the transport of suspended particles during the propagation of ISWs toward the shore [6]. However, their findings are confined to the interaction between ISWs and suspended particles within a limited range. Despite efforts to overcome these limitations, such as the study conducted by Masunaga et al., which explored the relationship between ISW-induced mixing effects and suspended particle transport in shallow bays using towed YODA, there remains a temporal discrepancy between observations of ISWs and suspended particles [57]. Consequently, establishing a direct correlation between the observed phenomena presents a challenge.

On the other hand, the impact of ISWs on suspended particles remains unclear. Currently, it has only been demonstrated that ISWs influence the concentration and distribution of suspended particles. Researchers have inferred the distribution of suspended particles through optical attenuation signals and measured changes in particle concentration using turbidity meters [58,59]. However, the processes of sedimentation and transport of suspended particles, as well as variations in particle size distribution and aggregation level during the action of ISWs, remain unknown.

Figure 6. Diagram illustrating the impact of ISWs on seabed suspended particles and biota. Panel is adapted from [59], with permission from Frontiers, 2023.

Figure 6. Diagram illustrating the impact of ISWs on seabed suspended particles and biota. Panel is adapted from [59], with permission from Frontiers, 2023.

3.4. Interaction Between ISWs and SPM in the Northern South China Sea

The northern SCS is widely regarded as a “natural laboratory” for the study of oceanic internal waves due to the typical and prominent features of ISWs (ISWs) in this region [60]. In 1990, scientists first observed internal solitary wave packets in the northern SCS [61], laying the foundation for subsequent related studies. Alford et al. [18] conducted more than a decade of continuous research on internal waves in the SCS and found that ISWs are the dominant form. These waves are primarily generated by internal tides or the Kuroshio current and are mainly formed near the Luzon Strait. During their propagation, ISWs rapidly dissipate energy over the continental shelf. Field observations show that, during their shoaling over the continental slope, ISWs can cause the thermocline to depress by over 150 m within just five minutes. Furthermore, ISWs in the SCS are considered among the largest in amplitude globally, with observed amplitudes reaching up to 240 m, horizontal velocities up to 2.5 m/s, and vertical velocities up to 0.5 m/s [62]. These findings are critical for understanding the dynamics of ISWs and their impact on seabed sediments.

As ISWs propagate from the Luzon Strait toward the continental shelf in the northern SCS, their interactions with the seabed result in changes in amplitude, speed, and other characteristics due to decreasing water depth (Figure 7). This process is also accompanied by significant energy dissipation and polarity shifts in ISWs [63,64]. Yang et al. [65] reported the presence of mode-2 ISWs on the continental slope, which are associated with the evolution of mode-1 ISWs. Orr and Mignerey [66] observed that as ISWs shoal toward the continental slope, their waveforms undergo complex transformations. In particular, during the upslope propagation, ISWs are affected by shear instabilities, leading to energy loss and polarity conversion—from concave-down to convex-up forms. These dynamic processes significantly influence the vertical dispersion of suspended particles and their residence time in the water column, ultimately altering local sedimentary dynamics and material flux structures.

Reeder et al. [67] used ship-mounted echosounders to detect a 125-m amplitude concave-down ISW propagating along the thermocline, which induced sediment resuspension at a depth of 600 m. Ma et al. [68] found that to the west of Dongsha Atoll in the northern SCS, ISWs passing through a water depth of 175 m induced near-bottom currents exceeding 0.8 m/s. The formation of sand ripples and scour grooves on the continental shelf break was closely related to the combined action of ISWs and internal tides. In the Dongsha region at a depth of 956 m, ISWs can resuspend sediments up to 8 m above the seabed, with suspended sediment concentrations reaching 0.62 mg/L. The annual resuspended sediment load is estimated at 787 Mt/a [69]. Tian et al. [70] observed that ISWs can induce sediment resuspension at depths of up to 1500 m, potentially influencing the evolution of seabed morphology.

4. Conclusions and Future Outlooks

Currently, the exploration of ISWs’ impact on SPM remains in its nascent stage both domestically and internationally. Advancements in observational technologies, such as high-frequency radar and acoustic measurements, have opened avenues for precise spatiotemporal assessments of ISWs and SPM. Nevertheless, the research landscape is characterized by disparate investigations, often isolating internal solitary wave dynamics from SPM studies, leading to incongruent research scales and a paucity of interrelation.

To further deepen the understanding of the dynamic processes of SPM under the influence of ISWs, this study proposes several future research directions from both modeling and observational perspectives:

(1)

Current models are still insufficient in capturing the nonlinear and unsteady interactions between ISWs and SPM. Future research should focus on developing Lagrangian–Eulerian coupled models, in which the wave-induced flow field is solved within an Eulerian framework, while the motion of individual particles or particle clouds is tracked using a Lagrangian approach. This method can explicitly account for particle inertia, size distribution, and settling behavior under wave-induced turbulence and density stratification.

(2)

There is an urgent need to develop more reasonable parameterization schemes to describe sediment resuspension, flocculation, and settling during shear and turbulence processes induced by ISWs. These parameterizations should be validated using high-resolution field and experimental data, especially during the steepening and breaking phases of ISWs—periods when sediment transport is most active.

(3)

Stronger collaboration with observational efforts is essential. Field observations should aim to provide key datasets with high temporal and spatial resolution, such as near-bottom current velocity, turbulence intensity, SPM concentration, and interface instability, to support model validation. In turn, model outputs can guide the timing and spatial layout of future observations, thereby improving the efficiency and focus of field surveys.

(4)

Although significant progress has been made in the South China Sea, future research should also focus on other regions with complex topography or ecological sensitivity, such as estuaries, deltas, and continental slopes, to further assess the ecological and geomorphological impacts of ISW–SPM interactions in diverse settings.

By advancing these research directions, we can better elucidate the impact of ISWs on the characteristics and behavior of suspended particulate matter. This will provide theoretical support and reference data for understanding the role of SPM in shaping seafloor topography, influencing water quality, and affecting marine ecological environments—demonstrating both academic value and practical significance.