Bottom and Intermediate Nepheloid Layer Induced by Shoaling Internal Solitary Waves: Impacts of the Angle of the Wave Group Velocity Vector and Slope Gradients

Abstract

The widely recognized global phenomena of bottom nepheloid layer (BNL) and intermediate nepheloid layer (INL) are ubiquitous in the ocean. These phenomena are induced by shoaling internal solitary waves (ISWs), as observed in many studies. In this study, we analyzed the BNLs and INLs induced by shoaling ISWs and their detailed processes using flume experiments and field observations. ISWs suspended seabed sediment by the horizontal velocity in the vortex, and the near‐bottom vertical velocity lifted sediment into the water column to create a BNL, which detached from the slope and diffused along the isopycnals, forming more than one INL. Considering the results of previous researchers, we found that the numbers of BNLs and INLs were principally determined by the relationship between the angle of the ISW group velocity vector (α) relative to horizontal and the slope gradients (γ). In transmissive regions (γ/α < 1), one BNL and more than one INL were formed. In critical regions (γ/α~1), only one BNL and less than one INL were observed. In reflective regions (γ/α > 1), less than one BNL was formed and no INL. The BNL in the critical regions should be the thickest where the sediment resuspension was the greatest. The concentrations and thicknesses of BNLs and INLs were related to the energy and amplitude of ISW and the sediment condition. The results in the transmissive regions were proved by field observation. Our research will help to predict the number and magnitude of transport channels formed by shoaling ISWs from the ocean margin to the ocean interior.

Calibrated Suspended Sediment Observations Beneath Large Amplitude Non‐Linear Internal Waves

“…Hosegood et al (2004), Cheriton et al (2014), Masunaga et al (2015), Bourgault et al (2014), and others have observed bottom and intermediate depth sediment layers (nepheloid layers) that formed due to NLIW interacting with a sloped shelf. This process has been demonstrated in laboratory studies (e.g., Tian et al, 2019) and qualitatively replicated in a 2D transect field-scale numerical model (Masunaga et al, 2017). The mechanisms for NLIW-induced sediment resuspension on flat bottoms and mild slopes remains speculative, with a recent review by Boegman and Stastna (2019) identifying the need for more process-oriented research in these areas.…”

Carbon Isotopic Constraints on Basin‐Scale Vertical and Lateral Particulate Organic Carbon Dynamics in the Northern South China Sea

“…As reported, vertical‐averaged geostrophic current speed from 2,400 m to the bottom in the northern SCS basin is slower than 1 cm/s (G. Wang et al., 2011). Sediment resuspension by internal waves in the northeastern SCS is observed at a maximum water depth of ∼1,500 m (Jia et al., 2019; Tian et al., 2019, 2021). Thus, direct riverine input or locus of active resuspension is unlikely.…”

Internal Solitary Waves Observed on the Continental Shelf in the Northern South China Sea From Acoustic Backscatter Data

“…seabed sediments, and controlling the grain size of the regional sediments (Ma et al, 2016;Tian et al, 2019aTian et al, ,b, 2021. The phenomena of sediment resuspension can be easily observed from high-resolution acoustic backscatter data (Cacchione et al, 2002;Reeder et al, 2011;Masunaga et al, 2015;Tian et al, 2019a).…”

“…seabed sediments, and controlling the grain size of the regional sediments (Ma et al, 2016;Tian et al, 2019aTian et al, ,b, 2021. The phenomena of sediment resuspension can be easily observed from high-resolution acoustic backscatter data (Cacchione et al, 2002;Reeder et al, 2011;Masunaga et al, 2015;Tian et al, 2019a). In this study, patches of weak scattering intensity (purple arrows) are often observed in the near-bottom layers after passing the ISWs (Figures 7A,G), showing the sediment resuspension process induced by the ISWs.…”

“…Sediment resuspension is larger than 5 m height above the seafloor, and affects over tens of kilometers. This phenomena, which called bottom nepheloid layer (BNL), is widely recognized in the global ocean (Masunaga et al, 2017;Tian et al, 2019a).…”