The mechanisms responsible for sediment resuspension and transport by nonlinear internal waves (NLIWs) remain poorly understood largely due to a dearth of detailed field measurements. We present novel observations of the turbulent benthic boundary-layer (BBL) beneath trains of NLIWs of depression in the ocean. At the 250 m deep, low-gradient (<0.2%) continental shelf site the BBL was near well mixed to an average height of about 10 m above the bottom. Above this bottom mixing-layer, stratification constrained the extent of vertical sediment transport. NLIWs drove sediment transport by a combination of bed-stress intensification, turbulent transport, and a vertical pumping mechanism associated with the compression and subsequent expansion of the mixing-layer. There was no evidence that the observed dynamics were associated with a global instability, as proposed in previous studies. The results have implications for cross-shelf mass transport and highlight future challenges for measuring and modeling boundary-layer processes within shelf seas. Plain Language Summary With wave heights reaching 100 m, nonlinear internal waves generate some of the strongest ocean currents on the world's continental shelves. These extreme currents penetrate down to the seabed, where they greatly enhance sediment resuspension, eject sediments high into the water column, and generate some of the strongest forces on subsea engineered structures. These waves likely redistribute settled biological material, dense plastics, and sediment-sorbed hydrocarbons on the continental shelf. Despite their significance, the details of these processes remain inadequately understood, owing to the challenges of detailed near-bed observation and equally the challenges of configuring laboratory and computational experiments to be representative of ocean conditions. We present new detailed near-bed observations under 70 m nonlinear internal waves in the ocean. The observations (1) show how these waves enhanced resuspension and transport of sediments; (2) identify a potential pathway for transport of terrestrial material from the continent toward the abyss; and (3) highlight some future challenges for modeling these processes in computer simulations of the ocean.

We present 15 days of both mean and turbulent field observations bottom mixing‐layer at a gently sloping 250 m deep continental shelf site, energized by tides and nonlinear internal waves (NLIWs). The tidal frequency forcing was due to the combined effects of the barotropic tide and a mode‐1 internal tide (IT), while the NLIWs were predominantly mode‐1 waves of depression. The bottom mixing‐layer thickness varied at both semidiurnal and sub‐tidal ∼O(10)d frequencies, with an average thickness of around 10 m. Compression and expansion of the mixing‐layer by both the IT and NLIWs affected the mean velocity profiles in the mixing‐layer, while the phasing between the barotropic and baroclinic flows led to an asymmetry in mean velocity profiles between periods of rising and falling isotherms. With the exception of periods of flow reversal, the turbulent kinetic energy balance and turbulent stress observations were consistent with the existence of an inertial‐sublayer with thickness of approximately 10%–15% of the mixing‐layer thickness ( ∼1 m), even beneath NLIWs. In the outer portion of the mixing‐layer—that is, above the inertial‐sublayer—NLIWs modulated the local turbulence spectra. We discuss the observations in the context of a predictive model for mixing‐layer thickness. The analysis suggests that the high‐frequency variability in mixing‐layer thickness was dominated by internal wave pumping, though strength of the ambient stratification and the frequency of the forcing were important controls on the time‐averaged (sub‐tidal) variation.

&lt;p&gt;The Bay of Biscay (Bob) is a hot spot for the generation of internal tides and nonlinear internal waves (NLIW). However, no studies have focused on internal waves on the continental shelf of the Bob. Here, we present 22 days of collocated temperature, velocity and backscatter profiles within a water depth H of 65 m. The background stratification evolved from two pycnoclines, with the strongest one near the sea bed, to a continuous profile due to wind-driven upwelling.&lt;/p&gt; &lt;p&gt;Under the double pycnocline situation, we observed trains of elevation emerging from each internal tidal front with amplitude reaching up to H/4 and propagating at speeds between 0.1 and 0.35 m/s. Sporadically depression waves were measured within the train and can propagate substantially faster (between 0.36 and 0.54 m/s). With the continuous stratification, the trains of NLIWs of elevation and containing opposite polarities were no longer observed.&lt;/p&gt; &lt;p&gt;These observations suggest that depression waves can cross the train of elevation waves. Resulting interactions could have significant impacts on sediment dynamics over the shelf. The double pycnocline regime and the impact of the stratification modification due to wind will be investigated numerically in future work.&lt;/p&gt;

No abstract