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Internal waves drive ocean mixing and enhance the transport of heat, momentum and other tracers in shelf seas. We collected observations of mixing over a 30‐day period from three cross‐shore moorings placed on the 330, 200 and 150 m isobaths on the offshore side of a pelagic ridge on the Australian North West Shelf. The region is forced by energetic surface and internal tides, exhibits non‐linear internal waves, experiences flow‐topography interactions, and is subject to episodic intense wind events. This complex forcing drove energetic diapycnal mixing at all sites. We identified five dominant internal wave forcing categories: mode‐1 waves at low‐frequency (time scales from double the buoyancy period to 4 hr), mode‐1 waves at high‐frequency (HF) (time scales between the buoyancy period and double the buoyancy period), mode‐2 waves, internal bores, and internal hydraulic jumps. Overall, just 15% of mixing events accounted for 90% of the total observed heat flux over the record. Mixing during internal wave events accounted for as much as 50% of the total heat flux in some locations. Of the internal wave categories, HF mode‐1 waves were the most significant contributors to the total heat flux at all sites (∼20%). On the other hand, internal bores made significant contributions to mixing only at the 200 and 150 m moorings; they made no contribution to mixing at the 330 m mooring. At the shallowest mooring, the different internal wave categories all made similar contributions to the total flux, indicating an increasingly complicated relationship between the evolving internal wavefield and the mixing. Plain Language Summary: Internal waves propagate along the density gradients found beneath the ocean's surface layer, analogous to surface waves propagating along the sharp density gradient where air and water meet. These waves play an important role in the distribution of nutrients, heat, contaminants, and other tracers in the ocean, especially in coastal regions where the waves break. The density structure of the ocean, the tidal and wind forcing, and the seabed features result in many different types of internal waves, each of which travel and break differently. In this work, we examine the mixing caused by different types of internal waves as they travel up and over a subsurface ridge on the Australian North West Shelf. We found that most of the significant mixing resulted from relatively rare events, which often occurred during internal wave forcing events. The magnitude of mixing increased in shallower waters due to internal waves breaking and causing energetic turbulent flows. However, the wave types that were the most significant for mixing changed depending on the location on the shelf and the depth. High‐frequency internal waves were generally the most significant contributor to mixing. Internal bores, a class of waves moving upslope near the seabed, did dominate the total ocean mixing near‐bottom at some locations. Key Points: Energetic but short‐lived nonlinear internal wave events drove most of the vertical turbulent heat flux over the month‐long recordInternal wave events evolved over relatively short time scales and relatively short spatial scalesEnergetic internal wave events contribute significantly to mixing in shelf seas [ABSTRACT FROM AUTHOR]

Non‐linear internal waves (NLIW) are important to processes such as heat transfer, nutrient replenishment and sediment transport on continental shelves. Our unique field observations of shoaling NLIW of elevation revealed a variety of different wave shapes, varying from relatively symmetric waves, to waves with either steepened leading‐ or trailing‐faces; many had evidence of trapped cores. The wave shape was related to the position of maximum density overturns and diapycnal mixing. We observed both shear (where sheared currents overcome the stabilizing effects of stratification) and convective (where the local velocity exceeds the wave propagation speed) instabilities. The elevated diapycnal mixing (>10−3 m2s−1) and heat flux (>500 Wm−2) were predominantly local to the NLIW of elevation packets, and were transported onshore 10s kilometers with the wave packets. We demonstrate that wave steepness may be a useful bulk property for the parameterization of wave‐averaged diapycnal heat flux. Plain Language Summary: Predicting the distribution of constituents such as nutrients, heat, sediment and pollutants on the continental shelf is key to processes such as: the safe operation of offshore infrastructure; understanding the variation in primary productivity; the prediction of marine heat waves; and environmental impact assessments of new activities. Non‐linear internal waves, waves that travel within the stratified water column, are likely to have a significant impact on constituent transport; however, they are poorly understood. Here we present unique observations that quantify the properties of a class of these nonlinear internal waves, known as waves of elevation. These waves can break in a similar way to surface waves at the beach, leading to dramatic increases in turbulent mixing over horizontal length scales of 10s kilometers. We have used our observations to define the amount of vertical heat transport induced by the breaking waves as a function of wave steepness. The vertical heat transport can be used as a proxy to estimate the vertical nutrient transport. This study has made a step change in quantifying the impact of NLIW of elevation on vertical transport. Key Points: Nonlinear internal waves of elevation can form both convective and shear instabilities, resulting in enhanced diapycnal mixing and heat fluxBreaking continues as the nonlinear waves propagate onshore, influencing diapycnal mixing and vertical heat flux over 10s of kilometersThe elevated heat flux associated with the nonlinear internal waves of elevation increases with the wave steepness [ABSTRACT FROM AUTHOR]

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. Key Points: Novel observations of the oceanic benthic boundary‐layer structure provide unique insight into sediment transport under NLIWs of depressionSediment transport is explained by strong turbulence and pumping by the NLIW without recourse to complex instability mechanismsThe findings have implications for cross‐shelf transport, RANS ocean modeling, and DNS/LES modeling of NLIWs in the ocean [ABSTRACT FROM AUTHOR]

We examine observations of turbulence in the geophysical environment, primarily from oceans but also from lakes, in light of theory and experimental studies undertaken in the laboratory and with numerical simulation. Our focus is on turbulence in density-stratified environments and on the irreversible fluxes of tracers that actively contribute to the density field. Our understanding to date has come from focusing on physical problems characterized by high Reynolds number flows with no spatial or temporal variability, and we examine the applicability of these results to the natural or geophysical-scale problems. We conclude that our sampling and interpretation of the results remain a first-order issue, and despite decades of ship-based observations we do not begin to approach a reliable sampling of the overall turbulent structure of the ocean interior. [ABSTRACT FROM AUTHOR]