The tides are a major source of the kinetic energy supporting turbulent mixing in the global oceans. The prime mechanism for the transfer of tidal energy to turbulent mixing results from the interaction between topography and stratified tidal flow, leading to the generation of freely propagating internal waves at the period of the forcing tide. However, poleward of the critical latitude (where the period of the principal tidal constituent exceeds the local inertial period), the action of the Coriolis force precludes the development of freely propagating linear internal tides. Here we focus on a region of sloping topography, poleward of the critical latitude, where there is significant conversion of tidal energy and the flow is supercritical (Froude number, Fr > 1). A high-resolution nonlinear modeling study demonstrates the key role of tidally generated lee waves and supercritical flow in the transfer of energy from the barotropic tide to internal waves in these high-latitude regions. Time series of flow and water column structure from the region of interest show internal waves with characteristics consistent with those predicted by the model, and concurrent microstructure dissipation measurements show significant levels of mixing associated with these internal waves. The results suggest that tidally generated lee waves are a key mechanism for the transfer of energy from the tide to turbulence poleward of the critical latitude. Plain Language Summary The decline in aerial extent of sea ice covering the Arctic Ocean in the recent years is perhaps one of the leading indications of climate change. Warm water enters the Arctic Ocean at depths of 100-200 m; however, it is isolated from melting the ice by the lack of mixing in the Arctic Ocean. This lack of mixing has been attributed to the ocean being isolated from the wind by ice, and the fact that much of the Arctic Ocean is north of the critical latitude, beyond which the type of internal tide that is believed to drive mixing across other major oceans on the planet cannot occur. However, new evidence has been found that suggests that the tide might be important in driving mixing in certain areas of the Arctic Ocean. Here we combine state-of-the-art numerical modeling with new turbulence measurements to identify the mechanism by which the tide can drive mixing at these high latitudes.

The combination of acoustic Doppler current profilers and the structure function methodology provides an attractive approach to making extended time series measurements of oceanic turbulence (the rate of turbulent kinetic energy dissipation ε) from moorings. However, this study shows that for deployments in the upper part of the water column, estimates of ε will be biased by the vertical gradient in wave orbital velocities. To remove this bias, a modified structure function methodology is developed that exploits the differing length scale dependencies of the contributions to the structure function resulting from turbulent and wave orbital motions. The success of the modified method is demonstrated through a comparison of ε estimates based on data from instruments at three depths over a 3-month period under a wide range of conditions, with appropriate scalings for wind stress and convective forcing.

The offshore wind energy sector has rapidly expanded over the past two decades, providing a renewable energy solution for coastal nations. Sector development has been led in Europe, but is growing globally. Most developments to date have been in well-mixed, i.e., unstratified, shallow-waters near to shore. Sector growth is, for the first time, pushing developments to deep water, into a brand new environment: seasonally stratified shelf seas. Seasonally stratified shelf seas, where water density varies with depth, have a disproportionately key role in primary production, marine ecosystem and biogeochemical cycling. Infrastructure will directly mix stratified shelf seas. The magnitude of this mixing, additional to natural background processes, has yet to be fully quantified. If large enough it may erode shelf sea stratification. Therefore, offshore wind growth may destabilize and fundamentally change shelf sea systems. However, enhanced mixing may also positively impact some marine ecosystems. This paper sets the scene for sector development into this new environment, reviews the potential physical and environmental benefits and impacts of large scale industrialization of seasonally stratified shelf seas and identifies areas where research is required to best utilize, manage, and mitigate environmental change.

Primary productivity in the Arctic Ocean is experiencing dramatic changes linked to the receding sea ice cover. The vertical transport of nutrients from deeper water layers is the limiting factor for primary production. Here, we compare coincident profiles of turbulence and nutrients from the Siberian Seas in 2007, 2008, and 2018. In all years, the water column structure in the upstream region of the Arctic Boundary Current promotes upward nutrient transport, in contrast to the regions further downstream, and there are first indications for an eastward progression of these conditions. In summer 2018, strongly enhanced vertical nitrate flux and primary production above the continental slope were observed, likely related to a remote storm. The estimated contribution of these elevated fluxes above the slope to the Pan‐Arctic vertical nitrate supply is comparable with the basin‐wide transport, and is predicted to increase with declining sea ice cover in the future.

The seasonal cycle of stratification in lakes results from the interaction between turbulent mixing, forced mainly by surface wind-stress, and surface heat exchange (Fischer et al., 1979;Imboden & Wüest, 1995). During the spring-summer period, the stratifying effect of surface heat input out-competes vertical mixing, leading to a robust stratified regime developing in all but very shallow polymictic lakes. This stable regime continues until the autumn period, when lakes start to lose heat to the atmosphere and both wind-stress and heat loss act together to erode stratification and induce the autumn overturn. Thereafter a vertically mixed regime prevails through the winter months and continues until surface heat input resumes, around the vernal equinox.The seasonal cycle of stratification and mixing exerts a major influence on lake biogeochemistry and ecology. For example, stable stratification increases the light received by phytoplankton by reducing the depth of the surface mixed layer and separates zones of primary production in the well-lit epilimnion from zones of decomposition in the darker hypolimnion. This decoupling of processes has consequences for nutrient availability in the epilimnion, oxygen depletion in the hypolimnion and consequent phosphorus release from the sediment and the distribution of organisms within a lake (Yankova et al., 2017). When the water column becomes vertically well-mixed during the autumn overturn, much higher mixing rates prevail, and nutrients are rapidly transported up the water column. These changes in the seasonal mixing regime also affects the vertical transfer rate of other scalar properties including, for example, the potent greenhouse gases carbon dioxide and methane (Vachon et al., 2019).