The Arctic Ocean differs from other oceans globally in several ways. Stratification is largely determined by changes in salinity, with cooler fresher water overlying warmer (intruding) saltier water. Until very recently the ocean was largely isolated from the atmosphere by sea ice restricting exchange of heat and momentum across the sea surface. As much of the Arctic Ocean lies poleward of the critical latitude for the dominant tidal forcing, preventing the formation of freely propagating internal tides, the major pathway of tidal energy to ocean mixing. As such mixing between layers in the Arctic Ocean is weak.An analogy is often drawn between the circulation in the Arctic Ocean and that in an estuary. Lateral gradients in density drive exchange through Arctic gateways with the exchange flow mediated by vertical mixing within the Arctic Ocean. Here we examine the potential impact of the recent decline sea ice extent on both the vertical mixing and the import of heat and export of freshwater through the Arctic gateways.

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer in response to deep water remineralisation of sunken organic matter. Model simulations suggest that a warming climate will accelerate this deficiency due to strengthening stratification and the increased duration of stratification. In the next decades the seasonally stratified seas will host the massive expansion of off-shore wind farms, and in particular the new generation of floating wind turbines. Here we integrate novel turbulence timeseries measurements with vertical profiles of water column properties from a seasonally stratified shelf sea to estimate oxygen and biogeochemical fluxes. These new results are used to investigate the role of vertical mixing in the development of the seasonal O2 deficit with particular reference to the potential impact of enhanced mixing associated with the wake of the floating wind turbines.

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer. Predictions suggest global warming will accelerate this deficiency. Here we integrate turbulence timeseries with vertical profiles of water column properties from a seasonal stratified shelf sea to estimate oxygen and biogeochemical fluxes. The profiles reveal a significant subsurface chlorophyll maximum and associated mid-water oxygen maximum. We show that the oxygen maximum supports both upward and downwards O2 fluxes. The upward flux is into the surface mixed layer, whilst the downward flux into the deep water will partially off-set the seasonal O2 deficit. The results indicate the fluxes are sensitive to both the water column structure and mixing rates implying the development of the seasonal O2 deficit is mediated by diapcynal mixing. Analysis of current shear indicate that the downward flux is supported by tidal mixing, whilst the upwards flux is dominated by wind driven near-inertial shear. Summer storminess therefore plays an important role in the development of the seasonal deep water O2 deficit. Oxygen deprivation in the marine environment is likely to be exacerbated by climate change. We present data to show the potential impact of changing weather patterns in the development of a seasonal oxygen deficit in seasonally stratifying shelf seas.

The ocean surface mixed layer represents a critical interface linking the ocean and atmosphere. The physical processes determining the surface mixed layer properties and mediate atmosphere-ocean exchange. Sub-mesoscale processes play a key role in cross-scale oceanic energy transformation and the determination of surface mixed-layer properties, including the enhancement of vertical nutrient transport, leading to increased primary productivity. Herein, we presented observations of the spiral chlorophyll-a filament and its influence on turbulence within an anticyclonic eddy in the western South China Sea during August 2021. The filament had a negative Ertel potential vorticity associated with strong upwelled/downward currents (approximately 20-40 m/ day). Across-filament sections of the in-situ profiles showed turbulent dissipation rates enhanced in the filament. We suggested this enhancement values can be attributed to submesoscale processes, which accounted for 25 % of the total parameterized turbulent dissipation rates. The present parametrized submesoscale turbulent scheme overestimated the in-situ values. The filament transferred kinetic energy upward to anticyclonic eddy via bar-otropic instability and gained energy from the anticyclonic eddy via baroclinic instability. After kinetic energy budget diagnostic, we suggested besides symmetric instability, centrifugal instability and mixed layer baroclinic instability should also be included in the turbulence scheme to overcome the overestimation. The observed dual energy transfers between the anticyclonic eddy and filament, and the observed high turbulent energy dissipation within the filament, emphasized the need for these processes to be accurately parameterized regional and climate models.

Historically, the Arctic Ocean has been considered an ocean of weak turbulent mixing. However, the decline in seasonal sea ice cover over the past couple of decades has led to increased coupling between the atmosphere and the ocean, with potential enhancement of turbulent mixing. Here, we review studies identifying energy sources and pathways that lead to turbulent mixing in an increasingly ice-free Arctic Ocean. We find the evolution of wind-generated, near-inertial oscillations is highly sensitive to the seasonal sea ice cycle, but that the response varies greatly between the continental shelves and the abyssal ocean. There is growing evidence of the key role of tides and continental shelf waves in driving turbulent mixing over sloping topography. Both dissipate through the development of unsteady lee waves. The importance of the dissipation of unsteady lee waves in driving mixing highlights the need for parameterization of this process in regional ocean models and climate simulations.

<p>The drive to achieve net zero carbon has motivated the development of offshore wind into deeper waters further from shore. The relatively weak tidal currents and deep water of future development sites means that infrastructure will, for the first time, be deployed at scale in seasonally stratified waters.   Current designs for floating turbines have sub-structures which penetrate this stratification.  Flow past such substructures generates turbulent wakes which can regionally enhance the very low levels of internal mixing observed in the seasonal thermocline. </p><p>These low natural mixing rates drive nutrient fluxes which sustain phytoplankton growth at the subsurface chlorophyl maximum through the summer months and are responsible for 50% of the primary production in shelf seas.  Since this production supports the marine food web, changes to the physical drivers will fundamentally impact the marine food web.  Therefore, an anthropogenic source of turbulent mixing at the seasonal thermocline, has the potential to cause fundamental biogeochemical changes, impacting ecosystems, and fisheries in shelf seas. </p><p>We present new measurements of strongly elevated turbulence within wakes at a shallow water wind farm.  Strongly enhanced mixing is observed in the wake, and across the wider wind farm area and is associated with reduced stratification.    These observations and our estimates for deeper water wakes suggest that mixing from these structures can be significant, and further research is essential to quantify the impact of this new source of anthropogenic mixing.</p>

<p>Whilst accounting for only 7% of the global ocean surface area, shelf seas are important links between multiple environments (including terrestrial, deep marine, and atmospheric) and modulate the freshwater influx from rivers before it reaches the open ocean. This freshwater acts as a buoyancy forcing, and, together with solar heating and tidal mixing, affects the seasonal stratification of shelf seas. As stratification impacts numerous processes within the shelf seas, such as heat uptake, ocean currents and biogeochemistry which may further be of global importance, it is important that freshwater fluxes are accurately simulated within models. <br />Despite their importance, due to coarse model resolution, shelf seas are generally poorly represented in intermediate-complexity global climate models. Here, we examine the accuracy of shelf sea representation in the intermediate-complexity UVic Earth System Climate Model, with a primary focus on the North Sea. Using observational data, we show that the river basin configuration and freshwater discharge in the control model set up has large errors. As a result, the North Sea receives almost double the expected freshwater discharge on an annual scale, impacting the flushing time, seasonal stratification and biogeochemistry of the region. Through a series of simulations rerouting freshwater through more realistic drainage basins, and removing excess freshwater, we improve simulation results, with variations in freshwater fluxes having a significant impact on shelf sea processes. Our results indicate that the over-freshening of shelf seas may not solely be restricted to the UVic model but may be an issue in other global Earth system models due to their low spatial resolution.</p>

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.

Abstract. Turbulent mixing is a key process in the transport of heat, salt and nutrients in the marine environment, with fluxes commonly derived directly from estimates of the turbulent kinetic energy dissipation rate, ϵ. Time series of ϵ estimates are therefore useful in helping to identify and quantify key biogeochemical processes. Estimates of ϵ are typically derived using shear microstructure profilers, which provide high resolution vertical profiles, but require a surface vessel, incurring costs and limiting the duration of observations and the conditions under which they can be made. The velocity structure function method can be used to determine time series of ϵ estimates using along-beam velocity measurements from suitably configured acoustic Doppler current profilers (ADCP). Shear in the background current can bias such estimates, therefore standard practice is to deduct the mean or linear trend from the along-beam velocity over the period of an observation burst. This procedure is effective if the orientation of the ADCP to the current remains constant over the burst period. However, if the orientation of a tethered ADCP varies, a proportion of the velocity difference between bins is retained in the structure function and the resulting ϵ estimates will be biased. Long-term observations from a mooring with three inline ADCP show the heading oscillating with an angular range that depends on the flow speed; from large, slow oscillations at low flow speeds to smaller, higher frequency oscillations at higher flow speeds. The mean tilt was also determined by the flow speed, whilst the tilt oscillation range was primarily determined by surface wave height. Synthesised along-beam velocity data for an ADCP subject to sinusoidal oscillation in a sheared flow indicates that the retained proportion of the potential bias is primarily determined by the angular range of the oscillation, with the impact varying between beams depending on the mean heading relative to the flow. Since the heading is typically unconstrained in a tethered mooring, heading oscillation is likely to be the most significant influence on the retained bias for a given level of shear. Use of an instrument housing designed to reduce oscillation would mitigate the impact, whilst if the shear is linear over the observation depth range, the bias can be corrected using a modified structure function method designed to correct for bias due to surface waves.

Historically, the Arctic Ocean has been considered an ocean of low variability and weak turbulent mixing. However, the decline in seasonal sea ice cover over the past couple of decades has led to increased coupling between the atmosphere and the ocean, with potential enhancement of turbulent mixing. Here, we review studies that allow identifying energy sources and pathways that lead to turbulent mixing in an increasingly ice-free Arctic Ocean. We find that the evolution of wind-generated, near-inertial oscillations is highly sensitive to the seasonal sea ice cycle, but the response varies greatly between the continental shelves and the abyssal ocean and between the eastern and western ocean basins. There is growing interest in the role of tides and continental shelf waves in driving mixing over sloping topography. Both dissipate through the development of unsteady lee waves. The role eddies play in transporting shelf water into the basins and in supporting mixing has become more apparent as technological advances have permitted higher resolution observations of sea ice retreat. The importance of the dissipation of unsteady lee waves and of eddies in driving mixing highlights the need for parameterizations of these phenomena in regional ocean models and climate simulations.

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.

Understanding the temporal and spatial characteristics of turbulent coherent structures is of interest to the emergent sector of marine renewable energy for power generation from tidal stream turbines, as loading due to these vortex structures has resulted in costly device failure. Here, methods for characterising these coherent structures are developed in the Menai Straits, Anglesey, using an off-the-shelf broadband acoustic Doppler current profiler (ADCP) vertical beam with the metrics fast Fourier transforms and a wavelet element model. Results indicate lengthscales fall in the range 2.5-51 m. Focused study on a 30-min window finds the 5 most powerful features have a median lengthscale of 13.2 m and the strongest signal lies at similar to 6.8 m, which scale to 0.86 and 0.44 times the water depth respectively, these features have a periodicity of similar to 105 s. Methods using variance across ADCP beams are common for turbulence characterisation within the tidal energy sector, with turbulence intensity being appropriated from the wind energy sector. However, turbulence intensity when using an ADCP is found to be a poor predictor of water column turbulence in the presence of coherent structures. Crown Copyright (c) 2022 Published by Elsevier Ltd. All rights reserved.

Intermediate nepheloid layers (INLs) form important pathways for the cross‐slope transport and vertical export of particulate matter, including carbon. While intermediate maxima in particle settling fluxes have been reported in the Eurasian Basin of the Arctic Ocean, direct observations of turbid INLs above the continental slope are still lacking. In this study, we provide the first direct evidence of an INL, coinciding with enhanced mid‐water turbulent dissipation rates, over the Laptev Sea continental slope in summer 2018. Current velocity data show a period of enhanced downslope flow with depressed isopcynals, suggesting that the enhanced turbulent dissipation is probably the consequence of the presence of an unsteady lee wave. Similar events occur mostly during ice free periods, suggesting an increasing frequency of episodic cross‐slope particle transport in the future. The discovery of the INL and the episodic generation mechanism provide new insights into particle transport dynamics in this rapidly changing environment.

) Abstract Observations of turbulent kinetic energy dissipation rate (  ) from a range of historical shelf seas data sets are viewed from the perspective of their forcing and dissipation mechanisms: barotropic to baroclinic tidal energy conversion, and pycnocline and bottom boundary layer (BBL) dissipation. The observations are placed in their geographical context using a high resolution numerical model (NEMO AMM60) in order to compute relevant maps of the forcing (conversion). We analyze, in total, 18 shear microstructure surveys undertaken over a 17 year period from 1996 to 2013 on the North West European shelf, consisting of 3,717 vertical profiles of shear microstructure: 2,013 from free falling profilers and 1,704 from underwater gliders. A robust positive relationship is found between model-derived barotropic to baroclinic conversion, and observed pycnocline integrated  .

The tides represent a highly predictable element of the Earth system, with the ebb and flow of the tide first linked to phases of the Moon over a millennium ago. However, it is only in the past 50 years that the key role of the tides in driving ocean mixing has been recognised. Here we review progress made in the identification and parameterisation of the pathways of tidal energy, from generation to dissipation and mixing, in a range of ocean environments. The review highlights the key role of tidal dissipation in driving heat, freshwater and biogeochemical fluxes across a range of scales and environments, highlighting the need for representation of the small-scale mixing processes supported by the tide in both regional and global ocean and climate models. We also consider the variation in tidal dissipation through different stages of the Earth's geological history and its impact on the evolution of the Earth-Moon system. We further present a number of examples of past climate states to demonstrate that present tides and tidal dissipation rates are a poor proxy for past and future levels of tidally driven oceanic mixing.

For over 150 years, plans to divert Arctic Ocean-draining rivers southwards in order to relieve an ongoing water supply crisis in central Asia have been discussed. Recent insights have identified t...

In the Arctic Ocean, limited measurements indicate that the strongest mixing below the atmospherically forced surface mixed layer occurs where tidal currents are strong. However, mechanisms of energy conversion from tides to turbulence and the overall contribution of tidally driven mixing to Arctic Ocean state are poorly understood. We present measurements from the shelf north of Svalbard that show abrupt isopycnal vertical displacements of 10-50 m and intense dissipation associated with cross-isobath diurnal tidal currents of similar to 0.15 m s(-1). Energy from the barotropic tide accumulated in a trapped baroclinic lee wave during maximum downslope flow and was released around slack water. During a 6-hr turbulent event, high-frequency internal waves were present, the full 300-m depth water column became turbulent, dissipation rates increased by a factor of 100, and turbulent heat flux averaged 15 W m(-2)compared with the background rate of 1 W m(-2).

A 15-yr duration record of mooring observations from the eastern (>70 degrees E) Eurasian Basin (EB) of the Arctic Ocean is used to show and quantify the recently increased oceanic heat flux from intermediate-depth (similar to 150-900m) warmAtlantic Water (AW) to the surfacemixed layer and sea ice. The upward release ofAWheat is regulated by the stability of the overlying halocline, which we show has weakened substantially in recent years. Shoaling of theAWhas also contributed, with observations in winter 2017-18 showing AWat only 80m depth, just below the wintertime surfacemixed layer, the shallowest in our mooring records. The weakening of the halocline for several months at this time implies that AW heat was linked to winter convection associated with brine rejection during sea ice formation. This resulted in a substantial increase of upward oceanic heat flux during the winter season, froman average of 3-4W m(-2) in 2007-08 to.10Wm(-2) in 2016-18. This seasonalAWheat loss in the easternEBis equivalent to a more than a twofold reduction of winter ice growth. These changes imply a positive feedback as reduced sea ice cover permits increased mixing, augmenting the summer-dominated ice-albedo feedback.