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Velocity and turbulence observations are used to estimate the forward cascade of kinetic energy from the internal tide to dissipation within a steep canyon. Two methods for computing cross‐frequency kinetic energy flux are compared to observed dissipation. One method, coarse graining, allows strongly nonlinear dynamics while the other assumes weak nonlinearity. Fluxes from both methods agree within a factor of 3 with dissipation estimates from a finescale parameterization which is often used in climate‐scale ocean models. Coarse graining predicts 68% of energy fluxing to dissipation from frequencies lower than 8cpd, while the weakly nonlinear method predicts 34%. The weighting of energy flux toward lower frequencies supports a shorter frequency‐space pathway to dissipation in the presence of topographic wave breaking than assumed by parameterizations. Enhanced near‐boundary mixing and upwelling has implications for the rate and spatial distribution of the upwelling branch of the global overturning circulation. Plain Language Summary Winds and tides contribute energy to the ocean which is either used to move deep water toward the surface or dissipated at small scales. It is important to understand the fate of ocean energy, so it can be properly incorporated into global models. To get to the small scales where dissipation occurs, energy undergoes a series of transfers through intermediate scales. The pathways energy takes on its way to dissipation are better understood in the open ocean than in areas near steep topography where dissipation is commonly the strongest, leaving a gap in our understanding. We use velocity data to examine the pathways that energy takes before being dissipated in a highly energetic environment, where waves created by the tide break on a canyon slope. The method we use has never been applied to small‐scale ocean observations before, but we are able to because of our closely spaced moorings and direct observations of small turbulent motions. This new approach reveals faster energy transfer (hours rather than days) than predictions from existing methods designed for the open ocean. By observing dissipation in areas where it is strongest, we can do a better job of estimating its impact on global systems in ocean‐climate models. Key Points Kinetic energy cascade from the internal tide to dissipation is mapped using velocity observations from a tidally active submarine canyon The timescale of energy flux to dissipation is 6 hr near the canyon floor, faster than previous open‐ocean estimates of many days A method allowing strong nonlinearity reveals energy flux to dissipation from frequencies lower than those predicted by weakly nonlinear methods

Two perpendicular microstructure turbulence and shipboard velocity sections were conducted at high horizontal resolution across an anticyclonic warm core ring. The observations showed elevated turbulence in the core of the eddy, coincident with regions of low Richardson number (Ri)$(Ri)$ . Shear leading to the low Ri$Ri$was associated with a downward‐propagating near‐inertial wave that appeared to be trapped in the negative vorticity associated with the eddy, as has been found previously. The magnitude of the turbulence production agreed well with the vertical divergence of the vertical energy flux of the wave. The mixing coefficient of the turbulence was near 0.2, which together with the correlation with low Ri$Ri$suggests that shear instability drives the turbulence. A high shear‐to‐strain ratio of 10.3 was found, as expected for a shear‐dominated near‐inertial wave. Fine‐structure parameterizations using strain only and both shear and strain overestimate the turbulence by factors of 2.7 and 12 respectively. Plain Language Summary We measured current velocities and turbulence inside an ocean eddy. A wind‐generated internal wave moved downward through the eddy and created shear in the velocity field that led to turbulence as the wave broke. The patterns of energy flux expected from the wave matched the measured magnitude of turbulent energy dissipation. Indirect methods of estimating the turbulence gave values about two and a half to 12 times too high. Key Points Enhanced turbulence was observed in an anticyclonic eddy, a warm core ring north of the Gulf Stream Observed turbulence was associated with alternating shear layers consistent with a downward‐propagating near‐inertial wave The internal wave shear‐to‐strain ratio is large within the eddy core, which has implications for fine‐structure parameterization estimates of turbulence

Unprecedented quantities of heat are entering the Pacific sector of the Arctic Ocean through Bering Strait, particularly during summer months. Though some heat is lost to the atmosphere during autumn cooling, a significant fraction of the incoming warm, salty water subducts (dives beneath) below a cooler fresher layer of near-surface water, subsequently extending hundreds of kilometers into the Beaufort Gyre. Upward turbulent mixing of these sub-surface pockets of heat is likely accelerating sea ice melt in the region. This Pacific-origin water brings both heat and unique biogeochemical properties, contributing to a changing Arctic ecosystem. However, our ability to understand or forecast the role of this incoming water mass has been hampered by lack of understanding of the physical processes controlling subduction and evolution of this this warm water. Crucially, the processes seen here occur at small horizontal scales not resolved by regional forecast models or climate simulations; new parameterizations must be developed that accurately represent the physics. Here we present novel high resolution observations showing the detailed process of subduction and initial evolution of warm Pacific-origin water in the southern Beaufort Gyre. Warming ocean water plays a significant role in accelerating Arctic sea ice melt. Here the authors present detailed observations of warm water of Pacific origin entering and diving beneath the Arctic ocean surface, and explore the dynamical processes governing its evolution.

We contend that ocean turbulent fluxes should be included in the list of Essential Ocean Variables (EOVs) created by the Global Ocean Observing System. This list aims to identify variables that are essential to observe to inform policy and maintain a healthy and resilient ocean. Diapycnal turbulent fluxes quantify the rates of exchange of tracers (such as temperature, salinity, density or nutrients, all of which are already EOVs) across a density layer. Measuring them is necessary to close the tracer concentration budgets of these quantities. Measuring turbulent fluxes of buoyancy (Jb), heat (Jq), salinity (JS) or any other tracer requires either synchronous microscale (a few centimeters) measurements of both the vector velocity and the scalar (e.g., temperature) to produce time series of the highly correlated perturbations of the two variables, or microscale measurements of turbulent dissipation rates of kinetic energy (ϵ) and of thermal/salinity/tracer variance (χ), from which fluxes can be derived. Unlike isopycnal turbulent fluxes, which are dominated by the mesoscale (tens of kilometers), microscale diapycnal fluxes cannot be derived as the product of existing EOVs, but rather require observations at the appropriate scales. The instrumentation, standardization of measurement practices, and data coordination of turbulence observations have advanced greatly in the past decade and are becoming increasingly robust. With more routine measurements, we can begin to unravel the relationships between physical mixing processes and ecosystem health. In addition to laying out the scientific relevance of the turbulent diapycnal fluxes, this review also compiles the current developments steering the community toward such routine measurements, strengthening the case for registering the turbulent diapycnal fluxes as an pilot Essential Ocean Variable.

The transition from winter to spring represents a major shift in the basal energy source for the Antarctic marine ecosystem from lipids and other sources of stored energy to sunlight. Because sea ice imposes a strong control on the transmission of sunlight into the water column during the polar spring, we hypothesized that the timing of the sea ice retreat influences the timing of the transition from stored energy to photosynthesis. To test the influence of sea ice on water column microbial energy utilization we took advantage of unique sea ice conditions in Arthur Harbor, an embayment near Palmer Station on the western Antarctic Peninsula, during the 2015 spring–summer seasonal transition. Over a 5-week period we sampled water from below land-fast sea ice, in the marginal ice zone at nearby Palmer Station B, and conducted an ice removal experiment with incubations of water collected below the land-fast ice. Whole-community metatranscriptomes were paired with lipidomics to better understand how lipid production and utilization was influenced by light conditions. We identified several different phytoplankton taxa that responded similarly to light by the number of genes up-regulated, and in the transcriptional complexity of this response. We applied a principal components analysis to these data to reduce their dimensionality, revealing that each of these taxa exhibited a strikingly different pattern of gene up-regulation. By correlating the changes in lipid concentration to the first principal component of log fold-change for each taxa we could make predictions about which taxa were associated with different changes in the community lipidome. We found that genes coding for the catabolism of triacylglycerol storage lipids were expressed early on in phytoplankton associated with a Fragilariopsis kerguelensis reference transcriptome. Phytoplankton associated with a Corethron pennatum reference transcriptome occupied an adjacent niche, responding favorably to higher light conditions than F. kerguelensis . Other diatom and dinoflagellate taxa had distinct transcriptional profiles and correlations to lipids, suggesting diverse ecological strategies during the polar winter–spring transition.

Small-scale turbulent mixing drives the upwelling of deep water masses in the abyssal ocean as part of the global overturning circulation 1 . However, the processes leading to mixing and the pathways through which this upwelling occurs remain insufficiently understood. Recent observational and theoretical work 2 – 5 has suggested that deep-water upwelling may occur along the ocean’s sloping seafloor; however, evidence has, so far, been indirect. Here we show vigorous near-bottom upwelling across isopycnals at a rate of the order of 100 metres per day, coupled with adiabatic exchange of near-boundary and interior fluid. These observations were made using a dye released close to the seafloor within a sloping submarine canyon, and they provide direct evidence of strong, bottom-focused diapycnal upwelling in the deep ocean. This supports previous suggestions that mixing at topographic features, such as canyons, leads to globally significant upwelling 3 , 6 – 8 . The upwelling rates observed were approximately 10,000 times higher than the global average value required for approximately 30 × 10 6 m 3 s −1 of net upwelling globally 9 . A dye-release experiment within a sloping submarine canyon provides direct evidence that vigorous mixing at topographic features, such as canyons, leads to rapid diapycnal upwelling of deep water.

Biological hotspots along the West Antarctic Peninsula (WAP) are characterized by high phytoplankton productivity and biomass as well as spatially focused penguin foraging activity. While unique physical concentrating processes were identified in one of these hotspots, understanding the mechanisms driving the blooms at these locations is of high importance. Factors posited to explain the blooms include the upwelling of macronutrient- and micronutrient-enriched modified Upper Circumpolar Deep Water (mUCDW) and the depth of the mixed layer influencing overall light availability for phytoplankton. Using shipboard trace-metal clean incubation experiments in three different coastal biological hotspots spanning a north-south gradient along the WAP, we tested the Canyon Hypothesis (upwelling) for enhanced phytoplankton growth. Diatoms dominated the Southern region, while the Northern region was characterized by a combination of diatoms and cryptophytes. There was ample concentration of macronutrients at the surface and no phytoplankton growth response was detected with the addition of nutrient-enriched mUCDW water or iron solution to surface waters. For all treatments, addition of mUCDW showed no enhancement in phytoplankton growth, suggesting that local upwelling of nutrient-enriched deep water in these hotspots was not the main driver of high phytoplankton biomass. Furthermore, the dynamics in the photoprotective pigments were consistent with the light levels used during these incubations showing that phytoplankton are able to photoacclimate rapidly to higher irradiances and that in situ cells are low light adapted. Light availability appears to be the critical variable for the development of hotspot phytoplankton blooms, which in turn supports the highly productive regional food web.

Three shipboard survey lines were occupied in Bering Strait during autumn of 2015, where high-resolution measurements of temperature, salinity, velocity, and turbulent dissipation rates were collected. These first-reported turbulence measurements in Bering Strait show that dissipation rates here are high even during calm winds. High turbulence in the strait has important implications for the modification of water properties during transit from the Pacific Ocean to the Arctic Ocean. Measured diffusivities averaging 2 × 10 −2 m 2 s −1 are capable of causing watermass property changes of 0.1°C and 0.1 psu during the ~1–2-day transit through the narrowest part of the strait. We estimate friction velocity using both the dissipation and profile methods and find a bottom drag coefficient of 2.3 (±0.4) × 10 −3 . This result is smaller than values typically used to estimate bottom stress in the region and may improve predictions of transport variability through Bering Strait.

Deep‐ocean upwelling, driven by small‐scale turbulence, plays a key role in climate by regulating the ocean's capacity to sequester heat and carbon. Recent theoretical studies have hypothesized that such upwelling may primarily occur within a bottom boundary layer (BBL) along the sloping seafloor. A dye experiment in a continental‐slope canyon during the BLT‐Recipes program revealed very rapid BBL‐focussed upwelling, endorsing this notion. Here, we elucidate the dynamical connection between the mixing and the upwelling. We show that along‐canyon upwelling stems from episodic turbulent mixing cells up to 250 m high, generated by tides sweeping up‐ and down‐canyon. The tidal currents support a vertical shear that periodically advects dense waters over slower‐flowing lighter waters, reducing BBL stratification. This triggers instabilities that mix the dense waters with neighboring lighter waters, resulting in net along‐boundary upwelling. Our findings substantiate the view that deep‐ocean upwelling can predominantly occur along the ocean's sloping boundaries.