We present a method to quantify total, horizontal kinetic and available potential energies of linear internal waves (IWs) and vortical mode (VM) using only two-dimensional (2D) (depth, along-track distance) measurements of horizontal velocity, such as those commonly taken by oceanic shipboard ADCP (SADCP). Previous IW and VM energy decomposition methods (Bühler et al., and their extensions) require both velocity and buoyancy measurements. Applying Helmholtz decomposition, 2D horizontal kinetic energy wavenumber ( k x , k z ) spectra are projected onto divergent K div and rotational K rot components. IW total energy spectrum is E IW = 2 K div . VM total energy is E VM = 1/Bu[(1 + Bu) K rot − K div ], where is Burger number with N and f the buoyancy and inertial frequencies, k h the horizontal wavenumber magnitude, and k z the vertical wavenumber. IW and VM horizontal kinetic energy ( K ) and available potential energy ( P ) can be inferred from E VM and E IW as functions of Bu. The proposed method, derived directly from IW and VM theoretical polarization relations, is demonstrated using two sets of velocity and density data. The E VM derived by this new method agrees with results computed using the 2014 and 2017 Bühler et al. methods at Bu ∼ O (1) and within a factor of ∼2–3 elsewhere, confirming that IW and VM energy can be separated using only velocity data. At Bu ≪ O (0.1), E VM is dominated by P VM , with K VM / P VM = Bu, and using K alone to extract E VM through the proposed method is challenging due to inherent uncertainty in spectral measurements. This method could be applied to global SADCP datasets to separate upper-ocean IW and VM energy contributions in different dynamical regions at horizontal scales O (100) m– O (100) km and vertical scales O (10)– O (100) m.

The spatial distribution of the flow field inferred from shipboard measurements often suffers from a spatial‐temporal aliasing effect. This study examined a detailed view of the horizontal divergence and relative vertical vorticity captured by a two‐vessel survey to characterize velocity gradients and the resulting process in the presence of nonlinear lee waves. The three‐dimensional structure of the horizontal velocity gradients and turbulent mixing within a nonlinear internal lee wave behind a shallow seamount was investigated. Synchronous two‐vessel Acoustic Doppler Current Profilers provide in‐situ measurements of velocity vectors that significantly minimize the spatial and temporal aliasing effect. The magnitude of horizontal divergence and relative vertical vorticity normalized by the planetary vorticity (, ∼ O (10)) is one order of magnitude greater than prior observations in the typical oceanic sub‐mesoscale flow field using a two‐vessel survey. Our analysis indicates that the spatial variations of horizontal divergence and relative vertical vorticity over the seamount are associated with flow‐topography interactions. Owing to the bottom Ekman effect, the deflected Kuroshio enhances the relative vertical vorticity, , and horizontal components of relative vorticity, and , resulting in vertical shear instability and symmetric instability. Instability hotspots are identified by the negative potential vorticity (PV) at the rear half of the nonlinear internal wave, where depressed isopycnals rebound. In situ observational surveys conducted on the lee of pinnacles further indicated that the enhanced turbulent eddy diffusivity and vertical nitrate gradient are collocated with the negative PV within a nonlinear internal lee wave.

Finescale properties of Kelvin-Helmholtz (KH)-like shear instabilities on the trailing edge of a nonlinear lee wave generated by the Kuroshio impinging on a seamount were measured using a towed CTD chain, shipboard ADCP, and echosounder. Lee-wave vertical velocity amplitudes vary in phase with the upstream semidiurnal along-stream current. The instabilities are analogous to atmospheric billows induced by a recirculation on the trailing edge of mountain lee waves. A total of 135 KH billows were identified in a 4-day-long time series roughly 300 m downstream of the center of the lee wave. The KH billows have heights H = 52 +/- 11 m, widths L = 162 +/- 72 m, and aspect ratios H/L = 0.39 +/- 0.18. Positive reduced shear squared S (2) - 4N (2) (where S is the vertical shear magnitude and N is the buoyancy frequency) in the shear-stratified billows suggests actively growing instabilities, with comparable contributions from across- and along-flow vertical shear. Billow cores are convectively unstable (N (2) < 0). Large turbulent kinetic energy dissipation rates similar to O(10-5)Wkg-1 are inferred from density overturns. Density, shear, and inferred turbulence properties vary with billow aspect ratios. As H/L increases, density gradients smear out. For 122 billows with H/L < 0.6, dissipation rates increase by one order of magnitude with increasing H/L. These observations of similar to 1-m vertical and similar to 5-m horizontal resolution billow structures and density overturn dissipation rates can provide a reference for future high-Reynolds-number direct numerical simulations.

Physical processes behind flow-topography interactions and turbulent transitions are essential for parameterization in numerical models. We examine how the Kuroshio cascades energy into turbulence upon passing over a seamount, employing a combination of shipboard measurements, tow-yo microstructure profiling, and high-resolution mooring. The seamount, spanning 5 km horizontally with two summits, interacts with the Kuroshio, whose flow speed ranges from 1 to 2 m s(-1), modulated by tides. The forward energy cascade process is commenced by forming a train of 2-3 nonlinear lee waves behind the summit with a wavelength of 0.5-1 km and an amplitude of 50-100 m. A train of Kelvin-Helmholtz (KH) billows develops immediately below the lee waves and extends downstream, leading to enhanced turbulence. The turbulent kinetic energy dissipation rate is O (10(-7)-10(-4)) W kg(-1), varying in phase with the upstream flow speed modulated by tides. KH billows occur primarily at the lee wave's trailing edge, where the combined strong downstream shear and low-stratification recirculation trigger the shear instability, Ri < 1/4. The recirculation also creates an overturn susceptible to gravitational instability. This scenario resembles the rotor, commonly found in atmospheric mountain waves but rarely observed in the ocean. A linear stability analysis further suggests that critical levels, where the KH instability extracts energy from the mean flow, are located predominantly at the strong shear layer of the lee wave's upwelling portion, coinciding with the upper boundary of the rotor. These novel observations may provide insights into flow-topography interactions and improve physics-based turbulence parameterization.

Shipboard ADCP velocity and towed CTD chain density measurements from the eastern North Paci fi c pycnocline are used to segregate energy between linear internal waves (IW) and linear vortical motion [quasigeostrophy (QG)] in 2D wavenumber space spanning submesoscale horizontal wavelengths l x - 1 - 50 km and fi nescale vertical wavelengths l z - 7 - 100 m. Helmholtz decomposition and a new Burger number (Bu) decomposition yield similar results despite different methodologies. While these wavelengths are conventionally attributed to internal waves, both QG and IW contribute signi fi cantly at all measured scales. Partition between IW and QG total energies depends on Bu. For Bu , 0.01, available potential energy E P exceeds horizontal kinetic energy E K and is contributed mostly by QG. In contrast, energy is nearly equipartitioned between QG and IW for Bu .. 1. For Bu , 2, E K is contributed mainly by IW, and E P by QG, while, for Bu . 2, contributions are reversed. Finescale near -inertial IW dominate vertical shear variance, implying negligible QG contribution to vertical shear instability. In contrast, both QG and IW at the smallest l x - 1 km contribute large horizontal shear variance, so that both may lead to horizontal shear instability, while QG, with its longer time scales, likely dominates isopycnal stirring. Both QG and IW contribute to vortex stretching at small vertical scales. For QG, the relative vorticity contribution to linear potential vorticity anomaly increases with decreasing horizontal and increasing vertical scales.

The Green Edge project was designed to investigate the onset, life, and fate of a phytoplankton spring bloom (PSB) in the Arctic Ocean. The lengthening of the ice-free period and the warming of seawater, amongst other factors, have induced major changes in Arctic Ocean biology over the last decades. Because the PSB is at the base of the Arctic Ocean food chain, it is crucial to understand how changes in the Arctic environment will affect it. Green Edge was a large multidisciplinary, collaborative project bringing researchers and technicians from 28 different institutions in seven countries together, aiming at understanding these changes and their impacts on the future. The fieldwork for the Green Edge project took place over two years (2015 and 2016) and was carried out from both an ice camp and a research vessel in Baffin Bay, in the Canadian Arctic. This paper describes the sampling strategy and the dataset obtained from the research cruise, which took place aboard the Canadian Coast Guard ship (CCGS) Amundsen in late spring and early summer 2016. The sampling strategy was designed around the repetitive, perpendicular crossing of the marginal ice zone (MIZ), using not only ship-based station discrete sampling but also high-resolution measurements from autonomous platforms (Gliders, BGC-Argo floats …) and under-way monitoring systems. The dataset is available at https://doi.org/10.17882/86417 (Bruyant et al., 2022).

Horizontal and vertical wavenumbers (k(x), k(z)) immediately below the Ozmidov wavenumber (N-3/epsilon)(1/2) are spectrally distinct from both isotropic turbulence (k(x), k(z) > 1 cpm) and internal waves as described by the Garrett-Munk (GM) model spectrum (k(z) < 0.1 cpm). A towed CTD chain, augmented with concurrent Electromagnetic Autonomous Profiling Explorer (EM-APEX) profiling float microstructure measurements and shipboard ADCP surveys, are used to characterize 2D wavenumber (k(x), k(z)) spectra of isopycnal slope, vertical strain, and isopycnal salinity gradient on horizontal wavelengths from 50 m to 250 km and vertical wavelengths of 2-48 m. For k(z) < 0.1 cpm, 2D spectra of isopycnal slope and vertical strain resemble GM. Integrated over the other wavenumber, the isopycnal slope 1D k(x) spectrum exhibits a roughly 11/3 slope for k(x) > 3 x 10(-3) cpm, and the vertical strain 1D k(z) spectrum a(-1) slope for k(z) > 0.1 cpm, consistent with previous 1D measurements, numerical simulations, and anisotropic stratified turbulence theory. Isopycnal salinity gradient 1D k(x) spectra have a 11 slope for k(x) > 2 x 10(-3) cpm, consistent with nonlocal stirring. Turbulent diapycnal diffusivities inferred in the (i) internal wave subrange using a vertical strain-based finescale parameterization are consistent with those inferred from finescale horizonal wavenumber spectra of (ii) isopycnal slope and (iii) isopycnal salinity gradients using Batchelor model spectra. This suggests that horizontal submesoscale and vertical finescale sub-ranges participate in bridging the forward cascade between weakly nonlinear internal waves and isotropic turbulence, as hypothesized by anisotropic turbulence theory.

Abstract. The Green Edge project was designed to investigate the onset, life and fate of a phytoplankton spring bloom (PSB) in the Arctic Ocean. The lengthening of the ice-free period and the warming of seawater, amongst other factors, have induced major changes in arctic ocean biology over the last decades. Because the PSB is at the base of the Arctic Ocean food chain, it is crucial to understand how changes in the arctic environment will affect it. Green Edge was a large multidisciplinary collaborative project bringing researchers and technicians from 28 different institutions in seven countries, together aiming at understanding these changes and their impacts into the future. The fieldwork for the Green Edge project took place over two years (2015 and 2016) and was carried out from both an ice-camp and a research vessel in the Baffin Bay, canadian arctic. This paper describes the sampling strategy and the data set obtained from the research cruise, which took place aboard the Canadian Coast Guard Ship (CCGS) Amundsen in spring 2016. The dataset is available at https://doi.org/10.17882/59892 (Massicotte et al., 2019a).

Microstructure and CTD/LADCP measurements from the Western Mediterranean basin east of 5 degrees E revealed two types of dynamical regions (Ferron et al., 2017), contrasted in terms of current magnitude, vertical shear, stratification and turbulent kinetic energy dissipation rate: energetic regions (Corsica Channel, Egadi Valley and Sicily Channel) and quiescent regions (Ligurian Sea, around Sardinia, and Tyrrhenian Sea). On average, the current speed and the buoyancy frequency in the energetic regions were twice as large as in the quiescent regions, and the vertical shear was five times as large. Turbulence properties inferred from the microstructure measurements were also contrasted, dissipation rates in the energetic regions being two orders of magnitude larger than in the quiescent regions. The present study investigates the variability of the dissipation flux coefficient, a measure of the mixing efficiency, in a rich assortment of dynamical regimes. This dataset covers the full range of turbulence intensities observed in previous studies based on field measurements, direct numerical simulations, and laboratory experiments alike. The dependency of the dissipation flux coefficient as a function of turbulence intensity for the quiescent and energetic regions frames the previously observed lower and upper bounds, respectively. A contrasting behaviour was revealed between the two types of regions. In the quiescent regions, the dissipation flux coefficient linearly decreases on average by one order of magnitude with turbulence intensity increasing by four orders of magnitude. On the other hand, in the energetic regions the dissipation flux coefficient exhibits a nearly constant value over 4 decades of turbulence intensity, before decreasing for very strong turbulence intensities. In contrast with other studies, this dataset shows no relationship between the Richardson number and the dissipation flux coefficient. This may be due to inadequate vertical sampling resolution of the currents, or to the high diversity of sampled turbulent regimes, contrary to previous studies focused on a single type of dynamical region or framework (such as the thermocline or shear instabilities).

The Green Edge initiative was developed to investigate the processes controlling the primary productivity and fate of organic matter produced during the Arctic phytoplankton spring bloom (PSB) and to determine its role in the ecosystem. Two field campaigns were conducted in 2015 and 2016 at an ice camp located on landfast sea ice southeast of Qikiqtarjuaq Island in Baffin Bay (67.4797∘ N, 63.7895∘ W). During both expeditions, a large suite of physical, chemical and biological variables was measured beneath a consolidated sea-ice cover from the surface to the bottom (at 360 m depth) to better understand the factors driving the PSB. Key variables, such as conservative temperature, absolute salinity, radiance, irradiance, nutrient concentrations, chlorophyll a concentration, bacteria, phytoplankton and zooplankton abundance and taxonomy, and carbon stocks and fluxes were routinely measured at the ice camp. Meteorological and snow-relevant variables were also monitored. Here, we present the results of a joint effort to tidy and standardize the collected datasets, which will facilitate their reuse in other Arctic studies. The dataset is available at https://doi.org/10.17882/59892 (Massicotte et al., 2019a).

During Marine Science Summer Course 2017, a continuous 24-h conductivity-temperature-depth (CTD) "yoyo" measurement has been carried out at the entrance of Padangbai Lombok Strait to investigate seawater properties variations on semidiurnal tidal-scale which is dominant in the strait. The SBE CTD 19 plus is equipped with optional sensors such as pH, turbidity and chlorophyll-a derived-fluoro. During 24-h field observation, 15 CTD casts from sea surface to about 60 m depth have been acquired. It is shown that observed seawater properties fluctuate strongly four times a day, following semidiurnal-tide period with two flood-tide and two ebb-tide conditions. During flood-tide, water mass is derived from open strait with colder, saltier, denser and low dissolved oxygen characteristics. In contrast, during ebb-tide, local water mass is recirculated back from the inner bay to the open strait. It is interesting to note that fluctuation of chlorophyll-a indicates a diurnal signal. In addition, fluctuation of pH, turbidity and dissolved oxygen showed a weak semidiurnal signal.

During summer, phytoplankton can bloom in the Arctic Ocean, both in open water and under ice, often strongly linked to the retreating ice edge. There, the surface ocean responds to steep lateral gradients in ice melt, mixing, and light input, shaping the Arctic ecosystem in unique ways not found in other regions of the world ocean. In 2016, we sampled a high-resolution grid of 135 hydrographic stations in Baffin Bay as part of the Green Edge project to study the ice-edge bloom, including turbulent vertical mixing, the under-ice light field, concentrations of inorganic nutrients, and phytoplankton biomass. We found pronounced differences between an Atlantic sector dominated by the warm West Greenland Current and an Arctic sector with surface waters originating from the Canadian archipelago. Winter overturning and thus nutrient replenishment was hampered by strong haline stratification in the Arctic domain, whereas close to the West Greenland shelf, weak stratification permitted winter mixing with high-nitrate Atlantic-derived waters. Using a space-for-time approach, we linked upper ocean dynamics to the phytoplankton bloom trailing the retreating ice edge. In a band of 60 km (or 15 days) around the ice edge, the upper ocean was especially affected by a freshened surface layer. Light climate, as evidenced by deep 0.415 mol m–2 d–1 isolumes, and vertical mixing, as quantified by shallow mixing layer depths, should have permitted significant net phytoplankton growth more than 100 km into the pack ice at ice concentrations close to 100%. Yet, under-ice biomass was relatively low at 20 mg chlorophyll-a m–2 and depth-integrated total chlorophyll-a (0–80 m) peaked at an average value of 75 mg chlorophyll-a m–2 only around 10 days after ice retreat. This phenological peak may hence have been the delayed result of much earlier bloom initiation and demonstrates the importance of temporal dynamics for constraints of Arctic marine primary production.

The dissipation flux coefficient, a measure of the mixing efficiency of a turbulent flow, was computed from microstructure measurements collected with a vertical microstructure profiler in the Sicily Channel. This hotspot for turbulence is characterised by strong shear in the transitional waters between the south-eastward surface flow and the north-westward deep flow. Observations from the two deep passages in the channel showed a contrast in turbulent kinetic energy dissipation rates, with higher dissipation rates at the location with the strongest deep currents. This study investigated the dissipation flux coefficient variability in the context of mechanically driven turbulence with a large range of turbulence intensities. The dissipation flux coefficient was shown to decrease on average with increasing turbulence intensity R e b , with median values of 0.74 for low R e b (< 8.5), 0.48 for moderate R e b (8.5≤ R e b < 400) and 0.30 for high R e b (≥ 400). The dissipation flux coefficient inferred from the measurements was systematically higher on average than the parameterisation as a function of turbulence intensity suggested by Bouffard and Boegman (Dyn Atmos Oceans 61:14–34, 2013 ). A plateau at moderate turbulence intensities was observed, followed by a decrease in the dissipation flux coefficient with increasing turbulence intensity as predicted by the parameterisation, but at higher turbulence intensity. The dissipation flux coefficient showed a strong variability with the water column stability regime for the different water masses. In particular, high dissipation flux coefficient (median 0.40) was found at R e b between 400 and 10 4 for the transitional waters at the northeastern passage, where dissipation rates were high, stratification and shear were strong but the Richardson number R i was sub-critical. Vertical diapycnal diffusive fluxes were computed, and upward salinity sustained density fluxes of the order of 9 × 10 −6 and 4 × 10 −6 kg m −2 s −1 were found to be characteristic of the transitional (28 < σ < 29 kg m −3 ) and intermediate ( σ > 29 kg m −3 ) waters, respectively. Turbulent mixing led to a lightening of the transitional and intermediate waters, which was consistent with previous estimates (Sparnocchia et al. J Mar Syst 20:301–317, 1999 ), but an order of magnitude lower when inferred from the (Bouffard and Boegman Dyn Atmos Oceans 61:14–34, 2013 ) parameterisation.

Arctic sea ice is experiencing a shorter growth season and an earlier ice melt onset. The significance of spring microalgal blooms taking place prior to sea ice breakup is the subject of ongoing scientific debate. During the Green Edge project, unique time-series data were collected during two field campaigns held in spring 2015 and 2016, which documented for the first time the concomitant temporal evolution of the sea ice algal and phytoplankton blooms in and beneath the landfast sea ice in western Baffin Bay. Sea ice algal and phytoplankton blooms were negatively correlated and respectively reached 26 (6) and 152 (182) mg of chlorophyll a per m2 in 2015 (2016). Here, we describe and compare the seasonal evolutions of a wide variety of physical forcings, particularly key components of the atmosphere–snow–ice–ocean system, that influenced microalgal growth during both years. Ice algal growth was observed under low-light conditions before the snow melt period and was much higher in 2015 due to less snowfall. By increasing light availability and water column stratification, the snow melt onset marked the initiation of the phytoplankton bloom and, concomitantly, the termination of the ice algal bloom. This study therefore underlines the major role of snow on the seasonal dynamics of microalgae in western Baffin Bay. The under-ice water column was dominated by Arctic Waters. Just before the sea ice broke up, phytoplankton had consumed most of the nutrients in the surface layer. A subsurface chlorophyll maximum appeared and deepened, favored by spring tide-induced mixing, reaching the best compromise between light and nutrient availability. This deepening evidenced the importance of upper ocean tidal dynamics for shaping vertical development of the under-ice phytoplankton bloom, a major biological event along the western coast of Baffin Bay, which reached similar magnitude to the offshore ice-edge bloom.

Small scale turbulence in the two main deep passages of the Sicily Channel was characterised for the first time with microstructure measurements collected during four cruises spanning a two year period. Large turbulent kinetic energy dissipation rates (epsilon) were observed, with averaged values below the mixed layer reaching 10(-7) W kg(-1), confirming that the Sicily Channel is a hotspot for turbulence. Contrasted depth-averaged epsilon were observed between the two passages below the mixed layer: enhanced epsilon in the northeastern passage ranging from 1.3 x 10(-8) - 2.7 x 10(-7) W kg(-1) over the different cruises, and much weaker epsilon in the southwestern passage ranging from 3.5 x 10(-9) to 7.7 x 10(-9) W kg(-1). This contrast in epsilon occurs due to a stronger deep flow at the northeastern passage, resulting in larger shear and stronger turbulence. Internal tides act as another important source of turbulence in both passages, modulating the subinertial flow and inducing shear instabilities. Enhanced turbulence was also revealed by additional measurements made downstream (with respect to the deep flow) in the northeastern passage towards the deeper Tyrrhenian Sea, as dense waters overflow above steep topography. A wave-wave parameterisation was tested for epsilon, which showed a reasonable consistency for the less turbulent southwestern passage, but not for the more turbulent northeastern passage, suggesting a difference in the mechanism of turbulence.