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It is generally understood that the origin of ocean diapycnal diffusivity is primarily associated with the stratified turbulence produced by breaking internal (gravity) waves (IW). However, it requires significant effort to verify diffusivity values in ocean general circulation models in any particular geographical region of the ocean due to the scarcity of microstructure measurements. Recent analyses of downscaled IW fields from an internal‐wave‐admitting global ocean simulation into higher‐resolution regional configurations northwest of Hawaii have demonstrated a much‐improved fit of the simulated IW spectra to the in‐situ profiler measurements such as the Garrett‐Munk (GM) spectrum. Here, we employ this dynamically downscaled ocean simulation to directly analyze the nature of the IW‐breaking and the wave‐turbulence cascade in this region. We employ a modified version of the Kappa Profile Parameterization (KPP) to infer what the horizontally averaged vertical profile of diapycnal diffusivity should be, and compare this to the background profile that would be employed in the ocean component of a low‐resolution coupled climate model such as the Community Earth System Model (CESM) of the US National Center for Atmospheric Research (NCAR). In pursuing this goal, we also demonstrate that the wavefield in the high‐resolution regional domain is dominated by a well‐resolved spectrum of low‐mode IWs that are predictable by solving an appropriate eigenvalue problem for stratified flow. We finally suggest a new tentative approach to improve the KPP parameterization. Plain Language Summary: A much‐improved spectrum of the simulated internal wave (IW) field has recently been obtained by downscaling a global ocean model into a higher‐resolution regional configuration. The global simulation is based on the Massachusetts Institute of Technology general circulation model (MITgcm) forced by both astronomical tidal potential and surface atmospheric processes. By employing a mathematical framework to predict the structure of IWs, we first demonstrate that the interior wavefield of the high‐resolution regional domain is well dominated by a series of low‐order IW modes. Then, we address the issue as to whether the component of the K‐Profile Parameterization (KPP) associated with IW shear might be able to explain the physical origins of the background depth dependence of diapycnal diffusivity that would normally be employed in the ocean component of a modern coupled climate model. Finally, we suggest a tentative approach to further improve KPP. Key Points: A representation of internal wave modes that includes dissipation is derived that explains spectra in a tidally forced numerical simulationEliminating the background component of KPP leads to a representation of the physics of internal wave breakingRealistic diapycnal diffusivity profiles can be obtained by minor adjustments to the shear component of KPP [ABSTRACT FROM AUTHOR]

This paper examines spectra of horizontal kinetic energy (HKE) in the surface and sub‐surface ocean, with an emphasis on internal gravity wave (IGW) motions, in global high‐resolution ocean simulations. Horizontal wavenumber‐frequency spectra of surface HKE are computed over seven oceanic regions from two global simulations of the HYbrid Coordinate Ocean Model (HYCOM) and three global simulations of the Massachusetts Institute of Technology general circulation model (MITgcm). In regions with high IGW activity, high surface HKE variance in the horizontal wavenumber‐frequency spectra is aligned along IGW linear dispersion curves. For both HYCOM and MITgcm, and in almost all regions, finer horizontal resolution yields more energetic supertidal IGW continuum spectra. The ratio of high‐horizontal‐wavenumber variance in semi‐diurnal and supertidal motions relative to lower‐frequency motions, a quantity of great interest for swath altimetry, depends on the model employed and the horizontal resolution within the model, implying that quantitative predictions of the partition between low‐ and high‐frequency motions taken from particular simulations should be treated with care. The frequency‐vertical wavenumber spectra, frequency spectra, and vertical wavenumber spectra from the models are compared to spectra computed from McLane profilers at nine locations. In general, MITgcm spectra match the McLane profiler spectra more closely at high frequencies (|ω| > 4.5 cpd). In both models, vertical wavenumber spectra roll off more steeply than observations at high vertical wavenumbers (m > 10−2 cpm). The vertical wavenumber spectra in such models is an important target for improvement, due to turbulence production and dissipation that takes place at high vertical wavenumbers. Plain Language Summary: Recently, a small but growing number of global ocean models have begun to employ simultaneous tidal and atmospheric forcing. At the same time, increasing supercomputer power has allowed for simulations of oceanic motions with increasing accuracy, increasing feature (spatial) resolution, and more frequent time slices. Global ocean models with fine grid spacing, and simultaneous tidal and atmospheric forcing, host a vigorous spectrum of high‐frequency waves that control mixing over most of the ocean water column, and are important for many operational oceanography challenges. As an example of the latter, high‐resolution global internal wave models have been used to study the relative partition of high‐frequency versus low‐frequency motions at the small horizontal scales that will be measured by the new Surface Water Ocean Topography mission. The partition described above depends on the model employed and the grid spacing employed within that model, meaning that conclusions about the partition are dependent on the model used to estimate it. Comparisons between the models and vertically profiling instruments indicate that resolving fine scale motions in the vertical direction, where ocean mixing takes place, is not yet handled well by the models. Modeling of fine‐vertical scale motions is therefore an important future research direction. Key Points: Vertical wavenumber spectra of internal gravity wave kinetic energy in two high‐resolution global models are compared to observed spectraModels under‐estimate motions at high vertical wavenumbers (small vertical scales), flagging this as a target for model improvementThe ratio of high‐ versus low‐frequency surface kinetic energy at small horizontal scales is dependent on the model and grid spacings employed [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) are powered by heat fluxes across the air‐sea interface, which are in turn influenced by subsurface physical processes that can modulate storm intensity. Here, we use data from 6 profiling floats to recreate 3D fields of temperature (T), salinity (S), and velocity around the fast‐moving Super Typhoon Mangkhut (western North Pacific, September 2018). Observational estimates of vorticity (ζ) and divergence (Γ) agree with output from a 3D coupled model, while their relation to vertical velocities is explained by a linear theoretical statement of inertial pumping. Under this framework, inertial pumping is described as a linear coupling between ζ and Γ, whose oscillations in quadrature cause periodic displacements in the ocean thermocline and generate near‐inertial waves (NIWs). Vertical profiles of T and S show gradual mixing of the upper ocean with diffusivities as high as κ ∼ 10−1 m2 s−1, which caused an asymmetric cold wake of sea surface temperature (SST). We estimate that ∼10% of the energy used by mixing was used to mix rainfall, therefore inhibiting SST cooling. Lastly, watermass transformation analyses suggest that κ > 3 × 10−3 m2 s−1 above ∼110 m depth and up to 600 km behind the TC. These analyses provide an observational summary of the ocean response to fast‐moving TCs, demonstrate some advantages of ζ and Γ for the study of internal wave fields, and provide conceptual clarity on the mechanisms that lead to NIW generation by winds. Plain Language Summary: Near‐inertial internal waves (NIWs) are generated by winds and lead to oscillations in the internal structure of ocean currents and stratification. Turbulence induced by the vertical current shear in these waves helps sustain the upper ocean stratification and circulation. In this study, we use data from six autonomous floats deployed ahead of Super Typhoon Mangkhut to reconstruct the ocean's 3D response and compare it to output from a coupled air‐sea model. Patterns in NIW are explained using simple linear equations based on vorticity and divergence rather than current velocities, providing an alternative view of how TC winds help generate waves in the stratified ocean interior. Measurements of temperature and salinity detail how turbulence mixed rainfall and thermocline waters into the upper ocean. Our analyses indicate that turbulent mixing rates are greatest within 100 km of the typhoon eye but remain elevated hundreds of kilometers in the TC wake. Theory and observations presented here provide a comprehensive view of the ocean response to fast‐moving tropical cyclones. Key Points: Float data, linear theory, and a 3D model reveal vorticity and divergence control on inertial pumping beneath a fast‐moving tropical cycloneRightward‐enhanced sea surface cooling of 1.2°C was dominated by mixing and modulated by rainfall, which suppressed cold water entrainmentEstimates of turbulent diffusivity explain sea surface cooling rates 0.1°C hr−1 under the tropical cyclone eye and thermocline mixing in its wake [ABSTRACT FROM AUTHOR]

The geographical variability, frequency content, and vertical structure of near‐surface oceanic kinetic energy (KE) are important for air‐sea interaction, marine ecosystems, operational oceanography, pollutant tracking, and interpreting remotely sensed velocity measurements. Here, KE in high‐resolution global simulations (HYbrid Coordinate Ocean Model; HYCOM, and Massachusetts Institute of Technology general circulation model; MITgcm), at the sea surface (0 m) and at 15 m, are compared with KE from undrogued and drogued surface drifters, respectively. Global maps and zonal averages are computed for low‐frequency (<0.5 cpd), near‐inertial, diurnal, and semidiurnal bands. Both models exhibit low‐frequency equatorial KE that is low relative to drifter values. HYCOM near‐inertial KE is higher than in MITgcm, and closer to drifter values, probably due to more frequently updated atmospheric forcing. HYCOM semidiurnal KE is lower than in MITgcm, and closer to drifter values, likely due to inclusion of a parameterized topographic internal wave drag. A concurrent tidal harmonic analysis in the diurnal band demonstrates that much of the diurnal flow is nontidal. We compute simple proxies of near‐surface vertical structure—the ratio 0 m KE/(0 m KE + 15 m KE) in model outputs, and the ratio undrogued KE/(undrogued KE + drogued KE) in drifter observations. Over most latitudes and frequency bands, model ratios track the drifter ratios to within error bars. Values of this ratio demonstrate significant vertical structure in all frequency bands except the semidiurnal band. Latitudinal dependence in the ratio is greatest in diurnal and low‐frequency bands. Plain Language Summary: It is important to map and understand ocean surface currents because they affect climate and marine ecosystems. Recent advances in global ocean models include the addition of astronomical tidal forcing alongside atmospheric forcing and the usage of more powerful computers that can resolve finer features. Here, we evaluate ocean surface currents in high‐resolution simulations of two different ocean models through comparison with observations from surface drifting buoys. We examine near‐inertial motions, forced by fast‐changing winds; semidiurnal tides, forced by the astronomical tidal potential; diurnal motions, arising from tidal and other sources; and low‐frequency currents and eddies, forced by atmospheric fields. Global patterns in the models and drifters are broadly consistent. The two models differ in their degree of proximity to drifter measurements in the near‐inertial band, most likely due to different update intervals for atmospheric forcing and in the semidiurnal band, most likely due to different damping schemes. A simple proxy for vertical structure of the currents, measured by differences in drifter flows at the surface versus 15 m depth, is tracked reasonably well by the models. Discrepancies between models and observations motivate future improvements in the models. Key Points: We examine frequency content of ocean kinetic energy (KE) at the sea surface (0 m) and 15 m depth with global drifter data and two modelsNear‐surface near‐inertial and tidal KE in numerical models are sensitive to atmospheric forcing frequency and dampingThe ratio 0 m KE/(0 m KE + 15 m KE) in models lies within error bars of the observational ratios over some latitudes and frequency bands [ABSTRACT FROM AUTHOR]

We present improvements in the modeling of the vertical wavenumber spectrum of the internal gravity wave continuum in high‐resolution regional ocean simulations. We focus on model sensitivities to mixing parameters and comparisons to McLane moored profiler observations in a Pacific region near the Hawaiian Ridge, which features strong semidiurnal tidal beams. In these simulations, the modeled continuum exhibits high sensitivity to the background mixing components of the K‐Profile Parameterization (KPP) vertical mixing scheme. Without the KPP background mixing, stronger vertical gradients in velocity are sustained in the simulations and the modeled kinetic energy and shear spectral slopes are significantly closer to the observations. The improved representation of internal wave dynamics in these simulations makes them suitable for improving ocean mixing estimates and for the interpretation of satellite missions such as the Surface Water and Ocean Topography mission. Plain Language Summary: Internal waves (IWs) exist in the ocean interior due to differences in fluid densities. Breaking IWs cause mixing, which has important effects on ocean temperatures and nutrients. Interactions between internal tides generated by tidal flow over bathymetric features and near‐inertial waves generated by wind yield a spectrum of IWs at many frequencies. Here, we compare the IW spectrum in high‐resolution numerical simulations of a region in the North Pacific with observations from moored instruments. We study the effects of the "background" mixing components of the widely used K‐Profile Parameterization (KPP) vertical mixing scheme on the vertical structure of the IW field. The KPP background parameterizes the mixing action of IWs, which is not resolved in coarser‐resolution global ocean models. In our high‐resolution simulations, the IW field is highly active, and the KPP background components turn out to be mostly redundant in this setting. The modeled IW field lies closer to observations when we turn off the KPP background. Improved IW representation in ocean models can play an important role in the accurate representation of IW‐driven mixing in ocean simulations and interpretation of IW signatures from the upcoming Surface Water and Ocean Topography mission. Key Points: Regional ocean simulations are ideal for examining sensitivity of internal gravity wave (IGW) spectra to model mixing parametersTurning off the background components of K‐Profile Parameterization yields more realistic IGW vertical structure in high‐resolution regional modelsIGW spectra are most correctly estimated in models away from tidal generation sites and lateral boundaries [ABSTRACT FROM AUTHOR]

Tidal and low‐frequency flows interact with abrupt topographic features, giving rise to a variety of small‐scale phenomena including lee waves, vortical motions and turbulence. With the goal of identifying the processes that transfer momentum and energy between the larger‐scale and smaller‐scale flows, detailed tidally‐resolving shipboard surveys were conducted near a sharp and steep headland north of Velasco Reef, Palau, in the Western Pacific. We present velocity, microstructure and rapid‐cycling conductivity and temperature measurements, contextualized with a nearby 10‐month moored record, to examine wake and vortex structures. Observed flow near Velasco Reef was layered and multi‐directional in depth such that the "downstream" side of the headland depended on depth. Downstream of the headland, during a period where mean flow dominated over the tide in the top 100 m of the water column, turbulent kinetic energy dissipation rates were enhanced by up to three orders of magnitude relative to those upstream. During a period dominated by tides, the flow reversed periodically and wake eddies were associated with turbulent dissipation rates up to O10−5Wkg−1 $\mathcal{O}\left(1{0}^{-5}\right)\ \mathrm{W}\hspace*{.5em}\mathrm{k}{\mathrm{g}}^{-\mathrm{1}}$. Turbulent dissipation rates in the wake increased with inverse Richardson number for both tidal and mean flows past the headland, and were larger for stronger vorticity flows. Our analysis suggests that the eddies themselves lead to shear instabilities and subsequent turbulence, either through tilting or their limited vertical extent. We associate the enhanced downstream dissipation with a combination of breaking internal tides and either these eddy processes or a multi‐scaled turbulent wake. Plain Language Summary: Tall, steep underwater features such as ridges and headlands interact with tides and large currents to alter the flow downstream. Here, we present observations near Velasco Reef, Palau, an example of a sharp headland in the western Pacific. Turbulence in the wake of the headland is higher than upstream. During a time when the tides were strong the flow changed direction such that eddies were shed from both sides of the headland. These patches of rotating flow were associated with stronger turbulence than elsewhere around the headland. Shearing of the flow in the vertical direction by the eddies and the internal tides are possible mechanisms by which the turbulence is generated. Key Points: Direct observations of turbulent dissipation rates show enhancement downstream of a headland during mean flow and tidal flow regimesRegions of strong vorticity in the wake are associated with the highest turbulent dissipation ratesShear instabilities are a likely mechanism for the generation of turbulence in the wake [ABSTRACT FROM AUTHOR]

The potential influences of turbulence on planktonic processes such as nutrient uptake, grazing, predation, infection, and mating have been explored in hundreds of laboratory and theoretical studies. However, the turbulence levels used may not represent those experienced by oceanic plankton, bringing into question their relevance for understanding planktonic dynamics in the ocean. Here, we take a data‐centric approach to understand the turbulence climate experienced by plankton in the ocean, analyzing over one million turbulence measurements acquired in the open ocean. Median dissipation rates in the upper 100 m were < 10−8 W kg−1, with 99% of the observations < 10−6 W kg−1. Below mixed layers, the median dissipation rate was ~ 10−10 W kg−1, with 99% of the observations < 10−7 W kg−1. Even in strongly mixing layers the median dissipation rates rarely reached 10−5 W kg−1, decreasing by orders of magnitude over 10 m or less in depth. Furthermore, episodes of intense turbulence were transient, transitioning to background levels within 10 min or less. We define three turbulence conditions in the ocean: weak (< 10−8 W kg−1), moderate (10−8–10−6 W kg−1), and strong (> 10−6 W kg−1). Even the strongest of these is much weaker than those used in most laboratory experiments. The most frequent turbulence levels found in this study are weak enough for most plankton—including small protists—to outswim them, and to allow chemical plumes and trails to persist for tens of minutes. Our analyses underscore the primary importance of planktonic behavior in driving individual interactions. [ABSTRACT FROM AUTHOR]

Characterizing how ocean turbulence transports heat is critically important for accurately parameterizing global circulation models. We present a novel data‐driven approach for identifying distinct regions of turbulent mixing within a microstructure data set that uses unsupervised machine learning to cluster fluid patches according to their background buoyancy frequency N, and turbulent dissipation rates of kinetic energy ϵ and thermal variance χ. Applied to data collected near the Velasco Reef in Palau, our clustering algorithm discovers spatial and temporal correlations between the mixing characteristics of a fluid patch and its depth, proximity to the reef and the background current. While much of the data set is characterized by the canonical mixing coefficient Γ = 0.2, elevated local mixing efficiencies are identified in regions containing large density fluxes derived from χ. Once applied to further datasets, unsupervised machine learning has the potential to advance community understanding of global patterns and local characteristics of turbulent mixing. Plain Language Summary: Turbulent fluid motions enhance the mixing of heat between different layers of the ocean, playing a critical role in driving large‐scale currents that influence the Earth's climate. Measurements of centimeter‐scale velocity and temperature fluctuations within the ocean, termed microstructure, are used to study the properties of turbulent mixing and characterize its variability in both space and time. We present a new technique for analyzing microstructure data that uses a machine learning algorithm to identify fluid regions with similar measured characteristics automatically, yielding insight into the underlying turbulent mechanisms driving mixing. Applied to a data set collected near the Velasco Reef in Palau, our algorithm highlights several distinct turbulent regions whose properties depend on their depth, proximity to the reef and the background current. Our findings demonstrate that machine learning is a valuable new technique for characterizing ocean turbulence that, applied to further data sets, has the potential to advance our understanding of global patterns and local characteristics of turbulent mixing. Key Points: Unsupervised machine learning is used to automatically cluster microstructure measurements collected off the Velasco Reef in PalauSeveral distinct turbulent regions are identified, highlighting strong spatial and temporal variability in the flowStrongly localized thermal gradients elevate the mixing coefficient above the canonical value 0.2 in a subset of the domain [ABSTRACT FROM AUTHOR]

A near‐inertial wave stalling and breaking in a critical layer was observed for a week by a pair of autonomous velocity and density profilers on concentric paths in an ocean mesoscale eddy. Profiler observations provide estimates of the eddy's vertical vorticity and shear, quantities needed to test theories of downward near‐inertial wave (NIW) energy flux and loss from inertial wave–eddy interactions. The unique observations of the wave's intrinsic frequency ωi and vertical wavenumber m provide a novel estimate of the vertical group velocity Cgz from changes in ωi with respect to m. Shear, strain, energy flux convergence, and parameterized turbulence are all elevated near 140 m depth, near the bottom of the strongest eddy velocities. Our observations are consistent with a downgoing NIW's group velocity decreasing owing to wave–eddy interactions, providing important clues on global energetics of NIW mixing. Plain Language Summary: Internal gravity waves with frequency near the Earth's Coriolis frequency, called a near‐inertial wave, was observed stalling and breaking as it propagated downward from the sea surface. The wave stalled because its downward propagation speed slowed in the presence of a background eddy that weakened with depth. We believe this is by far the clearest observation of such a process, which is an important aspect of interactions between internal gravity waves and eddies. A key finding of the paper is the first‐ever direct determination of the wave's propagation speed from our observations without assumptions or reliance on theory. Key Points: A pair of EM‐APEX floats moving with a subsurface turbulent zone observe mesoscale vorticity (by Kelvin's circulation theorem) and vertical shear; observations resolve near‐inertial wave phase and group velocities, density, and shear to diagnose dynamics of NIW stalling and dissipationFirst known determination of NIW vertical group velocity computed from ∂ωi/∂m in ocean observationsAnalytic theory of mesoscale vorticity and vertical shear predicts NIW kinetic properties that compares well with observations [ABSTRACT FROM AUTHOR]

Coastal physical processes are essential for the cross‐shore transport of meroplanktonic larvae to their benthic adult habitats. To investigate these processes, we released a swarm of novel, trackable, subsurface vehicles, the Mini‐Autonomous Underwater Explorers (M‐AUEs), which we programmed to mimic larval depth‐keeping behavior. The M‐AUE swarm measured a sudden net onshore transport of 30–70 m over 15–20 min, which we investigated in detail. Here, we describe a novel transport mechanism of depth‐keeping plankton revealed by these observations. In situ measurements and models showed that, as a weakly nonlinear internal wave propagated through the swarm, it deformed surface‐intensified, along‐isopycnal background velocities downward, accelerating depth‐keeping organisms onshore. These higher velocities increased both the depth‐keepers' residence time in the wave and total cross‐shore displacement, leading to wave‐induced transports twice those of fully Lagrangian organisms and four times those associated with the unperturbed background currents. Our analyses also show that integrating velocity time series from virtual larvae or mimics moving with the flow yields both larger and more accurate transport estimates than integrating velocity time series obtained at a point (Eulerian). The increased cross‐shore transport of organisms capable of vertical swimming in this wave/background‐current system is mathematically analogous to the increase in onshore transport associated with horizontal swimming in highly nonlinear internal waves. However, the mechanism described here requires much weaker swimming speeds (mm s−1 vs. cm s−1) to achieve significant onshore transports, and meroplanktonic larvae only need to orient themselves vertically, not horizontally. [ABSTRACT FROM AUTHOR]

Temperature variance and kinetic energy (KE) from two global simulations of the HYbrid Coordinate Ocean Model (HYCOM; 1/12° and 1/25°) and three global simulations of the Massachusetts Institute of Technology general circulation model (MITgcm; 1/12°, 1/24°, and 1/48°), all of which are forced by atmospheric fields and the astronomical tidal potential, are compared with temperature variance and KE from a database of about 2,000 moored historical observations (MHOs). The variances are computed across frequencies ranging from supertidal, dominated by the internal gravity wave continuum, to subtidal, dominated by currents and mesoscale eddies. The most important qualitative difference between HYCOM and MITgcm, and between simulations of different resolutions, is in the supertidal band, where the 1/48° MITgcm lies closest to observations. Across all frequency bands examined, the HYCOM simulations display higher spatial correlation with the MHO than do the MITgcm simulations. The supertidal, semidiurnal, and diurnal velocities in the HYCOM simulations also compare more closely with observations than do the MITgcm simulations in a number of specific continental margin/marginal sea regions. To complement the model‐MHO comparisons, this paper also compares the surface ocean geostrophic eddy KE in HYCOM, MITgcm, and a gridded satellite altimeter product. Consistent with the model‐MHO comparisons, the HYCOM simulations have a higher spatial correlation with the altimeter product than the MITgcm simulations do. On the other hand, the surface ocean geostrophic eddy KE is too large, relative to the altimeter product, in the HYCOM simulations. [ABSTRACT FROM AUTHOR]

Multisatellite altimetry and an eddy‐resolving model with tides are used to quantify the attenuation of the mode‐1 M2 internal tide as it propagates from three major sources in the North Pacific. The model is used to correct the altimetric fluxes for the nonstationary signal that altimeters cannot detect. Because internal tides in the North Pacific are highly stationary, these corrections do not materially impact the decay rate estimates. Fluxes are integrated in wedges extending from the sources to account for interference and radial spreading. Observed attenuation rates are consistent with e‐folding scales between 750 and 3,000 km, suggesting weak dissipation rates (≤10−10 W/kg or 0.75×10−3 W/m2) compared to typical open‐ocean turbulence levels, implicating near‐inertial waves and higher‐mode internal tides in providing the balance of the dissipation in the ocean interior. Key Points: Mode‐1 M2 internal tide attenuation across the North Pacific is quantifiedAssociated dissipation is weak compared to typical thermocline dissipation ratesE‐folding scales are 750–3,000 km [ABSTRACT FROM AUTHOR]

Wake eddies are frequently created by flow separation where ocean currents encounter abrupt topography in the form of islands or headlands. Most previous work has concentrated on wake eddy generation by either purely oscillatory (usually tidal) currents, or quasi‐steady mean flows. Here we report measurements near the point of flow separation at the northern end of the Palau island chain, where energetic tides and vertically sheared low‐frequency flows are both present. Energetic turbulence measured near the very steeply sloping ocean floor varied cubically with the total flow speed (primarily tidal). The estimated turbulent viscosity suggests a regime of flow separation and eddying wake generation for flows that directly feel this drag. Small‐scale (∼1 km), vertically sheared wake eddies of different vorticity signs were observed with a shipboard survey on both sides of the separation point, and significantly evolved over several tidal periods. The net production and export of vorticity into the wake, expected to sensitively depend on the interplay of tidal and low‐frequency currents, is explored here with a simple conceptual model. Application of the model to a 10‐month mooring record suggests that inclusion of high frequency oscillatory currents may boost the net flux of vorticity into the ocean interior by a depth dependent factor of 2 to 25. Models that do not represent the effect of these high frequency currents may not accurately infer the net momentum or energy losses felt where strong flows encounter steep island or headland topography. Key Point: Island wake generation is influenced by both low‐frequency and tidal flows [ABSTRACT FROM AUTHOR]

We describe a process called "squeeze dispersion" in which the squeezing of oceanic tracer gradients by waves, eddies, and bathymetric flow modulates diapycnal diffusion by centimeter to meter‐scale turbulence. Due to squeeze dispersion, the effective diapycnal diffusivity of oceanic tracers is different and typically greater than the average "local" diffusivity, especially when local diffusivity correlates with squeezing. We develop a theory to quantify the effects of squeeze dispersion on diapycnal oceanic transport, finding formulas that connect density‐averaged tracer flux, locally measured diffusivity, large‐scale oceanic strain, the thickness‐weighted average buoyancy gradient, and the effective diffusivity of oceanic tracers. We use this effective diffusivity to interpret observations of abyssal flow through the Samoan Passage reported by Alford et al. (2013, https://doi.org/10.1002/grl.50684) and find that squeezing modulates diapycnal tracer dispersion by factors between 0.5 and 3. Plain Language Summary: Turbulent vertical ocean mixing forms a key part of the Earth's climate system by drawing atmospheric carbon and heat into the massive reservoir that is the deep ocean. Quantifying vertical ocean mixing is difficult: vertical mixing is associated with turbulence at the tiny scales of centimeters to meters but affects the entire ocean on the long time scales of decades and centuries. We demonstrate that vertical ocean mixing depends not only on small‐scale turbulence, but on the combination of small‐scale turbulence and larger‐scale motions, such as currents, eddies, and waves similar to the jet streams and hurricanes of the atmosphere. In particular, when a patch of ocean is mixed by small‐scale turbulence while being "squeezed" in the vertical at the same time by currents and eddies, the patch ultimately mixes more quickly than the turbulence would cause alone. This means that estimating the total rate of oceanic vertical mixing requires knowledge both of the magnitude of ocean squeezing as well as the intensity of small‐scale ocean turbulence. Key Points: Squeezing and stretching of density layers modulates the diapycnal diffusion of oceanic tracersSqueeze dispersion enhances dispersion by 2–3 times across some isopycnals in the abyssal Samoan PassageDiapycnal transport is strongly affected by positive correlations between squeezing and turbulence [ABSTRACT FROM AUTHOR]

Abstract: The generation, propagation, and dissipation of nonlinear internal waves (NLIW) in sheared background currents is examined using 7 days of shipboard microstructure surveys and two moorings on the continental shelf offshore of Washington state. Surveys near the hypothesized generation region show semi‐diurnal (D2) energy flux is onshore and that the ratio of energy flux to group speed times energy ( F / c g E) increases sharply at the shelf break, suggesting that the incident D2 internal tide is partially reflected and partially transmitted. NLIW appear at an inshore mooring at the leading edge of the onshore phase of the baroclinic tide, consistent with nonlinear transformation of the shoaling internal tide as their generation mechanism. Of the D2 energy flux observed at the eastern extent of the generation region (133 ± 18 Wm−1), approximately 30% goes into the NLIW observed inshore (36 ± 11 Wm−1). Inshore of the moorings, 7 waves are tracked into shallow (30‐40 m) water, where a vertically sheared, southward current becomes strong. As train‐like waves propagate onshore, wave amplitudes of 25‐30 m and energies of 5 MJ decrease to 12 m and 10 kJ, respectively. The observed direction of propagation rotates from 30 ° N of E to ∼ 30 ° S of E in the strongly sheared region. Linear ray tracing using the Taylor‐Goldstein equation to incorporate parallel shear effects accounts for only a small portion of the observed rotation, suggesting that three‐dimensionality of the wave crests and the background currents is important here. [ABSTRACT FROM AUTHOR]

Abstract: Internal waves, which drive most ocean turbulence and add “noise” to lower‐frequency records, interact with low‐frequency current systems and topography in yet poorly known ways. Taking advantage of a heavily instrumented, 14 month mooring array, internal waves in the East Australian Current (EAC) are examined for the first time. Internal wave horizontal kinetic energy (HKE) is within a factor of 2 of the Garrett‐Munk (1976) spectrum. Continuum internal waves, near‐inertial waves, and internal tides together constitute a significant percentage of the total velocity variance. Mode‐1 internal tide energy fluxes are southward and much smaller than energy times group velocity, consistent with reflection at the continental slope of incident waves generated from near New Caledonia and the Solomon Islands. Internal tide HKE is highly phase variable, consistent with refraction by the variable EAC. Mode‐1 near‐inertial wave energy fluxes are of comparable magnitude and are equatorward and episodic, consistent with generation by storms farther poleward. These processes are considered together in the complex environment of the EAC. [ABSTRACT FROM AUTHOR]

Two global ocean models ranging in horizontal resolution from 1/12° to 1/48° are used to study the space and time scales of sea surface height (SSH) signals associated with internal gravity waves (IGWs). Frequency-horizontal wavenumber SSH spectral densities are computed over seven regions of the world ocean from two simulations of the HYbrid Coordinate Ocean Model (HYCOM) and three simulations of the Massachusetts Institute of Technology general circulation model (MITgcm). High wavenumber, high-frequency SSH variance follows the predicted IGW linear dispersion curves. The realism of high-frequency motions (> [ABSTRACT FROM AUTHOR]