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Abstract: Abyssal waters forming the lower limb of the global overturning circulation flow through the Samoan Passage and are modified by intense mixing. Thorpe-scale-based estimates of dissipation from moored profilers deployed on top of two sills for 17 months reveal that turbulence is continuously generated in the passage. Overturns were observed in a density band in which the Richardson number was often smaller than ¼, consistent with shear instability occurring at the upper interface of the fast-flowing bottom water layer. The magnitude of dissipation was found to be stable on long time scales from weeks to months. A second array of 12 moored profilers deployed for a shorter duration but profiling at higher frequency was able to resolve variability in dissipation on time scales from days to hours. At some mooring locations, near-inertial and tidal modulation of the dissipation rate was observed. However, the modulation was not spatially coherent across the passage. The magnitude and vertical structure of dissipation from observations at one of the major sills is compared with an idealized 2D numerical simulation that includes a barotropic tidal forcing. Depth-integrated dissipation rates agree between model and observations to within a factor of 3. The tide has a negligible effect on the mean dissipation. These observations reinforce the notion that the Samoan Passage is an important mixing hot spot in the global ocean where waters are being transformed continuously.

This study investigates three-dimensional semidiurnal internal tide (IT) energetics in the vicinity of La Jolla Canyon, a steep shelf submarine canyon off the Southern California coast, with the Stanford Unstructured Nonhydrostatic Terrain-Following Adaptive Navier–Stokes Simulator (SUNTANS) numerical simulator. Numerical simulations show vertical structure and temporal phasing consistent with detailed field observations. ITs induce large (approximately 34 m from peak to peak) isotherm displacements and net onshore IT energy flux up to 200 W m−1. Although the net IT energy flux is onshore, the steep supercritical slope around the canyon results in strong reflection. The model provides the full life span of internal tides around the canyon, including internal tide generation, propagation, and dissipation. ITs propagate into the canyon from the south and are reflected back toward offshore from the canyon's north side. In the inner part of the canyon, elevated mixing occurs in the middle layer due to an interaction between incident mode-1 ITs and reflected higher-mode ITs. The magnitude of IT flux, generation, and dissipation on the south side of the canyon are higher than those on the north side. An interference pattern in horizontal kinetic energy and available potential energy with a scale of approximately 20–50 km arises due to low-mode wave reflections. Our results provide new insight into IT dynamics associated with a small-scale canyon topography. Significance Statement: Internal waves play an important role in ocean circulations and ecosystems. In particular, internal waves with frequencies of tides, known as internal tides, strongly enhance energy, heat, and mass transport in coastal oceans. This study presents internal tide dynamics in La Jolla Canyon, California, using a high-resolution numerical model. Model results show energy convergence in the canyon leading to internal tide energy dissipation and mixing. Some parts of internal tide energy reflect back offshore resulting in standing internal waves off California. This study provides new insights into internal tide dynamics and energy budgets in submarine canyons. [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]

Moored observations and a realistic, tidally forced 3D model are presented of flow and internal-tide-driven turbulence over a supercritical 3D fan in southeastern Luzon Strait. Two stacked moored profilers, an acoustic Doppler current profiler, and a thermistor string measured horizontal velocity, density, and salinity over nearly the entire water column every 1.5 h for 50 days. Observed dissipation rate computed from Thorpe scales decays away from the bottom and shows a strong spring–neap cycle; observed depth-integrated dissipation rate scales as U BT 2.5 ± 0.6 where UBT is the barotropic velocity. Vertical velocities are strong enough to be comparable at times to the vertical profiling speed of the moored profilers, requiring careful treatment to quantify bias in dissipation rate estimates. Observations and the model are in reasonable agreement for velocity, internal wave displacement and depth-integrated dissipation rate, allowing the model to be used to understand the 3D flow. Turbulence is maximum following the transition from up-fan to down-fan flow, consistent with breaking lee waves advected past the mooring as seen previously at the Hawaiian Ridge, but asymmetric flow arises because of the 3D topography. Observed turbulence varies by a factor of 2 over the four observed spring tides as low-frequency near-bottom flow changes, but the exact means for inclusion of such low-frequency effects is not clear. Our results suggest that for the extremely energetic turbulence associated with breaking lee waves, dissipation rates may be quantitatively predicted to within a factor of 2 or so using numerical models and simple scalings. Significance Statement: This paper describes deep ocean turbulence caused by strong tidal and low-frequency meandering flows over and around a three-dimensional bump, using moored observations and a computer simulation. Such information is important for accurately including these effects in climate simulations. The observations and model agree well enough to be able to use both to synthesize a coherent picture. The observed and modeled turbulence scale as the cube of the tidal speed as expected from theory, but low-frequency flows complicate the picture. We also demonstrate the underestimation of the turbulence that can result when vertical profiling rates are comparable to the internal wave velocities. [ABSTRACT FROM AUTHOR]

Pacific Summer Water eddies and intrusions transport heat and salt from boundary regions into the western Arctic basin. Here we examine concurrent effects of lateral stirring and vertical mixing using microstructure data collected within a Pacific Summer Water intrusion with a length scale of ∼20 km. This intrusion was characterized by complex thermohaline structure in which warm Pacific Summer Water interleaved in alternating layers of O (1) m thickness with cooler water, due to lateral stirring and intrusive processes. Along interfaces between warm/salty and cold/freshwater masses, the density ratio was favorable to double-diffusive processes. The rate of dissipation of turbulent kinetic energy (ε) was elevated along the interleaving surfaces, with values up to 3 × 10−8 W kg−1 compared to background ε of less than 10−9 W kg−1. Based on the distribution of ε as a function of density ratio Rρ, we conclude that double-diffusive convection is largely responsible for the elevated ε observed over the survey. The lateral processes that created the layered thermohaline structure resulted in vertical thermohaline gradients susceptible to double-diffusive convection, resulting in upward vertical heat fluxes. Bulk vertical heat fluxes above the intrusion are estimated in the range of 0.2–1 W m−2, with the localized flux above the uppermost warm layer elevated to 2–10 W m−2. Lateral fluxes are much larger, estimated between 1000 and 5000 W m−2, and set an overall decay rate for the intrusion of 1–5 years. [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]

An intrahalocline eddy was observed on the Chukchi slope in September of 2015 using both towed CTD and microstructure temperature and shear sections. The core of the eddy was 6°C, significantly warmer than the surrounding −1°C water and far exceeding typical temperatures of warm-core Arctic eddies. Microstructure sections indicated that outside of the eddy the rate of dissipation of turbulent kinetic energy ε was quite low . However, at the edges of the eddy core, ε was elevated to . Three different processes were associated with elevated ε. Double-diffusive steps were found at the eddy's top edge and were associated with an upward heat flux of 5 W m−2. At the bottom edge of the eddy, shear-driven mixing played a modest role, generating a heat flux of approximately 0.5 W m−2 downward. Along the sides of the eddy, density-compensated thermohaline intrusions transported heat laterally out of the eddy, with a horizontal heat flux of 2000 W m−2. Integrating these fluxes over an idealized approximation of the eddy's shape, we estimate that the net heat transport due to thermohaline intrusions along the eddy flanks was 2 GW, while the double-diffusive flux above the eddy was 0.4 GW. Shear-driven mixing at the bottom of the eddy accounted for only 0.04 GW. If these processes continued indefinitely at the same rate, the estimated life-span would be 1–2 years. Such eddies may be an important mechanism for the transport of Pacific-origin heat, freshwater, and nutrients into the Canada Basin. [ABSTRACT FROM AUTHOR]

The M2 internal tide in the Tasman Sea is investigated using sea surface height measurements made by multiple altimeter missions from 1992 to 2012. Internal tidal waves are extracted by two-dimensional plane wave fits in 180 km by 180 km windows. The results show that the Macquarie Ridge radiates three internal tidal beams into the Tasman Sea. The northern and southern beams propagate respectively into the East Australian Current and the Antarctic Circumpolar Current and become undetectable to satellite altimetry. The central beam propagates across the Tasman Sea, impinges on the Tasmanian continental slope, and partially reflects. The observed propagation speeds agree well with theoretical values determined from climatological ocean stratification. Both the northern and central beams refract about 15° toward the equator because of the beta effect. Following a concave submarine ridge in the source region, the central beam first converges around 45.5°S, 155.5°E and then diverges beyond the focal region. The satellite results reveal two reflected internal tidal beams off the Tasmanian slope, consistent with previous numerical simulations and glider measurements. The total energy flux from the Macquarie Ridge into the Tasman Sea is about 2.2 GW, of which about half is contributed by the central beam. The central beam loses little energy in its first 1000-km propagation, for which the likely reasons include flat bottom topography and weak mesoscale eddies. [ABSTRACT FROM AUTHOR]

The reflection of a low-mode internal tide on the Tasman continental slope is investigated using simulations of realistic and simplified topographies. The slope is supercritical to the internal tide, which should predict a large fraction of the energy reflected. However, the response to the slope is complicated by a number of factors: the incoming beam is confined laterally, it impacts the slope at an angle, there is a roughly cylindrical rise directly offshore of the slope, and a leaky slope-mode wave is excited. These effects are isolated in simulations that simplify the topography. To separate the incident from the reflected signal, a response without the reflector is subtracted from the total response to arrive at a reflected signal. The real slope reflects approximately 65% of the mode-1 internal tide as mode 1, less than two-dimensional linear calculations predict, because of the three-dimensional concavity of the topography. It is also less than recent glider estimates, likely as a result of along-slope inhomogeneity. The inhomogeneity of the response comes from the Tasman Rise that diffracts the incoming tidal beam into two beams: one focused along beam and one diffracted to the north. Along-slope inhomogeneity is enhanced by a partially trapped, superinertial slope wave that propagates along the continental slope, locally removing energy from the deep-water internal tide and reradiating it into the deep water farther north. This wave is present even in a simplified, straight slope topography; its character can be predicted from linear resonance theory, and it represents up to 30% of the local energy budget. [ABSTRACT FROM AUTHOR]

The most comprehensive studies of the spatial and temporal scales of diffusivity rely on internal wave parameterizations that require knowledge of finescale shear and strain. Studies lacking either shear or strain measurements have to assume a constant ratio between shear and strain ( R ω). Data from 14 moorings collected during five field programs are examined to determine the spatial and temporal patterns in R ω and the influence of these patterns on parameterized diffusivity. Time-mean R ω ranges from 1 to 10, with changes of order 10 observed over a broad range of scales. Temporal variability in R ω is observed at daily, weekly, and monthly scales. Observed changes in R ω could produce a 2-3 times change in parameterized diffusivity. Vertical profiles of R ω, Eshear, and Estrain (shear or strain variance relative to Garret-Munk) reveal that both local topographic properties and wind variability impact the internal wave field. Time series of R ω from each mooring have strong correlations to either shear or strain, often only at a specific range of vertical wavenumbers. Sites fall into two categories, in which R ω variability is dominated by either shear or strain. Linear fits to the dominant property (i.e., shear or strain) can be used to estimate a time series of R ω that has an RMS error that is 30% less than the RMS error from assuming R ω = 3. Shear and strain level vary in concert, as predicted by the Garret-Munk model, at high Eshear values. However, at Eshear < 5, strain variations are 3 times weaker than shear. [ABSTRACT FROM AUTHOR]

The flow of dense water through the Samoan Passage accounts for the major part of the bottom water renewal in the North Pacific and is thus an important element of the Pacific meridional overturning circulation. A recent set of highly resolved measurements used CTD/LADCP, a microstructure profiler, and moorings to constrain the complex pathways and variability of the abyssal flow. Volume transport estimates for the dense northward current at several sections across the passage, calculated using direct velocity measurements from LADCPs, range from 3.9 × 106 to 6.0 × 106 ± 1 × 106 m3 s−1. The deep channel to the east and shallower pathways to the west carried about equal amounts of this volume transport, with the densest water flowing along the main eastern channel. Turbulent dissipation rates estimated from Thorpe scales and direct microstructure agree to within a factor of 2 and provide a region-averaged value of O(10−8) W kg−1 for layers colder than 0.8°C. Associated diapycnal diffusivities and downward turbulent heat fluxes are about 5 × 10−3 m2 s−1 and O(10) W m−2, respectively. However, heat budgets suggest heat fluxes 2-6 times greater. In the vicinity of one of the major sills of the passage, highly resolved Thorpe-inferred diffusivity and heat flux were over 10 times larger than the region-averaged values, suggesting the mismatch is likely due to undersampled mixing hotspots. [ABSTRACT FROM AUTHOR]

The Community Climate System Model, version 4 (CCSM4) is used to assess the climate impact of wind-generated near-inertial waves (NIWs). Even with high-frequency coupling, CCSM4 underestimates the strength of NIWs, so that a parameterization for NIWs is developed and included into CCSM4. Numerous assumptions enter this parameterization, the core of which is that the NIW velocity signal is detected during the model integration, and amplified in the shear computation of the ocean surface boundary layer module. It is found that NIWs deepen the ocean mixed layer by up to 30%, but they contribute little to the ventilation and mixing of the ocean below the thermocline. However, the deepening of the tropical mixed layer by NIWs leads to a change in tropical sea surface temperature and precipitation. Atmospheric teleconnections then change the global sea level pressure fields so that the midlatitude westerlies become weaker. Unfortunately, the magnitude of the real air-sea flux of NIW energy is poorly constrained by observations; this makes the quantitative assessment of their climate impact rather uncertain. Thus, a major result of the present study is that because of its importance for global climate the uncertainty in the observed tropical NIW energy has to be reduced. [ABSTRACT FROM AUTHOR]

Observations of breaking internal tides on the Oregon continental slope during a 40-day deployment of 5 moorings along 43°12′N are presented. Remotely generated internal tides shoal onto the slope, steepen, break, and form turbulent bores that propagate upslope independently of the internal tide. A high-resolution snapshot of a single bore is captured from lowered acoustic Doppler current profilers (LADCP)/CTD profiles in a 25-h time series at 1200 m. The bore is cold, salty, over 100 m tall, and has a turbulent head where instantaneous dissipation rates are enhanced ( ε > 10−6 W kg−1) and sediment is resuspended. At the two deepest slope moorings (1452 and 1780 m), similar borelike phenomena are observed in near-bottom high-resolution temperature time series. Mean dissipation rates and diapycnal diffusivities increase by a factor of 2 when bores are present ( W kg−1 and m s−1) and observed internal tides are energetic enough to drive these enhanced dissipation rates. Globally, the authors estimate an average of 1.3 kW m−1 of internal tide energy flux is directed onto continental slopes. On the Oregon slope, internal tide fluxes are smaller, suggesting that it is a relatively weak internal tide sink. Mixing associated with the breaking of internal tides is therefore likely to be larger on other continental slopes. [ABSTRACT FROM AUTHOR]

Internal tide generation, propagation, and dissipation are investigated in Luzon Strait, a system of two quasi-parallel ridges situated between Taiwan and the Philippines. Two profiling moorings deployed for about 20 days and a set of nineteen 36-h lowered ADCP-CTD time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals. Measurements were concentrated on a northern line, where the ridge spacing was approximately equal to the mode-1 wavelength for semidiurnal motions, and a southern line, where the spacing was approximately two-thirds that. The authors contrast the two sites to emphasize the potential importance of resonance between generation sites. Throughout Luzon Strait, baroclinic energy, energy fluxes, and turbulent dissipation were some of the strongest ever measured. Peak-to-peak baroclinic velocity and vertical displacements often exceeded 2 m s−1 and 300 m, respectively. Energy fluxes exceeding 60 kW m−1 were measured at spring tide at the western end of the southern line. On the northern line, where the western ridge generates appreciable eastward-moving signals, net energy flux between the ridges was much smaller, exhibiting a nearly standing wave pattern. Overturns tens to hundreds of meters high were observed at almost all stations. Associated dissipation was elevated in the bottom 500-1000 m but was strongest by far atop the western ridge on the northern line, where >500-m overturns resulted in dissipation exceeding 2 × 10−6 W kg−1 (implying diapycnal diffusivity Kρ > 0.2 m2 s−1). Integrated dissipation at this location is comparable to conversion and flux divergence terms in the energy budget. The authors speculate that resonance between the two ridges may partly explain the energetic motions and heightened dissipation. [ABSTRACT FROM AUTHOR]

The relative strength and spatiotemporal structure of near-inertial waves (NIW) and internal tides (IT) are examined in the context of recent moored observations made 19 km south of Mendocino Escarpment, an abrupt ridge/step feature in the eastern Pacific. In addition to strong internal tide generation, steps and ridges give rise to the possibility of ''shadowing,'' wherein near-inertial energy is prevented from reaching depths beneath a characteristic intersecting the ridge top. A combination of two moored profilers and a long-range acoustic Doppler current profiler (ADCP) yielded velocity and shear measurements from 100 to 3640 m (60 m above bottom) and isopycnal depth, strain, and overturn-inferred turbulence dissipation rate from 1000 to 3640 m. Sampling intervals (20 min in the upper 1000 m and 1.5 h below that) were fast enough to minimize aliasing of higher-frequency internal-wave motions. The 67-day-long record is easily sufficient to isolate NIW and IT via bandpass filtering and to capture low-frequency variability in all quantities. No near-inertial shadowing was observed. Instead, energetic near-inertial waves were observed at all depths, radiating both upward and downward. A strong upward internal tide beam, showing a pronounced spring--neap cycle, was also seen near the expected depth. Case studies of each of these are presented in depth and isopycnal-following coordinates. Except for immediately above the bottom and in the ''beam,'' where IT kinetic energy shows marked peaks, kinetic energy in the two bands is within a factor of 2 of each other. However, because of the redder NIW vertical wavenumber spectrum, NIW shear exceeded IT shear at all depths by a factor of 2--4. Dissipation rate was strongly enhanced in the bottom 1000 m and in the depth range of the internal tide beam. However, except for very near the bottom and possibly in one NIW event, no clear phase relationship was observed between dissipation rate and wave shear or strain, suggesting that turbulence occurs through a cascade process rather than by direct breaking at most locations. [ABSTRACT FROM AUTHOR]