Diurnal Internal Tides Research Articles

Investigation of Internal Tides Variability in the Andaman Sea: Observations and Simulations

AbstractThe Andaman Sea in the Indian Ocean is known for the presence of large‐amplitude Internal waves (IW). Internal tides (IT) are IW of tidal frequency whose temporal variability is unknown in this region. Therefore, we used in‐situ observations collected at (10.5°N, 94°E) from March 2017 to February 2018. The analysis shows that the kinetic energy of semidiurnal IT is dominant to that of diurnal IT by a factor of 4–5. The ellipticity of both semidiurnal and diurnal motions is dominated by rectilinear zonal flow, indicating generation at the slopes of the Andaman and Nicobar islands. Maximum isopycnal displacement reached 46 m for semidiurnal IT. The semidiurnal IT displayed significant seasonal variability—Stronger in summer and autumn but weaker in spring and winter, whereas the diurnal IT are relatively stronger in summer and winter. Furthermore, salinity plays a dominant role in controlling the near‐surface stratification, whereas the temperature variations control the subsurface stratification. This led to the formation of a strong double pycnocline during autumn and winter. Baroclinic coherent semidiurnal (diurnal) variance accounts for 49% (27%) of the semidiurnal (diurnal) motions. Model simulations carried out for March, June, September, and December of 2017 using the Massachusetts Institute of Technology General Circulation Model showed significant seasonal variability in the generation and dissipation of IT. The isopycnal displacement near the generation sites is about 88 m. The experiments suggest that the presence of the Andaman Nicobar Ridge contributes nearly 89% to the total IT generation.

Radiation Path of Diurnal Internal Tides in the Northwestern Pacific Controlled by Refraction and Interference

AbstractThe diurnal internal tides contribute nearly a quarter of global baroclinic tidal energy. However, their roles in shaping the spatiotemporal inhomogeneity of the tidal energy field are not well known. Based on observation‐supported numerical simulation and theoretical analyses, we clarify the combined and relative contributions of β refraction, subtidal circulation refraction and multiwave interference to the long‐range radiation and dissipation maps of diurnal internal tides in the northwestern Pacific. The diurnal tidal beams primarily emanate from the Luzon Strait (LS) and Talaud‐Halmahera Passage (THP). The β refraction shifts the mean path of the LS tidal beam equatorward by ∼40° when it arrives at the deep basin. This is consistent with previous altimetric observations. A second refraction effect by subtidal circulation with seasonal variability deflects the mean beam path by ∼10°. Multiwave interference of the tidal beams from the LS and THP further enhances the inhomogeneous pattern. It enhances or reduces the energy fluxes of the tidal beams with distinct vertical structures in the western Mariana basin. A modified line‐source model and theoretical ray‐tracing analysis explain the effects of refraction and interference well. The internal tidal dissipation map in the deep basin coincides well with the inhomogeneous and spreading radiation paths. The characterization of the world's most energetic diurnal internal tides in the northwestern Pacific could improve our understanding of global baroclinic tidal energy redistribution and tidal mixing geography.

High-Frequency Motions in the Southeastern South China Sea During Winter–Spring 2018/2019

High-frequency motions in the southeastern South China Sea (SCS) have rarely been investigated due to sparse field observations. The vertical distribution and temporal variation of internal tides (ITs) and near-inertial waves (NIWs) near the Nansha area in the southeastern SCS were studied using a mooring current dataset from December 2018 to June 2019 in this study. Results showed that ITs were mainly dominated by O1, K1, and M2. Tidal energy analysis indicates that the diurnal ITs were the most energetic components, followed by the semidiurnal ITs. Modal decomposition reveals that diurnal ITs were dominated by mode-3, rather than mode-1, as reported by previous studies. The horizontal kinetic energy (HKE) of diurnal ITs fluctuated within a limited range, almost unaffected by the background field. However, the HKE of semidiurnal ITs was dominated by mode-1 and more affected by the background field, especially at the end of March. Most observations showed the phase of the NIWs propagating upward and the energy propagating downward. During the northeast monsoon period, the near-inertial energy had a large magnitude due to strong wind forcing. In addition, the near-inertial energy peaked from the middle of March to the beginning of April because of the input of NIWs from afar. Overall, near-inertial energy was found concentrated above a 500–600 m depth in the southeastern SCS.

Application of S_TIDE in exploration of seasonal variations of internal tidal amplitudes in the northern South China Sea

AbstractAn enhanced harmonic analysis (S_TIDE) approach is adopted to examine the seasonal variations of internal tidal amplitudes in the northern South China Sea (SCS). Results of idealized experiments reveal that the seasonality can be captured by S_TIDE. By applying S_TIDE to mooring data, observed seasonality of internal tidal amplitudes in the northern SCS are explored. Not diurnal and semidiurnal internal tides (ITs), but overtides and long-period constituents of ITs exhibit clear seasonal cycles. However, differences between amplitudes of the eastward velocity and the northward counterpart are evident for K1, M2 and MK3, which may be caused by the intensification of background currents. Amplitudes of those ITs are stronger at intersection time between spring and summer in the eastward direction, but weaker in the northward direction. EOF analysis reveals that modes of diurnal ITs are higher than those of seimidiurnal ITs, which induces relatively more complicated seasonal variations. In addition to intensification of background currents, influences of surface tides and stratification will also induce variations of internal tidal amplitudes, introducing tremendous difficulty in predicting variation trends of internal tidal amplitudes, which greatly reduces predictability of ITs.

Generation of Shoreward Nonlinear Internal Waves South of the Hainan Island: Synthetic Aperture Radar Observations and Numerical Simulations

AbstractThe generation of shoreward nonlinear internal waves (NLIWs) on the continental shelf south of the Hainan Island (SHI) is investigated based on spaceborne synthetic aperture radar (SAR) observations and numerical simulations. Two types of shoreward NLIWs are identified from SAR images according to their distinct geographic distribution. One type of NLIWs, named Type‐N NLIWs, is distributed in the northern part of SHI, and the other one is named Type‐S NLIWs, distributed in the southern part of SHI. The SAR‐observed wave occurrence frequency during the spring and neap tides, combined with the calculated body force, suggests that the Type‐N NLIWs originate from the Xisha Islands, whereas the Type‐S NLIWs originate from both the Xisha Islands and the continental shelf break, and the shelf break has a larger contribution. The synergistic analyses of the internal tidal raypath, gamma parameter and earliest SAR‐observed NLIWs suggest that the Type‐N NLIWs are caused by the impingement of the diurnal internal tidal beams emanating from the Xisha Islands on the near‐surface pycnocline close to the continental shelf. Based on the realistic shelf‐slope topography and tidal forcing, the two‐dimensional numerical simulations using the MITgcm suggest that the Type‐S NLIWs result from the nonlinear disintegration of a mode‐1 diurnal internal tide, which develops from a depression formed at the continental shelf break. Furthermore, numerical experiments show that the background current can greatly affect the nonlinear evolution of the internal waves generated at the shelf break.

Variance of Bottom Water Temperature at the Continental Margin of the Northern South China Sea

AbstractHigh‐resolution temperature variation was examined about 40 days near the ocean bottom at 20 sites within an area of 110 × 120 km2 close to the continental margin of the northern South China Sea (depth ≈ 4,000 m). The monitoring depth (0.5 m above the bottom) was presumably within the lower part of the bottom mixed layer, compared to its median thickness of 63 m. At all sites, the bottom water had temperature variance less than 0.01°C, and it alternatively varied in the near‐quiescent and internal wave states with time interval of diurnal period or even longer. Spectra of temperature variation showed that diurnal frequency dominated over the internal wave band. At different sites, the transition between internal wave band of scaling −2 and the inertial subrange turbulence of scaling −5/3 cannot easily be discriminated, attributed to the different intermittent presences of internal waves (tides). However, the resolved viscous‐convective and viscous‐diffusive subranges in the temperature spectrum provide the opportunity to evaluate the thermal (χ) and kinetic energy (ɛ) dissipation rates near the ocean bottom at each site. Close to a seamount, χ and ɛ were inferred to be about 1.2 × 10−12 °C2s−1 and 7.2 × 10−10 m2s−3, respectively, and they were strongly modulated by the diurnal internal tides. In the flat abyssal plain, χ and ɛ were much weaker, about at 3.2 × 10−14 °C2s−1 and 1.7 × 10−11 m2s−3, respectively. The spatial distribution of the nominal thermal χ, kinetic energy ɛ and the turbulent diffusivity Kρ indicated that they varied in the ranges [3.9 × 10−15 1.9 × 10−12] °C2s−1, [2.0 × 10−12 1.1 × 10−9] m2s−3, and [8.3 × 10−6 5.2 × 10−3] m2s−1, respectively, representing the different influences of internal waves and local topography.

On the Predictability of Sea Surface Height around Palau

AbstractAs part of the Flow Encountering Abrupt Topography (FLEAT) program, an array of pressure-sensor equipped inverted echo sounders (PIESs) was deployed north of Palau where the westward-flowing North Equatorial Current encounters the southern end of the Kyushu–Palau Ridge in the tropical North Pacific. Capitalizing on concurrent observations from satellite altimetry, FLEAT Spray gliders, and shipboard hydrography, the PIESs’ 10-month duration hourly bottom pressure p and round-trip acoustic travel time τ records are used to examine the magnitude and predictability of sea level and pycnocline depth changes and to track signal propagations through the array. Sea level and pycnocline depth are found to vary in response to a range of ocean processes, with their magnitude and predictability strongly process dependent. Signals characterized here comprise the barotropic tides, semidiurnal and diurnal internal tides, southeastward-propagating superinertial waves, westward-propagating mesoscale eddies, and a strong signature of sea level increase and pycnocline deepening associated with the region’s relaxation from El Niño to La Niña conditions. The presence of a broad band of superinertial waves just above the inertial frequency was unexpected and the FLEAT observations and output from a numerical model suggest that these waves detected near Palau are forced by remote winds east of the Philippines. The PIES-based estimates of pycnocline displacement are found to have large uncertainties relative to overall variability in pycnocline depth, as localized deep current variations arising from interactions of the large-scale currents with the abrupt topography around Palau have significant travel time variability.

Investigation of the Internal Tides in the Northwest Pacific Ocean Considering the Background Circulation and Stratification

AbstractA global ocean circulation and tide model with nonuniform resolution is used in this work to resolve the ocean circulation globally as well as mesoscale eddies and internal tides regionally. Focusing on the northwest Pacific Ocean (NWP, 0°–35°N, 105°–150°E), a realistic experiment is conducted to simulate internal tides considering the background circulation and stratification. To investigate the influence of a background field on the generation and propagation of internal tides, idealized cases with horizontally homogeneous stratification and zero surface fluxes are also implemented for comparison. By comparing the realistic cases with idealized ones, the astronomical tidal forcing is found to be the dominant factor influencing the internal tide conversion rate magnitude, whereas the stratification acts as a secondary factor. However, stratification deviations in different areas can lead to an error exceeding 30% in the local internal tide energy conversion rate, indicating the necessity of a realistic stratification setting for simulating the entire NWP. The background shear is found to refract propagating diurnal internal tides by changing the effective Coriolis frequencies and phase speeds, while the Doppler-shifting effect is remarkable for introducing biases to semidiurnal results. In addition, nonlinear baroclinic tide energy equations considering the background circulation and stratification are derived and diagnosed in this work. The mean flow–baroclinic tide interaction and nonlinear energy flux are the most significant nonlinear terms in the derived equations, and nonlinearity is estimated to contribute approximately 5% of the total internal tide energy in the greater Luzon Strait area.

Variability of coherent and incoherent features of internal tides in the north South China Sea

The coherent and incoherent features of internal tides (ITs) in the north South China Sea (SCS) are investigated based on observations and numerical simulations. The 11-month (from May 2011 to March 2012) moored current observations indicate that coherent semidiurnal ITs are obviously amplified, which can be attributed to the interference of ITs. Interference enhances coherent motions of semidiurnal ITs, but weakens those of diurnal ITs. Moreover, observations also show that semidiurnal ITs are more incoherent than diurnal ITs. Variations of vertical stratification and surface tide forcing can hardly affect the incoherence of ITs. The increase of incoherent signal is largely due to the influence of mesoscale eddies. Mesoscale eddies affect both amplitude and phase of ITs, making them more incoherent. Mesoscale eddies not only increase the intensity of background currents, but also induce horizontal variations of density. Variations of horizontal density and the influence of background currents lead to the increase of incoherent signals. And semidiural ITs are more sensitive to the influence of mesoscale eddies, making them more incoherent than diurnal counterparts. Incoherent ITs, which induce strong current shear, play essential roles in cascading tidal energy to small-scale motions, and contribute to turbulent mixing eventually. The findings help to better understand ITs and may offer reference for the improvement of parameterization of ocean turbulent mixing in the northern SCS.

Seasonal Variability of Internal Tides Northeast of Taiwan

The spatial-temporal characteristics of the barotropic tides and internal tides (ITs) northeast of Taiwan Island are examined, based on a 1-year mooring current observations from May 23, 2017 to May 19, 2018. The results of harmonic tidal analysis show that the barotropic tides are dominated by semidiurnal tides, which is mainly controlled by M 2 tidal components. Moreover, the vertical structures of diurnal and semidiurnal ITs show that the semidiurnal IT shows notable seasonal variation, whereas seasonal variations of the diurnal IT energy is not significant. The semidiurnal IT energy in winter half year is twice that in summer half year. The seasonal variation of semidiurnal IT is mainly modulated by the direction change of the current rather than by the topographic features and stratification. In summer (winter) half year cyclonic (anti-cyclonic) eddies meanly control at this point, so the flow direction is mainly in the southwest (northeast) direction, causing the background flow to flow along (perpendicular to) the isobath. When crossing the isobath, the ITs are generated by the interaction of the barotropic tide and the topography, resulting in the increase of the tidal energy in the winter half year.

Observations of semidiurnal M2 internal tidal parametric subharmonic instability in the northeastern South China Sea

Near-diurnal vertically-standing waves with high vertical wavenumbers kz were observed in the velocity and shear fields from a set of 75-d long ADCP moored in the northeastern South China Sea (SCS) away from the “critical” latitude of 28.8°. These enhanced near-diurnal internal waves followed a fortnightly spring-neap cycle. However, they always happened during semidiurnal spring tides rather than diurnal springs although strong diurnal internal tides with the fortnightly spring-neap cycle were prevailing, suggesting that they were generated via subharmonic instability (PSI) of dominant semidiurnal M2 internal tides. When two semidiurnal internal tidal waves with opposite vertical propagation direction intersected, both semidiurnal subharmonic and super harmonic waves were largely intensified. The observed maximum diurnal velocity amplitudes were up to 0.25 m/s. The kinetic energy and shear spectra further suggested that frequencies of daughter waves were not always perfectly equal to M2/2. The superposition of two daughter waves with nearly equal frequencies and nearly opposite kz in a PSI-triad leaded to the vertically-standing waves.

The Influence of ENSO on the Structure of Internal Tides in the Xisha Area

AbstractBased on mooring data in the Xisha area of the South China Sea from August 2007 to December 2016, we found that the diurnal internal tides (ITs) had a tendency to weaken from 2008 to 2016, while the semidiurnal ITs were enhanced from 2011 to 2015. The diurnal and semidiurnal ITs in the Xisha area were negatively and positively correlated with the Niño 3.4 Index, respectively. It was also found that the large‐scale El Niño–Southern Oscillation (ENSO) can influence ITs. The link between ENSO and ITs is stratification. The upper‐ocean stratification of the Xisha area in El Niño years was stronger than that in La Niña years. Under the effects of stratification, the structures of diurnal and semidiurnal ITs showed different patterns during El Niño and La Niña years; both were dominated by low modes in La Niña years but displayed more signals of high modes in El Niño years.

Cross-Shore Deformation of a Surfzone-Released Dye Plume by an Internal Tide on the Inner Shelf

AbstractAn inner-shelf (IS) dye plume that formed following a 3.84-h early morning surfzone (SZ) dye release off of Imperial Beach, California, is analyzed with in situ and aerial remotely sensed observations. Midmorning, 5 h after release start, the IS plume extended 800 m offshore (or ≈8Lsz, where Lsz is the surfzone width) and was surface intensified. Over the next ≈2 h, the IS plume deformed (narrowed) cross-shore with the offshore front progressing onshore at ≈5 cm s−1, deepened by up to 3 m, and elongated alongshore at ≈4.5 cm s−1 km−1 (at ≈2.5Lsz). Coincident with IS plume deformation and deepening, IS isotherms also deepened, with relatively stable IS plume joint dye and temperature statistics. Offshore tracer transport and subsequent IS plume deformation and deepening likely resulted from two phases of the diurnal internal tide (DIT). During and after deformation, the IS plume did not reenter the warm surfzone, which potentially acted as a thermal barrier. High-frequency internal waves (HF IWs) propagated through the IS plume at ≈9 cm s−1 and dissipated onshore of 4Lsz. Surface HF IW signal was elevated in the plume elongation region, suggesting a linkage between plume elongation and either the DIT or HF IW. This IS plume evolution differs from previous SZ tracer releases, highlighting the effects of release timing relative to the solar cycle or the internal tide.

Impact of multiple tidal forcing on the simulation of the M2 internal tides in the northern South China Sea

Previous studies have indicated that internal tides (ITs) in the South China Sea (SCS) are dominated by the M2, K1, and O1, among which intense nonlinear interaction occurs. In this study, the impact of multiple tidal forcing on the simulation of the M2 ITs in the northern SCS is investigated based on a primitive equation model. Under the same model settings except the forcing at open boundaries, the run forced by the M2 individually yields larger M2 tidal currents and baroclinic energy fluxes than observations and the run forced by the M2, K1, and O1 together, although the M2 barotropic tidal forcing is the same in the two runs. The M2 barotropic to baroclinic energy conversion in the Luzon Strait (LS) is almost comparable in the two runs, suggesting that it is not the cause for the differences between the two runs. Indeed, the intense nonlinear interaction between the M2 and diurnal ITs should account for the differences. Results show that the nonlinear interaction is more intense in the SCS Basin than the western Pacific. Therefore, more energy is transferred from the M2 to tri-diurnal ITs in the SCS Basin. In addition, the nonlinear interaction enhances the energy loss of the M2 ITs in the LS.

Modal structure and propagation of internal tides in the northeastern South China Sea

The evolution of energy, energy flux and modal structure of the internal tides (ITs) in the northeastern South China Sea is examined using the measurements at two moorings along a cross-slope section from the deep continental slope to the shallow continental shelf. The energy of both diurnal and semidiurnal ITs clearly shows a ~14-day spring-neap cycle, but their phases lag that of barotropic tides, indicating that ITs are not generated on the continental slope. Observations of internal tidal energy flux suggest that they may be generated at the Luzon Strait and propagate west-northwest to the continental slope in the northwestern SCS. Because the continental slope is critical-supercritical with respect to diurnal ITs, about 4.6 kJ/m2 of the incident energy and 8.7 kW/m of energy flux of diurnal ITs are reduced from the continental slope to the continental shelf. In contrast, the semidiurnal internal tides enter the shelf because of the sub-critical topography with respect to semidiurnal ITs. From the continental slope to the shelf, the vertical structure of diurnal ITs shows significant variation, with dominant Mode 1 on the deep slope and dominant higher modes on the shelf. On the contrary, the vertical structure of the semidiurnal ITs is stable, with dominant Mode 1.

The Lifecycle of Nonlinear Internal Waves in the Northwestern South China Sea

AbstractThe life cycle of nonlinear internal waves (NIWs) to the southeast of Hainan Island in the northwestern South China Sea is investigated using synergistic satellite observations, in situ measurements, and numerical simulations. A three-dimensional, fully nonlinear and nonhydrostatic model with ultrafine resolution shows that a diurnal internal tide emanates from a sill in the Xisha Islands at approximately 215 km away from the local shelf break. The internal tide transits the deep basin toward the shelf break and reflects at the sea bottom and seasonal thermocline in the form of a wave beam. Arriving at the shelf break, the internal tide undergoes nonlinear transformation and produces an undular bore. Analyses of in situ measurements reveal that the undular bore appears as sharp depressions of the strong near-surface seasonal thermocline. The undular bore gradually evolves into an internal solitary wave train on the midshelf, which was detected by the spaceborne synthetic aperture radar. This finding has great implications for investigating NIWs in other coastal oceans where waves are controlled by remotely generated internal tides.

Features of internal tides observed near the shelf break in the northern South China Sea

Features of internal tides in the northern South China Sea are investigated using measurements of two moorings at 506 m (MR1) and 260 m (MR2) working between April and November of 2014. The diurnal internal tides at MR1 are significantly enhanced in June–July and display a regular fortnight modulation, whereas at shallower MR2, the June–July intensification is absent and the fortnight cycle becomes irregular. Fortnight cycle peaks of diurnal internal tides have a regular time-lag of ~ 2.8 days after those of diurnal barotropic tides (at MR1). We derive a mathematical relationship indicating that this time-lag of fortnight cycle peaks is around twice the time for the diurnal internal tides propagating from the generation place to the mooring site. Semidiurnal internal tides are dominated by the mode-1 (~ 70%), while diurnal internal tides exhibit multi-modal structures, with the contribution of ~ 20–30% for each of the first three modes. The diurnal mode-1 contribution varies fortnightly, and eddy passages are highly correlated with the increasing contribution of semidiurnal high modes (at MR1). Furthermore, we find that the nonlinear interaction between internal tides and near-inertial motions is weak in summer, even during periods of strong near-inertial motions (at MR2), because in summer, the shear of near-inertial currents is maximum at a depth shallower than the depth of maximum internal tide shear.

Dynamics and Energetics of Trapped Diurnal Internal Kelvin Waves around a Midlatitude Island

AbstractThe generation of trapped and radiating internal tides around Izu‐Oshima Island located off Sagami Bay, Japan, is investigated using the three-dimensional Stanford Unstructured Nonhydrostatic Terrain-following Adaptive Navier–Stokes Simulator (SUNTANS) that is validated with observations of isotherm displacements in shallow water. The model is forced by barotropic tides, which generate strong baroclinic internal tides in the study region. Model results showed that when diurnal K1 barotropic tides dominate, resonance of a trapped internal Kelvin wave leads to large-amplitude internal tides in shallow waters on the coast. This resonance produces diurnal motions that are much stronger than the semidiurnal motions. The weaker, freely propagating, semidiurnal internal tides are generated on the western side of the island, where the M2 internal tide beam angle matches the topographic slope. The internal wave energy flux due to the diurnal internal tides is much higher than that of the semidiurnal tides in the study region. Although the diurnal internal tide energy is trapped, this study shows that steepening of the Kelvin waves produces high-frequency internal tides that radiate from the island, thus acting as a mechanism to extract energy from the diurnal motions.

Diurnal Critical Latitude and the Latitude Dependence of Internal Tides, Internal Waves, and Mixing Based on Barcoo Seamount

AbstractVertical mixing is a key issue in ocean circulation modeling today. Mixing, particularly tidal mixing, is poorly represented in ocean and climate models, which generally ignore critical latitude effects. Critical latitude is the latitude where the inertial frequency equals the tidal frequency and differs for each tidal constituent. Critical latitudes strongly influence generation and propagation of internal tides. Using a model, latitude effects on tidal interactions with a seamount were examined by varying the latitude from 20° to 38°, through the range of the diurnal critical latitudes. The diurnal critical latitudes were found to strongly influence propagation of the diurnal internal tides, the magnitude of the semidiurnal tides, the energy in the harmonic and higher frequencies, the barotropic mean flow, and the diffusivities. The strongest effects occurred between the K1 and O1 critical latitudes. Here the semidiurnal tides, harmonics, and high frequencies were enhanced, barotropic mean velocities weakened, energy at the harmonics and higher frequencies increased, and diffusivities increased. Spectral techniques indicate that most of these impacts are the result of nonlinear wave‐wave interactions and resonant phenomena with the prominent mechanism harmonic transfers. There was no evidence of parametric subharmonic instabilities. The semidiurnal tides indicated a resonant response at 20°S, which is near the latitude for the combined M2 and K1 tidal period, ∼19°S.

Internal-Wave-Driven Mixing: Global Geography and Budgets

AbstractInternal-wave-driven dissipation rates ε and diapycnal diffusivities K are inferred globally using a finescale parameterization based on vertical strain applied to ~30 000 hydrographic casts. Global dissipations are 2.0 ± 0.6 TW, consistent with internal wave power sources of 2.1 ± 0.7 TW from tides and wind. Vertically integrated dissipation rates vary by three to four orders of magnitude with elevated values over abrupt topography in the western Indian and Pacific as well as midocean slow spreading ridges, consistent with internal tide sources. But dependence on bottom forcing is much weaker than linear wave generation theory, pointing to horizontal dispersion by internal waves and relatively little local dissipation when forcing is strong. Stratified turbulent bottom boundary layer thickness variability is not consistent with OGCM parameterizations of tidal mixing. Average diffusivities K = (0.3–0.4) × 10−4 m2 s−1 depend only weakly on depth, indicating that ε = KN2/γ scales as N2 such that the bulk of the dissipation is in the pycnocline and less than 0.08-TW dissipation below 2000-m depth. Average diffusivities K approach 10−4 m2 s−1 in the bottom 500 meters above bottom (mab) in height above bottom coordinates with a 2000-m e-folding scale. Average dissipation rates ε are 10−9 W kg−1 within 500 mab then diminish to background deep values of 0.15 × 10−9 W kg−1 by 1000 mab. No incontrovertible support is found for high dissipation rates in Antarctic Circumpolar Currents or parametric subharmonic instability being a significant pathway to elevated dissipation rates for semidiurnal or diurnal internal tides equatorward of 28° and 14° latitudes, respectively, although elevated K is found about 30° latitude in the North and South Pacific.