The internal wave climate in the southern Aegean Sea is examined with an array of two bottom-mounted acoustic Doppler current profilers and three profiling moorings deployed on the northern continental slope of the Cretan Sea for 3 months. Frequency spectra indicate an extremely weak internal wave continuum, about 4–10 times weaker than the Garrett-Munk and Levine reference levels. Spectra are instead dominated by semidiurnal internal tides and near-inertial waves, which are examined in detail by bandpass filtering. In the semidiurnal band, a barotropic tidal flow of –2 cm s^-1 is observed, with a pronounced spring/neap modulation in phase with the lunar fortnightly cycle. One to two days following several of these spring tide periods, a distinct internal tide featuring 10–20 m vertical displacements and 15–20 cm s^-1 baroclinic velocities is detectable propagating upward and to the southeast. Time-mean energy increases a factor of 2–5 within about 100 m from the bottom, implying generation and/or scattering from the bottom, whose slope is nearly critical to semidiurnal internal waves over much of the array. Several strong, downward propagating near-inertial events are also seen, each of which occurs following a period of work done by the wind on the mixed layer as estimated from a nearby surface mooring. The high-frequency internal wave continuum is more temporally constant but increases substantially toward the end of the deployment. Significant but unexplained differences in kinetic energy occur between successive spring tide periods in the case of the internal tides and between successive wind events in the case of the near-inertial signals. Substantial variability is observed in the low-frequency flows, which likely contributes to the time variability of the internal wave signals.

Satellite altimetric sea surface height anomaly (SSHA) data from Geosat Follow-on (GFO) and European Remote Sensing (ERS), as well as TOPEX/Poseidon (T/P), are merged to estimate M₂ internal tides around the Hawaiian Ridge, with higher spatial resolution than possible with single-satellite altimetry. The new estimates are compared with numerical model runs. Along-track analyses show that M₂ internal tides can be resolved from both 8 years of GFO and 15.5 years of ERS SSHA data. Comparisons at crossover points reveal that the M₂ estimates from T/P, GFO, and ERS agree well. Multisatellite altimetry improves spatial resolution due to its denser ground tracks. Thus M₂ internal tides can be plane wave fitted in 120 km x 120 km regions, compared to previous single-satellite estimates in 4° lon x 3° lat or 250 km x 250 km regions. In such small fitting regions the weaker and smaller-scale mode 2 M₂ internal tides can also be estimated.

The higher spatial resolution leads to a clearer view of the M₂ internal tide field around the Hawaiian Ridge. Discrete generation sites and internal tidal beams are clearly distinguishable, and consistent with the numerical model runs. More importantly, multisatellite altimetry produces larger M₂ internal tidal energy fluxes, which agree better with model results, than previous single-satellite estimates. This study confirms that previous altimetric underestimates are partly due to the more widely spaced ground tracks and consequently larger fitting region. Multisatellite altimetry largely overcomes this limitation.

Tidal currents in Luzon Strait south of Taiwan generate some of the largest internal waves anywhere in the ocean. Recent collaborative efforts between oceanographers from the United States and Taiwan explored the generation, evolution, and characteristics of these waves from their formation in the strait to their scattering and dissipation on Dongsha Plateau and the continental slope of mainland China. Nonlinear internal waves affect offshore engineering, navigation, biological productivity, and sediment resuspension. Observations within Luzon Strait identified exceptionally large vertical excursions of density (as expressed primarily in temperature profiles) and intense turbulence as tidal currents interact with submarine ridges. In the northern part of the strait, the ridge spacing is close to the internal semidiurnal tidal wavelength, allowing wave generation at both ridges to contribute to amplification of the internal tide. Westward radiation of semidiurnal internal tidal energy is predominant in the north, diurnal energy in the south. The competing effects of nonlinearity, which tends to steepen the stratification, and rotational dispersion, which tends to disperse energy into inertial waves, transform waves traveling across the deep basin of the South China Sea. Rotation inhibits steepening, especially for the internal diurnal tide, but despite the rotational effect, the semidiurnal tide steepens sufficiently so that nonhydrostatic effects become important, leading to the formation of a nonlinear internal wave train. As the waves encounter the continental slope and Dongsha Plateau, they slow down, steepen further, and are modified and scattered into extended wave trains. At this stage, the waves can "break," forming trapped cores. They have the potential to trap prey, which may account for their attraction to pilot whales, which are often seen following the waves as they advance toward the coast. Interesting problems remain to be explored and are the subjects of continuing investigations.

A thin gash in the continental slope northwest of Monterey Bay, Ascension Canyon, is steep, with sides and axis both strongly supercritical to M2 internal tides. A hydrostatic model forced with eight tidal constituents shows no major sources feeding energy into the canyon, but significant energy is exchanged between barotropic and baroclinic flows along the tops of the sides, where slopes are critical. Average turbulent dissipation rates observed near spring tide during April are half as large as a two week average measured during August in Monterey Canyon. Owing to Ascension's weaker stratification, however, its average diapycnal diffusivity, 3.9 x 10^-3 m^2 s^-1, exceeded the 2.5 x 10^-3 m^2 s^-1 found in Monterey. Most of the dissipation occurred near the bottom, apparently associated with an internal bore, and just below the rim, where sustained cross-canyon flow may have been generating lee waves or rotors. The near-bottom mixing decreased sharply around Ascension's one bend, as did vertically integrated baroclinic energy fluxes. Dissipation had a minor effect on energetics, which were controlled by flux divergences and convergences and temporal changes in energy density. In Ascension, the observed dissipation rate near spring tide was 2.1 times that predicted from a simulation using eight tidal constituents averaged over a fortnightly period. The same observation was 1.5 times the average of an M2-only prediction. In Monterey, the previous observed average was 4.9 times the average of an M2-only prediction.

A complex superposition of locally forced and shoaling remotely generated semidiurnal internal tides occurs on the Oregon continental slope. Presented here are observations from a zonal line of five profiling moorings deployed across the continental slope from 500 to 3000 m, a 24-h expendable current profiler (XCP) survey, and five 15-48-h lowered ADCP (LADCP)/CTD stations. The 40-day moored deployment spans three spring and two neap tides, during which the proportions of the locally and remotely forced internal tides vary. Baroclinic signals are strong throughout spring and neap tides, with 4-5-day-long bursts of strong cross-slope baroclinic semidiurnal velocity and vertical displacement . Energy fluxes exhibit complex spatial and temporal patterns throughout both tidal periods. During spring tides, local barotropic forcing is strongest and energy flux over the slope is predominantly offshore (westward). During neap tides, shoaling remotely generated internal tides dominate and energy flux is predominantly onshore (eastward). Shoaling internal tides do not exhibit a strong spring-neap cycle and are also observed during the first spring tide, indicating that they originate from multiple sources. The bulk of the remotely generated internal tide is hypothesized to be generated from south of the array (e.g., Mendocino Escarpment), because energy fluxes at the deep mooring 100 km offshore are always directed northward. However, fluxes on the slope suggest that the northbound internal tide is turned onshore, most likely by reflection from large-scale bathymetry. This is verified with a simple three-dimensional model of mode-1 internal tides propagating obliquely onto a near-critical slope, whose output conforms fairly well to observations, in spite of its simplicity.

A strong internal tide is generated in the Luzon Strait that radiates westward to impact the continental shelf of the South China Sea. Mooring data in 1500-m depth on the continental slope show a fortnightly averaged incoming tidal flux of 12 kW m^-1, and a mooring on a broad plateau on the slope finds a similar flux as an upper bound. Of this, 5.5 kW m^-1 is in the diurnal tide and 3.5 kW m^-1 is in the semidiurnal tide, with the remainder in higher-frequency motions. Turbulence dissipation may be as high as 3 kW m^-1. Local generation is estimated from a linear model to be less than 1 kW m^-1. The continental slope is supercritical with respect to the diurnal tide, implying that there may be significant back reflection into the basin. Comparing the low-mode energy of a horizontal standing wave at the mooring to the energy flux indicates that perhaps one-third of the incoming diurnal tidal energy is reflected. Conversely, the slope is subcritical with respect to the semidiurnal tide, and the observed reflection is very weak. A surprising observation is that, despite significant diurnal vertical-mode-2 incident energy flux, this energy did not reflect; most of the reflection was in mode 1.

The observations are consistent with a linear scattering model for supercritical topography. Large fractions of incoming energy can reflect depending on both the geometry of the shelfbreak and the phase between the modal components of the incoming flux. If the incident mode-1 and mode-2 waves are in phase at the shelf break, there is substantial transmission onto the shelf; if they are out of phase, there is almost 100% reflection. The observations of the diurnal tide at the site are consistent with the first case: weak reflection, with most of it in mode 1 and almost no reflection in mode 2. The sensitivity of the reflection on the phase between incident components significantly complicates the prediction of reflections from continental shelves.

Finally, a somewhat incidental observation is that the shape of the continental slope has large regions that are near critical to the dominant diurnal tide. This implicates the internal tide in shaping of the continental slope.

Internal wave measurements from moorings and profiling floats throughout the Philippine Archipelago, collected as part of the Office of Naval Research Philippine Straits Dynamics Experiment, reveal a wealth of subsurface processes, some of which have not been observed previously (in the Philippines or elsewhere). Complex bathymetry and spatially varying tide and wind forcing produce distinct internal wave environments within the network of seas and channels, ranging from quiescent interior basins to remotely forced straits. Internal tides in both the diurnal and semidiurnal bands dominate much of the velocity structure and are likely the dominant source of energy for mixing in the region. In addition, the transfer of energy from the internal tide directly to near-inertial motions through parametric subharmonic instability appears to be active and, rather than wind forcing, is the dominant source of near-inertial band energy.

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.

Four instances of persistent intrusive deformation of the North Pacific Subtropical Front were tagged individually by a Lagrangian float and tracked for several days. Each feature was mapped in three dimensions using repeat towed observations referenced to the float. Isohaline surface deformations in the frontal zone included sheetlike folds elongated in the alongfront direction and narrow tongues extending across the front. All deformations appeared as protrusions of relatively cold, and fresh, water across the front. No corresponding features of the opposite sign or isolated lenslike structures were observed. The sheets were O(10 m) thick, protruded about 10 km into the warm saline side of the front, and were coherent for 10–30 km along the front. Having about the same thickness and cross-frontal extent as the sheets, tongues extended less than 5 km along the front.

All of the intrusions persisted as long as they were followed, several days to one week. Their structures evolved on both inertial (23 h) and subinertial (10 days) time scales in response to differential lateral advection. The water mass surrounding the intrusions participated in gradual anticyclonic rotation as a part of a mesoscale meander of the subtropical front. The intrusions may be interpreted as a manifestation of three-dimensional submesoscale turbulence of the frontal zone, driven by the mesoscale. Absence of large features of the opposite sign may be indicative of the asymmetry of the underlying dynamics.

The northeastward progression of the semidiurnal internal tide from French Frigate Shoals (FFS), Hawaii, is studied with an array of six simultaneous profiling moorings spanning 25.5–37.1 deg N (~1400 km) and 13-yr-long Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter data processed by a new technique. The moorings have excellent temporal and vertical resolutions, while the altimeter offers broad spatial coverage of the surface manifestation of the internal tide's coherent portion. Together these two approaches provide a unique view of the internal tide's long-range propagation in a complex ocean environment. The moored observations reveal a rich, time-variable, and multimodal internal tide field, with higher-mode motions contributing significantly to velocity, displacement, and energy. In spite of these contributions, the coherent mode-1 internal tide dominates the northeastward energy flux, and is detectable in both moored and altimetric data over the entire array. Phase and group propagation measured independently from moorings and altimetry agree well with theoretical values. Sea surface height anomalies (SSHAs) measured from moorings and altimetry agree well in amplitude and phase until the northern end of the array, where phase differences arise presumably from refraction by mesoscale flows. Observed variations in SSHA, energy flux, and kinetic-to-potential energy ratio indicate an interference pattern resulting from superposed northeastward radiation from Hawaii and southeastward from the Aleutian Ridge. A simple model of two plane waves explains most of these features.

In the South China Sea (SCS), 14 nonlinear internal waves are detected as they transit a synchronous array of 10 moorings spanning the waves' generation site at Luzon Strait, through the deep basin, and onto the upper continental slope 560 km to the west. Their arrival time, speed, width, energy, amplitude, and number of trailing waves are monitored. Waves occur twice daily in a particular pattern where larger, narrower "A" waves alternate with wider, smaller "B" waves. Waves begin as broad internal tides close to Luzon Strait's two ridges, steepening to O(3–10 km) wide in the deep basin and O(200–300 m) on the upper slope.

Nearly all waves eventually develop wave trains, with larger/steeper waves developing them earlier and in greater numbers. The B waves in the deep basin begin at a mean speed of ~5% greater than the linear mode-1 phase speed for semidiurnal internal waves (computed using climatological and in situ stratification). The A waves travel ~5–10% faster than B waves until they reach the continental slope, presumably because of their greater amplitude. On the upper continental slope, all waves speed up relative to linear values, but B waves now travel 8–12% faster than A waves, in spite of being smaller.

Solutions of the Taylor–Goldstein equation with observed currents demonstrate that the B waves' faster speed is a result of modulation of the background currents by an energetic diurnal internal tide on the upper slope. Attempts to ascertain the phase of the barotropic tide at which the waves were generated yielded inconsistent results, possibly partly because of contamination at the easternmost mooring by eastward signals generated at Luzon Strait's western ridge. These results present a coherent picture of the transbasin evolution of the waves but underscore the need to better understand their generation, the nature of their nonlinearity, and propagation through a time-variable background flow, which includes the internal tides.

A monthlong field survey in July 2007, focused on the North Pacific subtropical frontal zone (STFZ) near 30°N, 158°W, combined towed depth-cycling conductivity-temperature-depth (CTD) profiling with shipboard current observations. Measurements were used to investigate the distribution and structure of thermohaline intrusions. The study revealed that local extrema of vertical salinity profiles, often used as intrusion indicators, were only a subset of a wider class of distortions in thermohaline fields due to interleaving processes. A new method to investigate interleaving based on diapycnal spiciness curvature was used to describe an expanded class of laterally coherent intrusions. STFZ intrusions were characterized by their overall statistics and by a number of case studies. Thermohaline interleaving was particularly intense within 5 km of two partially compensated fronts, where intrusions with both positive and negative salinity anomalies were widespread. The vertical and cross-frontal scales of the intrusions were on the order of 10 m and 5 km, respectively. Though highly variable, the slopes of these features were typically intermediate between those of isopycnals and isohalines. Although the influence of double-diffusive processes sometime during the evolution of intrusions could not be excluded, the broad spectrum of the observed features suggests that any role of double diffusion was secondary.

New estimates of mode-1 M₂ internal tide energy flux are computed from an extended Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter dataset that includes both the original and tandem tracks, improving spatial resolution over previous estimates from O(500 km) to O(250 km). Additionally, a new technique is developed that allows separate resolution of northward and southward components. Half-wavelength features previously seen in unseparated estimates are shown to be due to the superposition of northward and southward wave trains.

The new technique and higher spatial resolution afford a new view of mode-1 M₂ internal tides in the central North Pacific Ocean. As with all altimetric estimates, only the coherent or phase-locked signals are detectable owing to the long repeat period of the tracks. Emanating from specific generation sites consistent with predictions from numerical models, internal tidal beams 1) are as narrow as 200 km and 2) propagate a longer distance than previously observed. Two northward internal tidal beams radiating from the Hawaiian Ridge, previously obscured by coarse resolution and the southward Aleutian beam, are now seen to propagate more than 3500 km across the North Pacific Ocean to reach the Alaskan shelf. The internal tidal beams are much better resolved than in previous studies, resulting in better agreement with moored flux estimates.

Extending an earlier attempt to understand long-range propagation of the global internal-wave field, the energy E and horizontal energy flux F are computed for the two gravest baroclinic modes at 80 historical moorings around the globe. With bandpass filtering, the calculation is performed for the semidiurnal band (emphasizing M₂ internal tides, generated by flow over sloping topography) and for the near-inertial band (emphasizing wind-generated waves near the Coriolis frequency). The time dependence of semidiurnal E and F is first examined at six locations north of the Hawaiian Ridge; E and F typically rise and fall together and can vary by over an order of magnitude at each site. This variability typically has a strong spring–neap component, in addition to longer time scales. The observed spring tides at sites northwest of the Hawaiian Ridge are coherent with barotropic forcing at the ridge, but lagged by times consistent with travel at the theoretical mode-1 group speed from the ridge. Phase computed from 14-day windows varies by approximately ±45° on monthly time scales, implying refraction by mesoscale currents and stratification. This refraction also causes the bulk of internal-tide energy flux to be undetectable by altimetry and other long-term harmonic-analysis techniques. As found previously, the mean flux in both frequency bands is O(1 kW m^-1), sufficient to radiate a substantial fraction of energy far from each source. Tidal flux is generally away from regions of strong topography. Near-inertial flux is overwhelmingly equatorward, as required for waves generated at the inertial frequency on a β plane, and is winter-enhanced, consistent with storm generation. In a companion paper, the group velocity is examined for both frequency bands.

Using a set of 80 globally distributed time series of near-inertial and semidiurnal energy E and energy flux F computed from historical moorings, the group velocity is estimated. For a single free wave, observed group speed should equal that expected from linear wave theory. For comparison, the latitude dependence of perceived group speed for perfectly standing waves is also derived. The latitudinal dependence of observed semidiurnal group speed closely follows that expected for free waves at all latitudes, implying that 1) low-mode internal tides obey linear theory and 2) standing internal-tidal waves are rare in the deep ocean for latitudes equatorward of about 35°. At higher latitudes, standing waves cannot be easily distinguished from free waves using this method because their expected group speeds are similar. Near-inertial waves exhibit scattered group speed values consistent with the passage of events generated at various latitudes, with implied frequencies ω ≈ 1.05 – 1.25 x f, as typically observed in frequency spectra.

Energy flux is a fundamental quantity for understanding internal wave generation, propagation, and dissipation. In this paper, the estimation of internal wave energy fluxes <u'p'> from ocean observations that may be sparse in either time or depth are considered. Sampling must be sufficient in depth to allow for the estimation of the internal wave-induced pressure anomaly p' using the hydrostatic balance, and sufficient in time to allow for phase averaging. Data limitations that are considered include profile time series with coarse temporal or vertical sampling, profiles missing near-surface or near-bottom information, moorings with sparse vertical sampling, and horizontal surveys with no coherent resampling in time. Methodologies, interpretation, and errors are described. For the specific case of the semidiurnal energy flux radiating from the Hawaiian ridge, errors of 10% are typical for estimates from six full-depth profiles spanning 15 h.

Ocean mixing, which affects pollutant dispersal, marine productivity and global climate, largely results from the breaking of internal gravity waves—disturbances propagating along the ocean's internal stratification. A global map of internal-wave dissipation would be useful in improving climate models, but would require knowledge of the sources of internal gravity waves and their propagation. Towards this goal, I present here computations of horizontal internal-wave propagation from 60 historical moorings and relate them to the source terms of internal waves as computed previously. Analysis of the two most energetic frequency ranges—near-inertial frequencies and semidiurnal tidal frequencies—reveals that the fluxes in both frequency bands are of the order of 1 kW m^-1 (that is, 15–50% of the energy input) and are directed away from their respective source regions. However, the energy flux due to near-inertial waves is stronger in winter, whereas the tidal fluxes are uniform throughout the year. Both varieties of internal waves can thus significantly affect the space-time distribution of energy available for global mixing.