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This study presents remotely generated internal tides that propagate into shallow regions and are modulated by background flows using numerical simulations and field observation data. Numerical results indicate that strongly enhanced semi‐diurnal (∼M2) internal tide energy is generated over a shallow ridge, the Izu‐Ogasawara Ridge. The generated internal tides propagates toward the Kuroshio upstream and shoal toward shallow regions near Cape Shiono‐Misaki. The intensified internal tide energy flux toward the cape is explained by two mechanisms: (a) intensified internal tide generation over the ridge due to the interaction between tides and the Kuroshio and (b) wave energy convergence along the Kuroshio due to wave refractions. A 13‐year field data set obtained from the cape was used to investigate shoaling internal tides influenced by the Kuroshio. The observed results reveal a significant positive correlation between the kinetic energy of semi‐diurnal internal tides and the background flows caused by the Kuroshio, which evidently supports the intensified internal tides attributed to the interaction between background flows and tides, as proposed by recent studies. The intensity of shoaling internal tides is also largely influenced by the path of the Kuroshio and seasonal effects. The magnitude of shoaling internal tides is clearly weaken as the Kuroshio meander occurs. Shoaling internal tides modulated by the Kuroshio can provide new insights into energy transport and mixing processes in coastal oceans. Plain Language Summary: Oceans worldwide have internal waves that significantly contribute to the transport of mass and heat within the oceans. In particular, internal waves propagating into shallow coastal regions result in enhanced energy dissipation accompanied by strong mixing and transport, similar to ocean surface waves. Internal waves at tidal frequencies are called internal tides and energetically dominate other internal waves in coastal regions. However, difficulties in observing the physical processes associated with internal tides have hindered the comprehensive elucidation of internal tides. This study presents shoaling internal tides generated by the combined effects of the Kuroshio and tides over a shallow ridge using numerical simulations and field observations. The numerical results show clear processes of shoaling internal tides propagating toward the Cape, generated by the interaction between the Kuroshio and the tides. Long‐term (13‐year) field observation data obtained from the Cape show that the magnitude of internal tides is significantly intensified by the magnitude of the Kuroshio, clearly indicating that the Kuroshio path/meander modulates shoaling internal tides. The shoaling internal tides observed in this study are anticipated to contribute to mixing and transport in coastal oceans. Key Points: Shoaling internal tide are investigated with a numerical model and a long‐term field data setThe Kuroshio intensifies generation of internal tides over a shallow ridge resulting in shoaling internal tidesThe Kuroshio meander greatly influences the magnitude of shoaling internal tides [ABSTRACT FROM AUTHOR]

The Indonesian seas, with their complex passages and vigorous mixing, constitute the only route and are critical in regulating Pacific–Indian Ocean interchange, air–sea interaction, and global climate events. Previous research employing remote sensing and numerical simulations strongly suggested that this mixing is tidally driven and localized in narrow channels and straits, with only a few direct observations to validate it. The current study offers the first comprehensive temporal microstructure observations in the south of Lombok Strait with a radius of 0.05° and centered on 115.54oE and 9.02oS. Fifteen days of tidal mixing observations measured potential temperature and density, salinity, and turbulent energy dissipation rate. The results revealed significant mixing and verified the remotely sensed technique. The south Lombok temporal and depth averaged of the turbulent kinetic energy dissipation rate, and the diapycnal diffusivity from 20 to 250 m are ε = 4.15 ± 15.9) × 10–6 W kg–1 and K ρ = (1.44 ± 10.7) × 10–2 m2s–1, respectively. This K ρ is up to 104 times larger than the Banda Sea [ K ρ = (9.2 ± 0.55) × 10–6 m2s–1] (Alford et al. Geophys Res Lett 26:2741–2744, 1999) or the "open ocean" K ρ = 0.03 × 10–4 m2s−1 within 2° of the equator to (0.4–0.5) × 10–4 m2s−1 at 50°–70° (Kunze et al. J Phys Oceanogr 36:1553–1576, 2006). Therefore, nonlinear interactions between internal tides, tidally induced mixing, and ITF plays a critical role regulating water mass transformation and have strong implications to longer-term variations and change of Pacific–Indian Ocean water circulation and climate. [ABSTRACT FROM AUTHOR]

The evolution of submarine canyons is primarily controlled by turbidity currents, which erode and fill them over time; however, many other hydrodynamic currents operate within canyons. Bottom currents from these other hydrodynamic processes, including internal tides, can be dominant processes, but their deposits are seldom recognized in sediment cores or the rock record. This study combines autonomous underwater vehicle swath bathymetry imagery and sub‐bottom profiles, high‐resolution sediment core analyses (X‐ray imagery and thin sections), and previously collected seabed video and flow measurements within Logan Canyon head (eastern Canada) to provide a detailed, modern record of facies associated with hydrodynamic processes in a canyon head. These results suggest that bottom currents are responsible for maintaining gullies on canyon sidewalls and an axial channel on the canyon floor. Thin sections of sediment cores reveal that muddy sand in the canyon head consists of mud aggregates and silt and fine‐grained sand, both behaving similarly in terms of flow dynamics. Three facies are present at macro‐scale and micro‐scale: laminated, partially laminated and bioturbated sandy mud. Sedimentary structures include rhythmic sand and mud aggregate couplets, planar to wavy laminations, current ripple cross‐laminations and fining‐upward successions, which are attributed to bottom currents induced by internal tides. Bioturbated facies, characterized by discrete biogenic structures and cross‐cutting relationships, predominate and overprint a mottled background. A mottled bioturbation fabric also alternates with or locally disrupts layering within the partially laminated facies. Internal tide currents, capable of bedload transport and forming ripples, were measured during a monitoring period in the canyon head, followed by rapid re‐establishment of benthos and associated biogenic structures, supporting the core interpretations. Preservation of sedimentary facies associated with these internal tides occurs when the sedimentation rate outpaces the rate of bioturbation, likely during stormier conditions on the shelf. These results represent observations of sedimentary facies associated with modern bottom currents and internal tides, and can be used to interpret similar occurrences within the rock record. [ABSTRACT FROM AUTHOR]

Internal tides (ITs) radiated from the Luzon Strait (LS) to the South China Sea (SCS) exhibit variability closely related to changes of stratification. Based on the CanESM5 simulation from the sixth phase of Coupled Model Intercomparison Project, the stratification within the LS will increase in the following century due to climate change resulting from human activities. Here we conduct numerical experiments to investigate changes of the M2 ITs under different shared socioeconomic pathway (SSP) scenarios. Results show that the cumulative generation of ITs within the LS weakens with strengthening stratification. The amount of ITs generated in the LS will decrease by 22.7% (2.7%) and that propagating into the SCS will decrease by 27.9% (4.4%) in a century under the SSP5-8.5 (SSP1-2.6) scenario that represents the high (low) end of future emissions. The changes are attributed to the interference of the M2 ITs within the double ridges, which weakens as the stratification strengthens and finally reduces the IT generation. Considering that ITs are one of the most important energy sources for diapycnal mixing, the decrease in ITs would have significant physical and biogeochemical implications in the LS and the SCS. [ABSTRACT FROM AUTHOR]

When the barotropic tide encounters variable bathymetry, fluctuating flow along a topographic slope generates baroclinic tides, or internal tides. There is growing evidence that these internal tides can affect primary production in the euphotic zone, though the dominant mechanisms are unclear. Internal tides move passive phytoplankton through an exponentially varying light field, enhancing primary production near the base of the euphotic zone. In addition internal tides also increase primary production through vertical nutrient advection into the euphotic zone. Topographically generated internal tides can be separated into two regimes: 1) the often highly nonlinear near-field regime where tidal beams are observed and 2) the more linear far-field regime. This study examines the primary production response to these internal tide processes using the Regional Ocean Modeling System (ROMS) coupled to a simple Nutrient, Phytoplankton, Zooplankton, Detritus (NPZD) model configured for an oligotrophic system with the nutricline positioned below 50 m depth. These idealized simulations generate internal tide beams with an oscillating, horizontal body force at the M2 tidal frequency that is applied to domains with a bathymetric step and uniform stratification. Sensitivity of the primary production response to the energy content of the tidal beam is obtained by adjusting the height and slope of the bathymetric step. Simulation results reveal that primary production intensifies along tidal beams due to the local enhancement of parcel vertical displacement (light effect) and nutrient advective flux divergence (nutrient effect). In the near-field regime across the range of step heights and slopes in this study, the nutrient effect is an order of magnitude larger and explains 92% of the variance in primary production versus only 14% for the light effect. The geometry of the generating feature sets the kinematics of the tidal beam. The light effect is limited in the euphotic zone across our domains because realized changes in light experienced over a tidal cycle are small relative to the amount of light available at a particular depth. In contrast, the magnitude of the nutrient effect increases more substantially with tidal beam energy.

The characteristics of the energy flux of internal tides in the northern South China Sea (SCS) were explored by analyzing a fleet of underwater gliders. It was found that the low‐mode diurnal internal tides with ∼300 km wavelength in the middle basin are generated predominantly in the Luzon Strait (LS) and propagate over 1,000 km to the western SCS. However, the semidiurnal internal tides are generated in multiple regions, including the LS in the east part, the continental shelf, and the islands in the west. The energy flux of the mode‐1 diurnal internal tides attenuated rapidly within 450 km of the LS and was less pronounced after that. The estimated dissipation rate based on the mode‐1 energy flux is about 10−8 W/kg, underling the significant role of mode‐1 diurnal internal tides in bolstering far‐field mixing. This study provides a unique view of the spatial pattern, energy flux, and energy sink of the internal tides in the northern SCS, which could supplement the altimetry‐based results and improve the parameterization in ocean models. Plain Language Summary: The South China Sea (SCS), is situated near the Luzon Strait (LS)—the global ocean's most active region for internal tide generation. It is abundant with internal tides. However, there is a notable lack of understanding regarding the spatial distribution of the energy flux of these internal tides in the SCS, as determined by in‐situ observations. Our study, based on data collected from a fleet of underwater gliders, revealed that internal tides of diurnal frequency can propagate to the western continental shelf of the SCS, exhibiting a significant mode‐1 vertical structure. In contrast, semidiurnal internal tides originated from multiple sources. We observed that energy decay occurs rapidly within 500 km of the source, while it exhibited relatively slow decay in the central region of the basin. By correlating the energy attenuation with dissipation, we found that the depth‐averaged dissipation rate aligns with the directly observed value. This suggested that the mode‐1 diurnal internal tides may effectively enhance the far‐field mixing in the northern SCS. Key Points: The spatial distribution of internal tides in the northern South China Sea is revealed by a fleet of underwater glidersPropagation distances exceeding 1,000 km and wavelengths of ∼300 km for mode‐1 diurnal tides were directly observedThe dissipation of the mode‐1 diurnal internal tides induces a dissipation rate of ∼10−8 W/kg in the far‐field [ABSTRACT FROM AUTHOR]

Interactions between internal tides and mesoscale eddies are an important topic. However, examining modulations of internal tides inside a mesoscale eddy based on observations is difficult due to limited observation duration and inaccurate positioning within the eddy. In order to overcome these two practical limitations, we use the active navigation capability of underwater gliders to conduct measurements inside the targeted eddy and utilize the wavelet approach to investigate modulations of internal tides with diurnal and semidiurnal periods inside the eddy. Based on the wavelet's frequency–time locality, we construct scale-specific networks via wavelet coherence (WC) from multivariate timeseries with a small sample size. The modulation of internal tides is then examined in terms of temporal evolutionary characteristics of the WC network's topological structure. Our findings are as follows: (1) the studied eddy is vertically separated into two layers, the upper (<400 m) and lower (>400 m) layers, indicating that the eddy is surface intensified; (2) the eddy is also horizontally divided into two domains, the inner and outer centers, where the modulation of internal tides seems to actively occur in the inner center; and (3) diurnal internal tides are more strongly modulated compared to semidiurnal ones, indicating the influence of spatial scales on the strength of interactions between internal tides and eddies. [ABSTRACT FROM AUTHOR]

We used NASA’s high-resolution (1/48° or 2.3 km, hourly) Estimating the Circulation and Climate of the Ocean (ECCO) estimates of salinity at a 1 m depth from November 2011 to October 2012 to detect semi-diurnal and diurnal internal tides (ITs) in the Andaman Sea and determine their characteristics in three 2° × 2° boxes off the Myanmar coast (box A), central Andaman Sea (box B), and off the Thailand coast (box C). We also used observed salinity and temperature data for the above period at the BD12-moored buoy in the central Andaman Sea. ECCO salinity data were bandpass-filtered with 11–14 h and 22–26 h periods. Large variations in filtered ECCO salinity (~0.1 psu) in the boxes corresponded with near-surface imprints of propagating ITs. Observed data from the box B domain reveals strong salinity stratification (halocline) in the upper 40 m. Our analyses reveal that the shallow halocline affects the signatures of propagating semi-diurnal ITs reaching the surface, but diurnal ITs propagating in the halocline reach up to the surface and bring variability in ECCO salinity. In box A, the semi-diurnal IT characteristics are higher speeds (0.96 m/s) with larger wavelengths (45 km), that are closer to theoretical mode 2 estimates, but the diurnal ITs propagating in the box A domain, with a possible source over the shelf of Gulf of Martaban, attain lower values (0.45 m/s, 38 km). In box B, the propagation speed is lower (higher) for semi-diurnal (diurnal) ITs. Estimates for box C are closer to those for box A.

A 3D high-resolution general ocean circulation model was implemented and validated to study the characteristics and seasonal variability of the internal tides in the Arabian Sea (AS). Three major source locations of internal tides were identified: Socotra Island, the northeastern shelf area of AS, and the Maldives. Around Socotra Island, internal tides propagate both southward and northward, before quickly dissipating. The internal tides generated in the northeastern AS split into two branches: Branch-I propagates perpendicular to the shelf, whereas Branch-II propagates more southernly. The internal tides originated in the Maldives propagate almost latitudinally both eastwards and westwards. Generally, the internal tides in the AS are more pronounced in January as shown by the forcing function, energy flux, and conversion rate. The hourly average conversion rate for the entire domain, including the AS, the Red Sea, and the Arabian Gulf - was 34.28 GW in January and 20.51 GW in July, suggesting a slightly larger barotropic-to-baroclinic conversion rate in January, probably due to the strong stratification around 100 meters in winter. [ABSTRACT FROM AUTHOR]

When the barotropic tide encounters variable bathymetry, fluctuating flow along a topographic slope generates baroclinic tides, or internal tides. There is growing evidence that these internal tides can affect primary production in the euphotic zone, though the dominant mechanisms are unclear. Internal tides move passive phytoplankton through an exponentially varying light field, enhancing primary production near the base of the euphotic zone. In addition internal tides also increase primary production through vertical nutrient advection into the euphotic zone. Topographically generated internal tides can be separated into two regimes: 1) the often highly nonlinear near-field regime where tidal beams are observed and 2) the more linear far-field regime. This study examines the primary production response to these internal tide processes using the Regional Ocean Modeling System (ROMS) coupled to a simple Nutrient, Phytoplankton, Zooplankton, Detritus (NPZD) model configured for an oligotrophic system with the nutricline positioned below 50 m depth. These idealized simulations generate internal tide beams with an oscillating, horizontal body force at the M2 tidal frequency that is applied to domains with a bathymetric step and uniform stratification. Sensitivity of the primary production response to the energy content of the tidal beam is obtained by adjusting the height and slope of the bathymetric step. Simulation results reveal that primary production intensifies along tidal beams due to the local enhancement of parcel vertical displacement (light effect) and nutrient advective flux divergence (nutrient effect). In the near-field regime across the range of step heights and slopes in this study, the nutrient effect is an order of magnitude larger and explains 92% of the variance in primary production versus only 14% for the light effect. The geometry of the generating feature sets the kinematics of the tidal beam. The light effect is limited in the euphotic zone across our domains because realized changes in light experienced over a tidal cycle are small relative to the amount of light available at a particular depth. In contrast, the magnitude of the nutrient effect increases more substantially with tidal beam energy.

The northern Andaman Sea (AS) continental shelf is unique due to the diverse marine ecosystem and existences of both hypoxia and internal tides, but limited in situ observations restrict our understanding of the hydrographic dynamic process. Based on the turbidity, mean volumes of backscattering strength (MVBS), we qualitatively studied the mean distribution characteristics and temporal variation in suspended matters on the northern AS continental shelf and their relation to hypoxia and internal tides. The results of both MVBS and turbidity revealed that the suspended matters exhibited a three-layer vertical structure. The upper and lower layers exhibited high values, while the middle layer had low values. The upper boundary of the high-value region in the upper layer descended below the surface to a depth of 30 m after sunrise and returned to the surface after sunset, indicating a diel vertical migration of zooplankton and micronekton. Daytime migration depth was likely constrained by hypoxia's upper boundary. In the lower layer, three MVBS enhancements and attenuations correlated with vertical upward and downward velocities, respectively, primarily driven by uplift or suppression. We proposed vertical velocity patterns resulted from internal bores, possibly triggered by shoaling semidiurnal internal tides. [ABSTRACT FROM AUTHOR]

Internal waves (IWs) are generated in all the oceans, and their amplitudes are large, especially in regions that receive a large amount of freshwater from nearby rivers, which promote highly stratified waters. When barotropic tides encounter regions of shallow bottom-topography, internal tides (known as IWs of the tidal period) are generated and propagated along the pycnocline due to halocline or thermocline. In the North Indian Ocean, the Bay of Bengal (BoB) and the Andaman Sea receive a large volume of freshwater from major rivers and net precipitation during the summer monsoon. This study addresses the characteristics of internal tides in the BoB and Andaman Sea using NASA's Estimating the Circulation and Climate of the Ocean (ECCO) project's high-resolution (1/48° and hourly) salinity estimates at 1 m depth (hereafter written as ECCO salinity) during September 2011–October 2012, time series of temperature, and salinity profiles from moored buoys. A comparison is made between ECCO salinity and NASA's Soil Moisture Active Passive (SMAP) salinity and Aquarius salinity. The time series of ECCO salinity and observed salinity are subjected to bandpass filtering with an 11–14 h period and 22–26 h period to detect and estimate the characteristics of semi-diurnal and diurnal period internal tides. Our analysis reveals that the ECCO salinity captured well the surface imprints of diurnal period internal tide propagating through shallow pycnocline (~50 m depth) due to halocline, and the latter suppresses the impact of semi-diurnal period internal tide propagating at thermocline (~100 m depth) reaching the sea surface. The semi-diurnal (diurnal) period internal tides have their wavelengths and phase speeds increased (decreased) from the central Andaman Sea to the Sri Lanka coast. Propagation of diurnal period internal tide is dominant in the northern BoB and northern Andaman Sea. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) cooling is a typical ocean response to tropical cyclones (TCs). Previous studies have shown that TC-induced SST cooling is influenced by the TC characteristics, ocean stratification, and background vorticity. However, whether internal tides (ITs) affect TC-induced SST cooling remains unclear. Based on a series of numerical simulations in the South China Sea, the influence of ITs on TC-induced SST cooling is explored in this study. Results indicate that ITs can enhance TC-induced SST cooling by 0.2 − 0.4 °C in the Luzon Strait. Temperature budget analysis shows that ITs modulate the SST cooling mainly through changing the vertical mixing and advection. In the Luzon Strait, ITs enhance the vertical mixing but suppress the vertical advection. Because the contribution of vertical mixing to SST cooling is one order of magnitude higher than that of vertical advection, ITs enhance TC-induced SST cooling in the Luzon Strait mainly through enhancing the vertical mixing. Moreover, the enhancements of vertical mixing and SST cooling induced by ITs are found to be related to the TC characteristics. One reason is that in addition to ITs, the high-frequency waves generated by the interaction between TC-induced near-inertial waves and ITs also contribute to the shear and hence the turbulent mixing. [ABSTRACT FROM AUTHOR]

Internal tides are key players in ocean dynamics above mid‐ocean ridges. The generation and propagation of internal tides over the Mid‐Atlantic Ridge (MAR) have been studied through theoretical and numerical models, as well as through moored, that is, one‐dimensional, observations. Yet, observations remain sparse and often restricted to the vertical direction. Here we report on the first two‐dimensional in situ observation of an internal tide beam sampled by a shipboard acoustic Doppler current profiler through a vertical section over the MAR. The beam is generated by the interaction of the barotropic tidal current with a supercritical abyssal hill that sits in the rift valley of the MAR. A vertical mode decomposition is carried out to characterize the spatio‐temporal variability of the beam. Although the modal content of the velocity field is dominated by modes 1 to 3, higher modes display localized and not persistent bursts of energy. The use of an analytical theory for linear internal waves allows us to rationalize the observed velocity field and interpret it as the superposition of modal waves generated on the hill and propagating in the same direction. The observed beam is qualitatively reconstructed as the superposition of waves of modes 2 to 6. The velocity field was sampled seven times across the same section and displayed qualitatively different patterns, unveiling the complexity of the dynamics above the MAR. A ray tracing of modal waves shows that the refraction by mesoscale currents could explain the observed variability of the tidal beam. Plain Language Summary: In the stratified ocean, the interaction of tidal currents with the seafloor topography generates waves that propagate along and across density layers. Those waves, called internal tides, play important roles in the equilibrium of the ocean. Yet, their fine‐scale observations are sparse and uncertainties remain on their lifecycle. Using a shipboard current profiler through a section over the Mid‐Atlantic Ridge, we sampled a clear signal of an internal tide beam, a structure whose existence has been unveiled by theories and models but never observed on the Mid‐Atlantic Ridge. We use a simple linear theory to characterize the beam and rationalize the observed velocity field, which can be interpreted as the superposition of many waves with distinct spatial structures. Key Points: An internal tide beam generated by an abyssal hill is observed by a Shipboard Acoustic Doppler Current Profiler through a section over the Mid‐Atlantic RidgeA linear wave theory quantitatively reconstructs the beam structure, interpreted as the superposition of modal internal tidesModes 1–3 are the most energetic but modes 2–6 are the ones that fit best the horizontal structure of modal internal tides [ABSTRACT FROM AUTHOR]

The accurate assessment of the energy dissipation of internal tides (ITs) is of great importance because ITs contribute significantly to abyssal mixing. Thus, in this study, the IT-driven dissipation and diapycnal diffusion in the northern Pacific are estimated using parameterizations proposed by St. Laurent et al. (2002), Koch-Larrouy et al. (2007), and de Lavergne et al. (2020) (hereafter referred to as LSJ02, KL07, and dL20, respectively). The performances of the three parameterizations are evaluated by comparing the calculated results with fine structure observations. In particular, the dissipation estimated by LSJ02 parameterization shows a bottom-intensified characteristic, with the patterns showing good agreement with the observations near seamounts. Moreover, 43% of the results calculated using the LSJ02 parameterization have errors lower than one order of magnitude in the generation sites of ITs. Meanwhile, the strongest dissipation estimated by the KL07 parameterization shifts to the thermocline, with the results showing the highest level of consistency with observations in the generation sites. The proportion of results with errors lower than one order of magnitude is 80.7%. Furthermore, the results calculated by dL20 parameterization agree well with the observations in the upper and middle layers, with the parameterization showing an accurate estimation of the remote dissipation. The percentages of the errors lower than one order of magnitude between the dL20 parameterization and observations account for 77.1% and 88.7% in the generation sites and far-field regions, respectively. [ABSTRACT FROM AUTHOR]

Abstract Barotropic and baroclinic tides were simulated for the Coral and Tasman Seas off eastern Australia and compared to a simulation without tides. Both simulations included geostrophic currents and replicated a current analogous to the East Australian Current (EAC). Tides and tidal currents in most of this region are weak, generally 1–2 cm s−1, with the exception of the far northwest portion of the Coral Sea. Even these weak tides were found to impact the mean EAC-like current transport, enhancing it by 1–4 Sv in some areas. Southward flow increased over the continental shelf. Tides did not appear to impact eddy formation, size, or rotational speed; however, they affected eddy propagation. Cyclonic eddies propagated northward faster with tides than without tides. Tides impacted cross-shelf transport of colder water, with significantly more on-shore transport occurring with tides, particularly equatorward of the diurnal critical latitudes. Cross-shelf transport of nutrient rich water onto the shelf is important in this oligotrophic region. Although the prime source of vertical shear and mixing were mean currents and eddies, tides played a secondary role. Tides influenced mixing by increasing vertical temperature diffusivities to 10−4 to 10−3 m2 s−1 over portions of the continental slope and over rough topography, particularly in regions near the diurnal critical latitudes (27°–30°). In conclusion, even small tides can significantly impact the circulation through their effects on the mean currents, eddy rotation velocities, eddy propagation, and mixing.

Internal tides are studied in the central Mediterranean Sea using observational data and numerical experiments. Both numerical results and observations indicate that the baroclinic variability in this area is dominated by the K1 diurnal tide. In agreement with previous studies, the diurnal internal tides have the characteristics of Kelvin-like bottom trapped waves. They are mainly generated by the interaction of the induced barotropic tidal flow with the steep bathymetric gradient connecting the Ionian Sea with the shallow Sicily Channel. The bathymetric gradient appears to be the major forcing shaping the propagation paths of the internal tides. The most energetic internal tides follow the steep bathymetric gradient, propagating southward and tending to dissipate rapidly. Other waves cross the continental shelf south of Malta and then split with one branch moving toward the southern coast of Sicily and the other moving toward the west. Internal tides propagate with a variable phase velocity of about 1 ms−1 and a wavelength of the order of 100 km. During their journey, the internal waves appear to be subject to local processes that can modify their characteristics. The induced vertical shear strongly dominates the vertical turbulence and generates vertical mixing that alters the properties of the water masses traversing the area. Barotropic and internal tides remove heat from the ocean surface, increasing atmospheric heating, and redistributing energy through increased lateral heat fluxes. Lateral heat fluxes are significantly greater in the presence of internal tides due to the simultaneous increase in volume fluxes and water temperatures. [ABSTRACT FROM AUTHOR]

Here we show how ultra‐high resolution seabed mapping using new technology can help to understand processes that sculpt submarine canyons. Time‐lapse seafloor surveys were conducted in the axis of Monterey Canyon, ∼50 km from the canyon head (∼1,840 m water depth) over an 18‐month period. These surveys comprised 5‐cm resolution multibeam bathymetry, 1‐cm resolution lidar bathymetry, and 2‐mm resolution stereophotographic imagery. Bathymetry data reveal centimeter‐scale textures that would be undetectable by more traditional survey methods. Upward‐looking Acoustic Doppler Current Profilers at the site recorded the flow character of internal tides and the passage of three turbidity currents, while sediment cores collected from the site record flow deposits. Combined with flow and core data, the bathymetry shows how turbidity currents and internal tides modify the seabed. The turbidity currents drape sediment across the site, infilling bedform troughs and smoothing erosional features carved by the internal tides (e.g., rippled scours). Turbidity currents with speeds of 0.9–3.3 m/s failed to cause notable bedform movement, which is surprising given that flows with similar speeds produced rapid bedform migration elsewhere, including the upper Monterey Canyon. The lack of migration may be related to the character of the underlying substrate or indicate that turbidity currents at the site lack dense, near‐bed layers. The scale of scours produced by the internal tides (≤0.7 m/s) approaches the scale of features recorded in the ancient rock record. Thus, these results illustrate how the scale gap between seabed mapping technology and the rock record may eventually be bridged. Plain Language Summary: This study presents the highest resolution repeat seafloor mapping surveys to date in a submarine canyon. Centimeter‐scale seafloor surveys were conducted over an 18‐month period at a location ∼50 km from the head of Monterey Canyon. Turbidity currents (rapid, down‐slope density‐driven flows of water) and internal tides (tidal currents that flow along the seabed) were recorded by a seafloor instrument node deployed in the study area. Turbidity currents drape sediment across the bedforms, infill troughs, and smooth out erosional features carved by internal tides. While the turbidity currents caused considerable migration of bedforms in the upper canyon, there was surprisingly no notable migration at the study site. The lack of migration may be related to the relatively coarse nature of the seafloor underlying the bedforms and suggests that the turbidity currents at the site lacked a dense near‐bed layer characteristic of flows farther up‐canyon. The internal tides carved centimeter‐scale scours in the seabed, a scale that approaches that of features preserved in the rock record. This study, therefore, shows how new mapping technology may eventually bridge the scale gap between modern seafloor surveys and the ancient rock record. Key Points: Ultra‐high resolution time‐lapse mapping is combined with flow monitoring and sediment cores to document submarine canyon activityFast turbidity currents fail to cause notable bedform migration, though flows with similar speeds cause significant migration up‐canyonSub‐meter to meter‐scale scours, likely carved by internal tides, approach the scale of features preserved in the rock record [ABSTRACT FROM AUTHOR]

Tides and internal tides (IT) in the ocean can significantly affect local to regional ocean temperature, including sea surface temperature (SST), via physical processes such as diffusion (vertical mixing) and advection (vertical and horizontal) of water masses. Offshore of the Amazon River, strong IT have been detected by satellite observations and well modelled; however, their impact on temperature, SST and the identification of the associated processes have not been studied so far. In this work, we use high resolution (1/36°) numerical simulations with and without the tides from an ocean circulation model (NEMO). This model explicitly resolves the internal tides (IT) and is therefore suitable to assess how they can affect ocean temperature in the studied area. We distinguish the analysis for two contrasted seasons, from April to June (AMJ) and from August to October (ASO), since the seasonal stratification off the Amazon River modulates the IT's response and their influence in temperature. The generation and the propagation of the IT in the model are in good agreement with observations. The SST reproduced by the simulation including tides is in better agreement with satellite SST data compared to the simulation without tides. During ASO season, stronger meso-scale currents, deeper and weaker pycnocline are observed in contrast to the AMJ season. The observed coastal upwelling during ASO season is better reproduced by the model including tides, whereas the no-tide simulation is too warm by +0.3 °C for the SST. In the subsurface above the thermocline, the tide simulation is cooler by -1.2 °C, and warmer below the thermocline by +1.2 °C compared to the simulation without the tides. The IT induce vertical mixing on their generation site along the shelf break and on their propagation pathways towards the open ocean. This process mainly explains the cooler temperature at the ocean surface and is combined with vertical and horizontal advection to explain the cooling in the subsurface water above the thermocline and a warming in the deeper layers below the thermocline. The surface cooling induced in turn an increase of the net heat flux from the atmosphere to the ocean surface, which could induce significant changes in the local and even for the regional tropical Atlantic atmospheric circulation and precipitation. We therefore demonstrate that IT, via vertical mixing and advection along their propagation pathways, and tides over the continental shelf, can play a role on the temperature structure off the Amazon River mouth, particularly in the coastal cooling enhanced by the IT. Keywords: internal tides, Amazon continental shelf and slope, temperature, modeling, satellite data, mixing, heat flux. [ABSTRACT FROM AUTHOR]

Transport processes in coastal oceans are important for understanding mass transport between land and ocean. Investigations of mixing processes are essential to reveal the transport processes. This paper presents mixing processes due to internal tides and the associated mass transport, on the basis of field observations and numerical simulations conducted by the author who is a recipient of Okada Prize from the Oceanographic Society of Japan, 2022. The Yoing Ocean Data Acquisition (YODA) Profiler, has been developed to observe small scale dynamics and mixing processes in coastal oceans. Results from the profiler revealed mixing in river plumes and breaking of nonlinear internal tides on a shallow slope. Breaking of internal tides results in strong turbulent mixing, sediment resuspension and generation of intermediate nepheloid layers in Otsuchi Bay, Japan. It was found that vertical mixing was enhanced by a collision of a receding internal tide and a subsequent run-up nonlinear internal bore on a gentle slope. In addition, numerical models were developed to investigate internal tides breaking. Large scale oceanic numerical models showed enhanced diurnal internal tides caused by a resonance of Kelvin waves around islands over the Izu-Ogasawara Ridge and strongly enhanced the internal tide energy flux toward the upstream of the Kuroshio owing to an interaction of the Kuroshio and tides. This study also applies methods established in oceanic studies to analysis of mixing processes in a lake. [ABSTRACT FROM AUTHOR]

An unconstrained global ocean simulation for 2020 supports past observations of diurnal internal tides by acoustic tomography during the 1991–1992 Acoustic Mid‐Ocean Dynamics Experiment in the Western North Atlantic. Explicitly representing the tides, the simulation reproduces the functional form and resonant state of K1 and O1 internal‐tide standing waves, while providing a more realistic physical picture of them. The tomographic data were used to predict the tides in 2020. Not surprisingly, the characteristics of the barotropic and internal tides of the unconstrained simulation deviate from observations. The simulated barotropic tidal currents have excessive, irregular amplitude and lead the acoustic tidal predictions by about 2 hr. While internal‐tide phase coherence is apparent, the simulated internal‐tide variations were irregular in amplitude and phase, unlike the observations. The tomographic tidal measurements therefore provide a quantitative benchmark for improved model representation of tides, internal tides, and dissipation. Plain Language Summary: Oceanic internal waves, supported by stratification and the local rate of rotation of the earth (Coriolis frequency), are ubiquitous, generated by winds, tides, and uneven seafloor. Over the past 30 years, internal‐tide waves of the gravest vertical mode (half‐cosine vertically, 150–500‐km wavelength) were discovered to travel coherently across ocean basins, observed by their signals in temperature or sea‐surface height. Internal tides in a high‐resolution ocean model were compared to acoustical observations made in the western North Atlantic in 1991, with encouraging agreement. Internal waves are refracted back toward the equator at the latitude where their frequency equals twice the earth's local rotation. In the model, internal tides with 24‐hr periods (diurnal) are found to be standing waves with Airy function form, trapped between Puerto Rico and their turning latitude at 30°N, confirming an earlier interpretation of the acoustical observations. The acoustical tidal observations are an excellent benchmark for the development and adjustment of ocean models. Key Points: Past interpretations that signals in acoustic data were caused by standing‐wave, diurnal internal tides are supported by an ocean modelThe historical acoustic data can be used to predict model internal‐tide variability much like ordinary tide predictionThe acoustical observations provide a quantitative benchmark for improved model representation of tides, internal tides, and dissipation [ABSTRACT FROM AUTHOR]

High biological productivity and the efficient export of carbon-enriched subsurface waters to the open ocean via the continental shelf pump mechanism make mid-latitude continental shelves like the northwest European shelf (NWES) significant sinks for atmospheric CO2. Tidal forcing, as one of the regionally dominant physical forcing mechanisms, regulates the mixing-stratification status of the water column that acts as a major control for biological productivity on the NWES. Because of the complexity of the shelf system and the spatial heterogeneity of tidal impacts, there still are large knowledge gaps on the role of tides for the magnitude and variability of biological carbon fixation on the NWES. In our study, we utilize the flexible cross-scale modeling capabilities of the novel coupled hydrodynamic–biogeochemical modeling system SCHISM–ECOSMO to quantify the tidal impacts on primary production on the NWES. We assess the impact of both the barotropic tide and the kilometrical-scale internal tide field explicitly resolved in this study by comparing simulated hindcasts with and without tidal forcing. Our results suggest that tidal forcing increases biological productivity on the NWES and that around 16% (14.47 Mt C) of annual mean primary production on the shelf is related to tidal forcing. Vertical mixing of nutrients by the barotropic tide particularly invigorates primary production in tidal frontal regions, whereas resuspension and mixing of particulate organic matter by tides locally hinders primary production in shallow permanently mixed regions. The tidal impact on primary production is generally low in deep central and outer shelf areas except for the southwestern Celtic Sea, where tidal forcing substantially increases annual mean primary production by 25% (1.53 Mt C). Tide-generated vertical mixing of nutrients across the pycnocline, largely attributed to the internal tide field, explains one-fifth of the tidal response of summer NPP in the southwestern Celtic Sea. Our results therefore suggest that the tidal NPP response in the southwestern Celtic Sea is caused by a combination of processes likely including tide-induced lateral on-shelf transport of nutrients. The tidally enhanced turbulent mixing of nutrients fuels new production in the seasonally stratified parts of the NWES, which may impact the air–sea CO2 exchange on the shelf.

Interactions between internal tides and mesoscale eddies are an important topic. However, examining modulations of internal tides inside a mesoscale eddy based on observations is difficult due to limited observation duration and inaccurate positioning within the eddy. In order to overcome these two practical limitations, we use the active navigation capability of underwater gliders to conduct measurements inside the targeted eddy and utilize the wavelet approach to investigate modulations of internal tides with diurnal and semidiurnal periods inside the eddy. Based on the wavelet’s frequency–time locality, we construct scale-specific networks via wavelet coherence (WC) from multivariate timeseries with a small sample size. The modulation of internal tides is then examined in terms of temporal evolutionary characteristics of the WC network’s topological structure. Our findings are as follows: (1) the studied eddy is vertically separated into two layers, the upper (400 m) layers, indicating that the eddy is surface intensified; (2) the eddy is also horizontally divided into two domains, the inner and outer centers, where the modulation of internal tides seems to actively occur in the inner center; and (3) diurnal internal tides are more strongly modulated compared to semidiurnal ones, indicating the influence of spatial scales on the strength of interactions between internal tides and eddies.

Vertical walls of submarine canyons represent features of high conservation value that can provide natural areas of protection for vulnerable marine ecosystems under increasing anthropogenic pressure from deep-sea trawling. Wall assemblages are spatially heterogeneous, attributed to the high environmental heterogeneity over short spatial scales that is a typical feature of canyons. Effective management and conservation of these assemblages requires a deeper understanding of the processes that affect faunal distribution patterns. Canyons are recognised as sites of intensified hydrodynamic regimes, with focused internal tides enhancing near-bed currents, turbulent mixing and nepheloid layer production, which influence faunal distribution patterns. Faunal patterns also respond to broad-scale hydrodynamics and gradients in water mass properties (e.g. temperature, salinity, dissolved oxygen concentration). Oscillating internal tidal currents can advect such gradients, both vertically and horizontally along a canyon's walls. Here we take an interdisciplinary approach using biological, hydrodynamic and bathymetry-derived datasets to undertake a high-resolution analysis of a subset of wall assemblages within Whittard Canyon, North-East Atlantic. We investigate if, and to what extent, patterns in diversity and epibenthic assemblages on deep-sea canyon walls can be explained by spatial and temporal variability induced by internal tides. Vertical displacement of water mass properties by the internal tide was calculated from autonomous ocean glider and shipboard CTD observations. Spatial patterns in faunal assemblage structure were determined by cluster analysis and non-metric Multi-Dimensional Scaling plots. Canonical Redundancy Analysis and Generalised Linear Models were then used to explore relationships between faunal diversity and assemblage structure and a variety of environmental variables. Our results support the hypothesis that internal tides influence spatial heterogeneity in wall faunal diversity and assemblages by generating both spatial and temporal gradients in hydrodynamic properties and consequently likely food supply.

Abstract An unconstrained global ocean simulation for 2020 supports past observations of diurnal internal tides by acoustic tomography during the 1991–1992 Acoustic Mid‐Ocean Dynamics Experiment in the Western North Atlantic. Explicitly representing the tides, the simulation reproduces the functional form and resonant state of K1 and O1 internal‐tide standing waves, while providing a more realistic physical picture of them. The tomographic data were used to predict the tides in 2020. Not surprisingly, the characteristics of the barotropic and internal tides of the unconstrained simulation deviate from observations. The simulated barotropic tidal currents have excessive, irregular amplitude and lead the acoustic tidal predictions by about 2 hr. While internal‐tide phase coherence is apparent, the simulated internal‐tide variations were irregular in amplitude and phase, unlike the observations. The tomographic tidal measurements therefore provide a quantitative benchmark for improved model representation of tides, internal tides, and dissipation.

Barotropic and baroclinic tides were simulated for the Coral and Tasman Seas off eastern Australia and compared to a simulation without tides. Both simulations included geostrophic currents and replicated a current analogous to the East Australian Current (EAC). Tides and tidal currents in most of this region are weak, generally 1–2 cm s−1, with the exception of the far northwest portion of the Coral Sea. Even these weak tides were found to impact the mean EAC-like current transport, enhancing it by 1–4 Sv in some areas. Southward flow increased over the continental shelf. Tides did not appear to impact eddy formation, size, or rotational speed; however, they affected eddy propagation. Cyclonic eddies propagated northward faster with tides than without tides. Tides impacted cross-shelf transport of colder water, with significantly more on-shore transport occurring with tides, particularly equatorward of the diurnal critical latitudes. Cross-shelf transport of nutrient rich water onto the shelf is important in this oligotrophic region. Although the prime source of vertical shear and mixing were mean currents and eddies, tides played a secondary role. Tides influenced mixing by increasing vertical temperature diffusivities to 10−4 to 10−3 m2 s−1 over portions of the continental slope and over rough topography, particularly in regions near the diurnal critical latitudes (27°–30°). In conclusion, even small tides can significantly impact the circulation through their effects on the mean currents, eddy rotation velocities, eddy propagation, and mixing. [ABSTRACT FROM AUTHOR]

The variability in the tidal current in the Jeju Strait was investigated based on sea surface current fields observed by a high-frequency (HF) radar and vertical current profiles observed by three bottom-moored acoustic Doppler current profilers (ADCPs). The monthly M2 tidal current, the most dominant constituent in the study area, had significant temporal variability, and there were spatial changes in the temporal variation trend. While the semi-major axis of the M2 tidal current in winter was stronger than that in summer in some parts of the HF radar observation range, the summertime semi-major axis was stronger than that in wintertime in other parts of the observation range. Both opposite trend areas coexisted within the HF radar coverage area of several tens of kilometers. The vertical structure of the tidal currents in winter was similar to the theoretical structure in which Ekman dynamics was applied under depth-independent vertical eddy viscosity, and the year-to-year change was weak. In contrast, the vertical structure of the tidal currents in summer significantly varies over depth and years. Furthermore, the seasonal variation patterns at the three ADCP stations differed spatially. The temporal and spatial variability of tidal currents could be explained as a result of the strengthening stratification and, consequently, semidiurnal internal tides in summer. In summer, an internal tide with a wavelength of tens of kilometers could be generated in the vicinity of and propagate into the study area. The overlapping and offsetting of the barotropic and internal tidal components within the HF radar observation range caused complex spatial variations. [ABSTRACT FROM AUTHOR]

The northern Andaman Sea (AS) continental shelf is unique due to the diverse marine ecosystem and existences of both hypoxia and internal tides, but limited in situ observations restrict our understanding of the hydrographic dynamic process. Based on the turbidity, mean volumes of backscattering strength (MVBS), we qualitatively studied the mean distribution characteristics and temporal variation in suspended matters on the northern AS continental shelf and their relation to hypoxia and internal tides. The results of both MVBS and turbidity revealed that the suspended matters exhibited a three-layer vertical structure. The upper and lower layers exhibited high values, while the middle layer had low values. The upper boundary of the high-value region in the upper layer descended below the surface to a depth of 30 m after sunrise and returned to the surface after sunset, indicating a diel vertical migration of zooplankton and micronekton. Daytime migration depth was likely constrained by hypoxia’s upper boundary. In the lower layer, three MVBS enhancements and attenuations correlated with vertical upward and downward velocities, respectively, primarily driven by uplift or suppression. We proposed vertical velocity patterns resulted from internal bores, possibly triggered by shoaling semidiurnal internal tides.

Recent mooring observations on the continental slope on the east side of the Dongsha Island in the northeastern South China Sea (SCS) showed that an along-slope bottom current can be generated when internal tides obliquely incident to the slope are dissipated near the seafloor. In this study, new mooring data collected on the south side of the Dongsha Island are used to explore the universality of internal wave driven the bottom currents and test the ability of the previous theory in estimating the along-slope current. The data show strong near-bottom energy dissipation due to the critical reflection of diurnal internal tides on the continental slope, with a time-mean depth-integrated dissipation rate of ~4.8×10-3 W/m2. Because of the obliquely incident of diurnal internal tides to the slope, near-bottom dissipation of internal tides generates a southwestward along-slope current, with the maximum velocity exceeding 6 cm/s. By comparing the observations, the previous theory for internal wave induced mean flows developed by Thorpe (1999) shows a good ability to estimate the along-slope bottom current velocity. Based on the theory, as well as modelled energy dissipation on the entire continental slope in the northeastern SCS, a map is obtained to quantitatively describe the along-slope bottom flow caused by internal tide breaking on the slope.

The generation and propagation of internal tides in the Andaman Sea are investigated using a three-dimensional high-resolution numerical model. Three categories of experiments, including driving the model with four main semidiurnal tides (M2, S2, N2, and K2), four main diurnal tides (K1, O1, P1, and Q1), and eight main tides (M2, S2, N2, K2, K1, O1, P1, and Q1), are designed to examine the effects of barotropic tides. The results show that the semidiurnal internal tides are dominant in the Andaman Sea, and the inclusion of diurnal barotropic tides negligibly modulates this result. That is partly due to the strength of the diurnal barotropic tides is generally one order smaller than that of the semidiurnal barotropic tides in this region. The sensitivity experiments put this on a firmer footing. In terms of the internal tidal energy, the experiments driven by the diurnal barotropic tides are three orders and one order smaller than those driven by the semidiurnal barotropic tides, respectively, during the spring and neap tides. In addition, the experiments result in total barotropic-to-baroclinic energy conversion rates over the Andaman Sea 29.15 GW (driven by the eight tides), 29.24 GW (driven by the four semidiurnal tides), and 0.05 GW (driven by the fourdiurnal tides) in the spring tidal period and 3.08 GW, 2.56 GW, and 0.31 GW in the neap tidal period, respectively. Four potential generation regions of internal tides are found, three of which are in the Andaman and Nicobar Islands and one in the northeastern Andaman Sea.

The wavenumber spectral slope of sea surface height (SSH) computed within the mesoscale range (70–250 km) from satellite altimetry exhibits a large spatial variability which, until now, has not been reproduced in numerical ocean models. This study documents the impacts of including internal tides, high‐resolution bathymetry, and high‐frequency atmospheric variability on the SSH wavenumber spectra in the Atlantic Ocean, using a series of 1/50° North and equatorial Atlantic simulations with a realistic representation of barotropic/baroclinic tides and mesoscale‐to‐submesoscale variability. The results show that the inclusion of internal tides does increase high frequency SSH variability (with clear peaks near 120 and 70 km) and flattens the spectra slope in the mesoscale range in a good agreement with observations. The surface signature of internal tides, mostly in the equatorial Atlantic but also in subtropical regions in the eastern North Atlantic, is the primary reason behind the observed large spatial variability of the spectral slope in the Atlantic. Internal tides are stronger in the tropical regions when compared to higher latitudes because of the stronger barotropic tides and stronger stratification in the upper layer of the water column. High‐resolution bathymetry does play an important role in the internal tide generation on a local scale, but its impact on the modeled large‐scale SSH variability and SSH wavenumber spectra is quite small. High‐frequency wind variability plays only a minor role on the generation of the simulated high‐frequency SSH variability. Impact of altimetry sampling on the computation of the SSH wavenumber spectra is also discussed. Plain Language Summary: The sea surface height (SSH) wavenumber spectrum is a quantification of the magnitude of SSH variability on different spatial scales. The slope of SSH wavenumber spectra is essential to understanding how kinetic energy is transferred across different scales in the turbulent ocean. The most striking feature of the SSH spectral slope as derived from satellite observations is that it has a wide range of values over different parts of the Atlantic Ocean: from an order of −5 in the western boundary current, to an order of −4 to −3 in the interior of subtropical North Atlantic, and to an order of −1 to −0.5 in the tropical Atlantic. In this study, we reproduced the observed spatial variability of the SSH spectral slope in numerical ocean models, and further documented that internal tide is the main driver behind the scenes. The surface manifestation of internal tides flattens the spectral slope of the SSH. This flattening is most significant in the tropical Atlantic, but also occurs in the subtropical eastern North Atlantic. The impacts from other factors such as high‐resolution bathymetry and high‐frequency atmospheric forcing are relatively small. Key Points: The slope of the sea surface height (SSH) power spectra as derived from satellite observations exhibits a large spatial variability, which is now reproduced in our numerical simulationsThe surface manifestation of internal tides with energy peaks near 120 and 70 km is the main driver for spatial variability in the slope of the SSH power spectraHigh‐resolution bathymetry plays an important role in the internal tide generation locally but has a relatively small impact on the SSH power spectra on a broader scale [ABSTRACT FROM AUTHOR]

This study proposes new insights for the interpretation of GEOVIDE particle measurements. Potential resuspension sites due to internal tides (ITs) have been identified along the western Iberian shelf slope using a tidal regional model. Iberian shelf is mainly a source of lithogenic material whereas Galicia Bank is a source of biogenic matter. The potential pathways for particles from the resuspension sites toward the GEOVIDE stations are identified using IBIRYS reanalysis to infer their transport by the ocean circulation. It appears that each station is influenced by a specific pathway and mixed particles from several resuspension sites. This methodology reveals that the criticality of the bathymetry slope, commonly used to track resuspension sites from internal waves, is not sufficient to investigate ITs. An explicit resolution of ITs using a numerical tidal model is required for an exhaustive identification of the resuspension sites. Plain Language Summary: Internal tides (ITs) are waves with a tidal period generated when tides cross a steep slope of the sea floor. ITs then propagate in every stratified layer of the open ocean over hundreds to thousands of kilometers. The encounter of ITs with underwater relief, either at the generation site or after propagation, results in high bottom friction that enhances sediment resuspension. Evidence of lithogenic particles was found at various depths during the GEOVIDE cruise, from the Iberian continental slope to thousands of kilometers away. The present study identifies the sites where ITs are responsible for sediment resuspension. Additionally, the oceanic circulation is investigated to establish the trajectories of the resuspended sediment that could explain the GEOVIDE measurements. Key Points: Sites of high bottom tidal velocity along the Iberian continental slope were identified using a semi‐realistic internal tides modelSediment resuspension was confirmed on five sites based on earlier sediment surveys and Shields criterion thresholdParticle trajectories were proposed based on circulation modeling to improve the interpretation of geochemical tracers measurements [ABSTRACT FROM AUTHOR]

Internal waves (IWs) are generated in all the oceans, and their amplitudes are large, especially in regions that receive a large amount of freshwater from nearby rivers, which promote highly stratified waters. When barotropic tides encounter regions of shallow bottom-topography, internal tides (known as IWs of the tidal period) are generated and propagated along the pycnocline due to halocline or thermocline. In the North Indian Ocean, the Bay of Bengal (BoB) and the Andaman Sea receive a large volume of freshwater from major rivers and net precipitation during the summer monsoon. This study addresses the characteristics of internal tides in the BoB and Andaman Sea using NASA’s Estimating the Circulation and Climate of the Ocean (ECCO) project’s high-resolution (1/48° and hourly) salinity estimates at 1 m depth (hereafter written as ECCO salinity) during September 2011–October 2012, time series of temperature, and salinity profiles from moored buoys. A comparison is made between ECCO salinity and NASA’s Soil Moisture Active Passive (SMAP) salinity and Aquarius salinity. The time series of ECCO salinity and observed salinity are subjected to bandpass filtering with an 11–14 h period and 22–26 h period to detect and estimate the characteristics of semi-diurnal and diurnal period internal tides. Our analysis reveals that the ECCO salinity captured well the surface imprints of diurnal period internal tide propagating through shallow pycnocline (~50 m depth) due to halocline, and the latter suppresses the impact of semi-diurnal period internal tide propagating at thermocline (~100 m depth) reaching the sea surface. The semi-diurnal (diurnal) period internal tides have their wavelengths and phase speeds increased (decreased) from the central Andaman Sea to the Sri Lanka coast. Propagation of diurnal period internal tide is dominant in the northern BoB and northern Andaman Sea.

We present a modeling study of an idealized island with sloping sides in a stratified ocean with internal tides propagating through the domain, which transform into nonlinear internal waves as they encounter shallow depths. In the base condition, representative of a mid‐latitude island with relatively steep super critical slopes, the nonlinear internal waves shoaling on the island wrap around the island, scatter, and create an asymmetric wavefield. Radiation stress gradients from this wavefield and rotation are balanced by pressure gradients and turbulent vertical diffusion in the momentum equations which drive residual mean currents around the island. We explore the effects of different physical parameters including no rotation, higher Froude number, subcritical slope, addition of barotropic tides, larger excursion number, and addition of large‐scale mean currents. All simulations showed formation of residual currents of varying intensity, upwelled deep waters in shallow regions, and cooler waters on the side of the island consistent with the direction of incoming internal tides. While the no rotation and strong mean flow conditions created some upwelling, of all the modeled conditions, the subcritical slope has the greatest potential for creating favorable conditions for benthic organisms through enhanced upwelling. Areas of the world's oceans with stratified waters sufficient to sustain internal tide motions, strong internal tide energy, and local subcritical bottom slopes are likely to have cooler waters and more upwelled deep waters than the surrounding areas, and potentially serve as thermal refugia from future ocean warming. Plain Language Summary: Tides oscillate the world's oceans, and because warmer lighter water sits on cooler dense water, wave energy travels along this interface. This process, called internal tides, creates waves which propagate in the ocean interior and are quite common in the ocean. When these waves encounter a coastline or island they function similarly to waves on a beach: they can turn, change shape, break, or reflect. These changes to the waves can create currents, often quite strong, and pull deeper cooler water into the shallow coastal areas. We performed a modeling study to determine which factors are most important in creating strong currents from these internal tides across a range of possible conditions on islands worldwide. The results show that the internal tides create complex currents and bring cooler dense water into the shallow regions under most conditions. The factor that creates potentially the best conditions for organisms living on the ocean floor is gradual ocean floor slopes (compared to steeper). The results for this condition had strong currents, increased deep ocean waters, and cooler temperatures in the shallow coastal areas which may buffer effects from future ocean warming. Key Points: Internal tides interacting with an island force complex residual currents and upwellingRotation, barotropic tides, large‐scale mean currents and bottom slope all have an influence on the currents and upwellingSubcritical slope has the largest upwelling from internal tides and the greatest potential for creating thermal refugia in a warming ocean [ABSTRACT FROM AUTHOR]

As a ubiquitous movement in the ocean, tides are vital for marine life and numerous marine activities such as fishing and ocean engineering. Tidal dynamics are complicated in the East Asian marginal seas (EAMS) due to changing complex topography and coastlines related to human activities (e.g., land reclamation and channel deepening) and natural variability (e.g., seasonal variations of ocean stratification and river flow). As an important tool, numerical models are widely used because they can provide basin-scale patterns of tidal dynamics compared to point-based tide gauges. This paper aims to overview the development history of the numerical simulation of tides in the EAMS, including the Bohai Sea, the Yellow Sea, the East China Sea, the East/Japan Sea, and the South China Sea, provide comprehensive understanding of tidal dynamics, and address contemporary research challenges. The basic features of major tidal constituents obtained by tidal models are reviewed, and the progress in the inversion of spatially and temporally changing model parameters via the adjoint method are presented. We review numerical research on how a changing ocean environment induces tidal evolution and how tides and tidal mixing influence ocean environment in turn. The generation, propagation, and dissipation of internal tides in the EAMS are also reviewed. Although remarkable progresses in tidal dynamics have been made, nonstationary tidal variations are not fully explained yet, and further efforts are needed. In addition, tidal influences on ocean environment still receive limited attention, which deserves special attention. [ABSTRACT FROM AUTHOR]

The vertical structure and seasonal variability of shear were examined using nearly three years of mooring ADCP (acoustic Doppler current profiler) data on the southwestern continental slope of the East China Sea (ECS). Shear spectra suggest that the sub-inertial currents (SICs); near-inertial waves (NIWs); and diurnal (D1), semidiurnal (D2), and tridiurnal (D3) internal tides (ITs) dominate the local shear field. The shear exhibits a remarkable surface-intensified pattern with high values occurring mostly in the upper 200 m. Significant seasonal variations can be found in the shear, but with differences between the upper (50–200 m averaged) and lower layers (210–570 m averaged). Satellite altimeter data indicate that the meander of the Kuroshio mainstream and the Kuroshio intrusion affect the seasonal variation of total shear by mainly influencing the shear caused by SICs. In addition, the shear efficiency (SE) of D2 ITs is obviously less than that of NIWs and that of D1 and D3 ITs via analyzing the kinetic energy (KE) densities and shear caused by these motions, since the predominant mode of the former is the first baroclinic mode, while the latter is dominated by higher baroclinic modes with large vertical wavenumbers. Moreover, the SE of incoherent ITs is relatively stronger than that of coherent ITs as a result of a larger proportion of high baroclinic modes in the incoherent component compared to the coherent component, based on modal decomposition. [ABSTRACT FROM AUTHOR]

A proper extraction of internal tidal signals is central to the interpretation of Sea Surface Height (SSH) data. The increased spatial resolution of future wide‐swath satellite missions poses a challenge for traditional harmonic analysis, due to prominent and unsteady wave‐mean interactions at finer scales. However, the wide swaths will also produce SSH snapshots that are spatially two‐dimensional, which allows us to treat tidal extraction as an image translation problem. We design and train a conditional Generative Adversarial Network, which, given a snapshot of raw SSH from an idealized numerical eddying simulation, generates a snapshot of the embedded tidal component. We test it on data whose dynamical regimes are different from the data provided during training. Despite the diversity and complexity of data, it accurately extracts tidal components in most individual snapshots considered and reproduces physically meaningful statistical properties. Predictably, Toronto Internal Tide Emulator's performance decreases with the intensity of the turbulent flow. Plain Language Summary: Wide‐swath satellite observations of Sea Surface Height (SSH) data at high spatial resolutions will be available in abundance thanks to advances of instrumental technologies. Embedded in the observed SSH are internal tides, a dynamical component that plays a crucial role in ocean circulation. As they are entangled with background currents and eddies, such tidal signals are challenging to extract. Methods that worked with previous‐generation altimeters will break down at the resolutions that the new generation promises. On the other hand, the wide satellite swaths provide new opportunities as they allow us to regard the observations as spatially two‐dimensional. Here we treat the tidal extraction solely as an image translation problem. We train a deep neural net so that given a snapshot of a raw SSH signal, it produces a "fake" snapshot of the tidal SSH signal that is meant to reproduce the original. The data we use in this article is generated by idealized numerical simulations. Once adapted to realistic data, the network has the potential to become a new tidal extraction tool for satellite observations. More broadly, successes in our experiments can inspire other applications of generative networks to disentangle dynamical components in data where classical analysis may fail. Key Points: A deep conditional Generative Adversarial Network is trained to extract tidal components in Sea Surface Height snapshots from an idealized modelThe network can extract tidal signals accurately in a snapshot whose underlying dynamics are different from training dataPerformance of the network degrades when extracting tidal signals entangled with higher turbulence energies [ABSTRACT FROM AUTHOR]

Breaking internal tides and induced mixing are critical to shelf dynamics, including heat and mass exchanges. Spatiotemporal variability of internal tides and modulation factors for the southern East China Sea shelf were examined based on a combination of a three-month mooring velocity and satellite altimeter data. Semidiurnal and diurnal internal tides exhibited distinct temporal trends, with the semidiurnal internal tides enhanced by an order of magnitude during the latter half of the record, while the diurnal internal tides followed quasi spring-neap cycles with a generally stable intensity except for two specific periods of strengthening. These internal tides probably originated remotely over the shelf-slope area northeast of Taiwan. Time-varying stratification was the most important factor for the internal tidal magnitude. In addition, varying background currents influenced the diurnal critical latitude band, which explains the slightly enhanced diurnal internal tides during the two periods. Although both semidiurnal and diurnal internal tides were mode-1 dominated, the semidiurnal internal tides were surface intensified while the diurnal tides were bottom intensified. The proportion of higher mode internal tides increased during robust eddy activities. Stronger background vertical shear corresponded to high-frequency events and energy transfers from tidal frequencies to high frequencies associated with turbulent mixing. [ABSTRACT FROM AUTHOR]

Using velocity and high-resolution temperature mooring data from the fore-reef slope of Yongxing Island in the northwest South China Sea (water depth of 69 m), we examine the effects of internal waves on the temporal variations in temperature, bottom mixed layer (BML) and turbulent mixing. The diurnal tide is found to be the dominant tidal force and the baroclinic tide is highly active, which would account for the 11 d abnormal spring-neap cycle in the barotropic tidal current. During the ebb period (tidal elevation decreases), the bottom diurnal baroclinic current transports cold water upslope, resulting in a decrease in temperature, and vice versa during the flood period. The BML thickness H b m l widely varies around 1. 5 m , approximately 2% of the water depth. The bottom turbulent mixing is not so active, indicated by the bulk dissipation E ɛ of 10 mW m − 2 and turbulent diffusivity κ z of 2 × 1 0 − 4 m 2 s − 1 . Both H b m l and E ɛ approximate a log-normal distribution, demonstrating strong intermittency. The high-frequency (ω) internal bores can increase H b m l by four times and enhance the turbulent mixing by one order, which should be responsible for the slow cascade of H b m l (∼ ω − 1. 5) and E ɛ (∼ ω − 1. 0). In the downslope phase, H b m l is about 30% thicker, and turbulent mixing is enhanced by 3 times stronger than those in the upslope phase. It is revealed that under the forcing of internal waves, turbulent mixing corresponds to a thick BML with H b m l ∼ 〈 κ z 〉 0. 25 , and stratification (N) has a significantly negative correlation with the development of BML, H b m l ∼ 〈 N 〉 − 1. 8 . • Barotropic tidal current exhibits an abnormal spring-neap cycle, is likely modulated by internal tide. • Bottom baroclinic currents induce the temperature increase (decrease) in the flood (ebb) period. • Bottom mixed layer thickness H b m l and turbulent diffusivity κ z are correlated with H b m l ∼ 〈 κ z 〉 0. 25 . [ABSTRACT FROM AUTHOR]

The Rio Grande Rise (RGR) is a plateau located at 31°S in the Southwestern Atlantic, rising from 5916 m up to 161 m below the sea level. The RGR is an important site for future mining of Fe-Mn crusts and can lead to an expansion of Brazil's Exclusive Economic Zone. The Cruzeiro do Sul Rift (CSR) fault cuts through the RGR from southeast to northwest. In this study we characterize the RGR circulation, showing that M2 tides are the main source of variability in the region, with an amplitude that can reach 0.3 m s−1, larger than the mean flow. These M2 tides are dominated by the baroclinic component and intensified near the bottom. The generation of M2 internal tides occurs mainly in the CSR slopes, with most energy converted from the barotropic tide being radiated away in the form of tidal beams. In addition, the impingement of the mean southern South Equatorial Current and tidal rectification generates anticyclonic circulations around the RGR peaks, with the latter mechanism being responsible for a bottom intensified anticyclonic circulation of 0.2 m s−1. Finally, our results reveal that the RGR is a hotspot of internal tide generation in the Southwestern Atlantic. • Most barotropic to baroclinic tide energy conversion occurs on the rift slopes. • 47.1% of the baroclinic energy converted from barotropic tides is radiated away. • Tidal rectification is the primary mechanism of generating bottom intensification. [ABSTRACT FROM AUTHOR]

The 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, 90°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. Plain Language Summary: Internal waves (IW), the subsurface analog of surface ocean waves, are present everywhere in the ocean. One of the largest IW ever observed in the world's ocean is seen in the Andaman Sea. Internal tides (IT) are IW at tidal frequencies. Internal waves have been documented in the Andaman Sea since 1965, although their temporal variability is largely unknown. From our investigations, we discovered that the internal wave spectrum is dominated by semidiurnal IT, which occurs twice a day. The amplitudes of semidiurnal IT can be as high as 44 m along the Andaman Nicobar Ridge, where they are generated. Moreover, salinity changes near the surface and temperature changes in the subsurface determine the density stratification. The numerical studies using a 3‐dimensional model showed that changes in density stratification mainly control the formation of IT. It is interesting to note that different sub‐regions exhibited distinct seasonal variability in terms of IT generation and dissipation. Seasonal variability of IT can have a significant effect on the diverse corals present in this region. Key Points: Temporal variability of internal tides (IT) in the Andaman Sea is comprehensively documented for the first time using in‐situ observationsChanges in the stratification mainly control the seasonal variability in the generation of ITMaximum baroclinic energy conversion is observed in December 2017 at the main generation sites [ABSTRACT FROM AUTHOR]

Data from Argo floats equipped with Iridium communications are used to obtain a global map of the total amplitude (or variance) of the semidiurnal internal tide at 1,000 dbar. The results are confirmed by a comparison with data from an historical collection of moored instruments. The obtained amplitude is in turn compared with the High‐Resolution Empirical Tide (HRET) model, based on satellite altimetry. While HRET only contains the stationary component, with a fixed phase difference to the astronomical tide, the present results capture the total amplitude, including the nonstationary component. We estimate the global average ratio of total (Argo) to stationary (HRET) semidiurnal internal tide variance to be 6.5, and the amplitude ratio to be 3.6. Our estimate of the stationary fraction of the semidiurnal internal tide is subject to significant uncertainties. In particular, HRET is thought to mainly represent baroclinic mode‐1 waves, while Argo data contain contributions from all modes. Plain Language Summary: Internal tides are internal waves generated by the barotropic tide. As they propagate they are affected by the eddying general circulation. This causes the waves to decorrelate: their phase shifts and is no longer locked to the phase of the generating force. Satellite altimetry can only measure the stationary wavefield, that is, the component phase locked to the exactly known astronomical forcing. The total amplitude is therefore poorly known. Here, we describe the total internal tide field using a global collection of autonomous floats recording data as they freely drift at 1,000 m below the surface. By comparing these data to a satellite altimetry product we find that the nonstationary fraction of the semidiurnal internal tide is significantly larger than commonly thought. Quantifying the nonstationary internal tides is crucial for a better understanding of the abyssal circulation, and therefore of the global climate system. Key Points: Time series from Argo floats are analyzed in the time domain to produce a global map of the total semidiurnal internal tide varianceThe results are validated by a comparison with an historical collection of mooringsA comparison with satellite altimetry shows that 85% of the variance of the internal semidiurnal tide becomes decorrelated in 400 hr [ABSTRACT FROM AUTHOR]

The Mindanao Current (MC) bridges the North Pacific low-latitude western boundary current system region and the Indonesian Seas by supplying the North Pacific waters to the Indonesian Throughflow. Although the previous study speculated that the diapycnal mixing along the MC might be strong on the basis of the water mass analysis of the gridded climatologic dataset, the real spatial distribution of diapycnal mixing along the MC has remained to be clarified. We tackle this question here by applying a finescale parameterization to temperature and salinity profiles obtained using two rapid-sampling profiling Argo floats that drifted along the MC. The western boundary (WB) region close to the Mindanao Islands and the Sangihe Strait are the two mixing hotspots along the MC, with energy dissipation rate ε and diapycnal diffusivity Kρ enhanced up to ~ 10–6 W kg−1 and ~ 10–3 m2 s−1, respectively. Except for the above two mixing hotspots, the turbulent mixing along the MC is mostly weak, with ε and Kρ to be 10–11–10–9 W kg−1 and 10–6–10–5 m2 s−1, respectively. Strong mixing in the Sangihe Strait can be basically attributed to the existence of internal tides, whereas strong mixing in the WB region suggests the existence of internal lee waves. We also find that water mass transformation along the MC mainly occurs in the Sangihe Strait where the water masses are subjected to strong turbulent mixing during a long residence time. [ABSTRACT FROM AUTHOR]

The author has employed ocean acoustic tomography over the past 30 years to examine problems in physical oceanography, often with surprising results. Tomography offers an accurate measurement of current or sound speed (temperature) averaged over 100s to 1000s of kilometers. It has been used to test the fundamental equation for the speed of sound in seawater and to show that the normal state of the ocean has smooth, stable, well-behaved characteristics of acoustic propagation. This latter property has been challenging for ocean models to reproduce. Tomography measures barotropic tidal currents with remarkable accuracy, and it was used to discover that low-mode internal tides radiate far into the ocean’s interior, while retaining a surprising coherence. Tomographic measurements of large-scale temperature are complementary to point measurements by hydrography; the present observing system, relying principally on altimetry and Argo floats, has little skill in predicting the available tomographic data. The addition of tomographic data to the system would substantially reduce the uncertainty of ocean state estimates. The information content of integral acoustic observations is best exploited as a constraint on ocean models by data assimilation. The applications of tomography for measuring large-scale barotropic current, relative vorticity, and temperature have been under-exploited. Dedicated research programs, supporting novel acoustical oceanography, associated instrumentation development, and multidisciplinary research would further the applications of this observational approach.

The author has employed ocean acoustic tomography over the past 30 years to examine problems in physical oceanography, often with surprising results. Tomography offers an accurate measurement of current or sound speed (temperature) averaged over 100s to 1000s of kilometers. It has been used to test the fundamental equation for the speed of sound in seawater and to show that the normal state of the ocean has smooth, stable, well-behaved characteristics of acoustic propagation. This latter property has been challenging for ocean models to reproduce. Tomography measures barotropic tidal currents with remarkable accuracy, and it was used to discover that low-mode internal tides radiate far into the ocean's interior, while retaining a surprising coherence. Tomographic measurements of large-scale temperature are complementary to point measurements by hydrography; the present observing system, relying principally on altimetry and Argo floats, has little skill in predicting the available tomographic data. The addition of tomographic data to the system would substantially reduce the uncertainty of ocean state estimates. The information content of integral acoustic observations is best exploited as a constraint on ocean models by data assimilation. The applications of tomography for measuring large-scale barotropic current, relative vorticity, and temperature have been under-exploited. Dedicated research programs, supporting novel acoustical oceanography, associated instrumentation development, and multidisciplinary research would further the applications of this observational approach. [ABSTRACT FROM AUTHOR]

We present 15 days of both mean and turbulent field observations bottom mixing‐layer at a gently sloping 250 m deep continental shelf site, energized by tides and nonlinear internal waves (NLIWs). The tidal frequency forcing was due to the combined effects of the barotropic tide and a mode‐1 internal tide (IT), while the NLIWs were predominantly mode‐1 waves of depression. The bottom mixing‐layer thickness varied at both semidiurnal and sub‐tidal ∼O(10)d frequencies, with an average thickness of around 10 m. Compression and expansion of the mixing‐layer by both the IT and NLIWs affected the mean velocity profiles in the mixing‐layer, while the phasing between the barotropic and baroclinic flows led to an asymmetry in mean velocity profiles between periods of rising and falling isotherms. With the exception of periods of flow reversal, the turbulent kinetic energy balance and turbulent stress observations were consistent with the existence of an inertial‐sublayer with thickness of approximately 10%–15% of the mixing‐layer thickness (∼1 m), even beneath NLIWs. In the outer portion of the mixing‐layer—that is, above the inertial‐sublayer—NLIWs modulated the local turbulence spectra. We discuss the observations in the context of a predictive model for mixing‐layer thickness. The analysis suggests that the high‐frequency variability in mixing‐layer thickness was dominated by internal wave pumping, though strength of the ambient stratification and the frequency of the forcing were important controls on the time‐averaged (sub‐tidal) variation. Plain Language Summary: The bottom boundary layer (BBL) is the portion of an ocean flow that "feels" the presence of the seabed. Oceanic bottom boundary layer dynamics are globally significant as they control many important physical, biological, and chemical transport processes in the ocean, and furthermore, most man‐made sub‐sea structures are located within this relatively thin layer. We present 15 days of detailed observations of a near well‐mixed BBL at a 250 m deep continental shelf site, where the ocean flow was characterized by strong tides and large internal waves. The boundary layer thickness varied at tidal and sub‐tidal frequencies, and was ∼10 m thick on average. In the lowermost 1 m of this layer, the flow conformed to conventional boundary layer dynamics as observed in the lowest layers of the atmosphere, for example. More than 1 m above the bed, however, the overlying internal waves modified both the nature of the turbulent flow and the time‐averaged properties of the bottom mixing‐layer. We also discuss a simple empirical model to estimate the layer thickness. Key Points: A full spring‐neap cycle of mean and turbulent observations within a tide and internal wave driven bottom mixing‐layer (BML) are presentedInfluenced by stratification and unsteady flow above, BML thickness varied at semidiurnal and sub‐tidal ∼O(10)d timescalesTurbulence observations consistent with an inertial sublayer whose thickness was ∼10%–15% of the total BML thickness (∼1 m) [ABSTRACT FROM AUTHOR]

Measurement data of the internal-wave and acoustic-fluctuation experiments conducted in the Yellow Sea in summer 2009 are analyzed herein. The internal-wave activity dominated by semidiurnal tides is observed. For the source and 16-element vertical linear array with fixed distance, the received energy fluctuations of 260-Hz continuous wave signals reach 20 dB in a single channel of the receiving array, and the fluctuation period is about 2–3 h across 10-h continuous acoustic measurement. The energy depth distribution varies with time, and the energy fluctuations are less than 10 dB post-depth-integration, evidencing that part of the energy transfers between channels. Acoustic normal modes are calculated according to the temperature chain observation results obtained at the receiving station and subsequently used to analyze the acoustic energy fluctuations: the difference between the average energy depth distribution in 0–5 h and 5–10 h indicates that the energy fluctuations of the single channel may be related to the dominant-mode alternation occurring between the first- and second-order modes; the modal decomposition results of the received acoustic field prove such alternation and explain the single-channel and depth-integral energy fluctuations caused by the first two modes’ amplitude variations. [ABSTRACT FROM AUTHOR]