Diurnal Internal Waves Research Articles

Generation of diurnal internal solitary waves (ISW-D) in the Sulu Sea: From geostationary orbit satellites and numerical simulations

Our recent study reported the existence of internal solitary waves with the diurnal tidal cycle (ISW-D) in the Sulu Sea, however, the three-dimensional characteristics and generation mechanism of ISW-D are still unclear (Huang et al., 2023). In this work, the spatial–temporal characteristics, generation mechanism and propagating process of ISW-D in the Sulu Sea are first preliminary investigated based on high-temporal-resolution Geostationary Orbiting Satellite (GOS) images and high-spatial-resolution two-dimensional numerical model (MITgcm). GOS images from 2018-2022 are retrieved and a total of 13 pairs of ISW-D packets are found. ISW-D occur at spring tide during May–August, with an average interpacket distance of 198 km and a phase speed of 2.30 m/s. To further knowledge of the generation mechanism and propagation process of ISW-D, the non-linear and non-hydrostatic numerical simulations are conducted. The comparison of ISW-D parameters with GOS images proves the validity of numerical simulations. The results of numerical simulations and theoretical parameters indicate that ISW-D are generated at the sill near Pearl Bank by internal tide release mechanism. Moreover, three sensitivity experiments are designed to investigate the effects of tidal force and seawater stratification on the generation and propagation of ISW-D. The results reveal that the ISW-D is generated by diurnal tides with stronger intensity than semidiurnal tides. Seawater stratification does not influence the generation of ISW-D but modulates the propagation process. Phase speeds from GOS images and theoretical model show a positive correlation between the phase speeds of ISW-D and the intensity of seawater stratification.

Diurnal and semidiurnal internal waves on the southern slope of the Yermak Plateau

The Yermak Plateau (YP) is located across the Arctic–Atlantic gateway in the northwest of the Svalbard archipelago. In this region, internal waves are believed to cause intense turbulent mixing and hence influence the heat budget in the Arctic Ocean. Based on year-long observations from three moorings, the characteristics and energetics of diurnal and semidiurnal internal waves on the southern slope of the YP are investigated in this study. Diurnal internal waves induce large isothermal displacements exceeding 100 m, which are nearly one order of magnitude greater than those of semidiurnal internal waves. In addition, diurnal internal waves are strong in winter but weak in summer, while the semidiurnal internal waves exhibit complicated temporal variation. For the diurnal internal waves, their available potential energy is greater than the horizontal kinetic energy; whereas the situation is opposite for the semidiurnal ones. This feature is further clarified with two-dimensional numerical simulations. Due to the larger tidal excursion, diurnal tidal forcing yields the generation of stronger higher harmonics, i.e., the semidiurnal internal waves. In contrast, higher harmonics are rather weak under the semidiurnal forcing. Moreover, a large proportion of energy for both diurnal and semidiurnal internal waves is dissipated locally. Results of this study can provide useful insight on the dynamics of internal waves in the Arctic Ocean.

Behavior of Diurnal Internal Waves in the Southwestern East/Japan Sea

ABSTRACT Chae, J.-Y.; Jeon, C.; Park, S.J.; Choi, Y., and Park, J.-H., 2018. Behavior of diurnal internal waves in the southern East/Japan Sea. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 486–490. Coconut Creek (Florida), ISSN 0749-0208. The diurnal internal waves cannot freely propagate and are trapped near their generation site in the East/Japan Sea due to its sub-inertial environment. Previous studies show that diurnal internal shelf waves (ISWs) are generated around the southern Ulleung Basin (UB) with their energy concentrated there. To investigate the spatio-temporal variation of diurnal ISWs, we use a high-resolution numerical ocean model that is forced by realistic 16 tidal constituents and involves a data assimilation scheme. The baroclinic total energy and the conversion rate of barotropic to baroclinic energy show higher values at the southern s...

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

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

Propagation of internal tides generated near Luzon Strait: Observations from autonomous gliders

The vertical isopycnal displacements associated with internal waves generated by the barotropic tidal currents in the vicinity of Luzon Strait are estimated using measurements collected by autonomous underwater gliders. Nearly 23,000 profiles from Seagliders and Spray gliders, collected during 29 different missions since 2007, are used to estimate the amplitude and phase of the linear semidiurnal and diurnal internal waves in this energetic region, particularly in the previously poorly sampled area near the eastern ridge and on the Pacific side of Luzon Strait. The mean and variability of the internal wave field in the upper 1000 m of the water column are described. The phase progression of internal waves as they propagate away from their generation sites is captured directly. The glider‐based observations are used to map the mode‐1 semidiurnal and diurnal internal wave fields, providing the baroclinic energy flux over a roughly 600 km × 800 km region based strictly on in situ observations.

On the mechanism of A-type and B-type internal solitary wave generation in the northern South China Sea

Internal solitary waves (ISWs) in the northern South China Sea (SCS) are investigated using historical observational data, linear theory, global inverse tidal model, and a fully nonlinear nonhydrostatic numerical model. It was found on the basis of mooring data that the appearance of ISWs in the northern SCS is strongly correlated with the maximum of semidiurnal barotropic tidal forcing in the Luzon Strait (LS), but not with the largest peaks of diurnal tidal currents. The role of the latter lies in the modulation of the baroclinic tidal signal in such a way that the diurnal intermittency of ISW characteristics known as A-type waves (large-amplitude rank-ordered ISW packets) and B-type waves (single weak ISWs) is introduced into the internal wave fields.Both A- and B-waves were reproduced in a series of numerical experiments. It was found that due to the neap-spring variability of the tidal forcing, a permanent transition of A-type waves into B-type waves and vice versa takes place. This effect is treated in terms of a multi-harmonic approach. It is assumed that the internal wave field in the SCS is a sum of progressive semidiurnal and diurnal internal tidal waves radiated from the LS. Their superposition in space creates an alternation of large and small troughs. In the course of nonlinear steepening and ultimate disintegration the large and small troughs give rise to type A and type B internal wave packets, respectively. The neap-spring variability of tidal forcing alters the specific contribution of diurnal and semidiurnal constituents and leads to the wave transition from one type to another.

Energy Transfer from High-Shear, Low-Frequency Internal Waves to High-Frequency Waves near Kaena Ridge, Hawaii

Abstract Evidence is presented for the transfer of energy from low-frequency inertial–diurnal internal waves to high-frequency waves in the band between 6 cpd and the buoyancy frequency. This transfer links the most energetic waves in the spectrum, those receiving energy directly from the winds, barotropic tides, and parametric subharmonic instability, with those most directly involved in the breaking process. Transfer estimates are based on month-long records of ocean velocity and temperature obtained continuously over 80–800 m from the research platform (R/P) Floating Instrument Platform (FLIP) in the Hawaii Ocean Mixing Experiment (HOME) Nearfield (2002) and Farfield (2001) experiments, in Hawaiian waters. Triple correlations between low-frequency vertical shears and high-frequency Reynolds stresses, 〈uiw∂Ui/∂z〉, are used to estimate energy transfers. These are supported by bispectral analysis, which show significant energy transfers to pairs of waves with nearly identical frequency. Wavenumber bispectra indicate that the vertical scales of the high-frequency waves are unequal, with one wave of comparable scale to that of the low-frequency parent and the other of much longer scale. The scales of the high-frequency waves contrast with the classical pictures of induced diffusion and elastic scattering interactions and violates the scale-separation assumption of eikonal models of interaction. The possibility that the observed waves are Doppler shifted from intrinsic frequencies near f or N is explored. Peak transfer rates in the Nearfield, an energetic tidal conversion site, are on the order of 2 × 10−7 W kg−1 and are of similar magnitude to estimates of turbulent dissipation that were made near the ridge during HOME. Transfer rates in the Farfield are found to be about half the Nearfield values.

A study of internal solitary waves observed on the continental shelf in the northwestern South China Sea

Based on in-situ time series data from the acoustic Doppler current profiler (ADCP) and thermistor chain in Wenchang area, a sequence of internal solitary wave (ISW) packets was observed in September 2005, propagating northwest on the continental shelf of the northwestern South China Sea (SCS). Corresponding to different stratification of the water column and tidal condition, both elevation and depression ISWs were observed at the same mooring location with amplitude of 35 m and 25 m respectively in different days. Regular arrival of the remarkable ISW packets at approximately the diurnal tidal period and the dominance of diurnal internal waves in the study area, strongly suggest that the main energy source of the waves is the diurnal tide. Notice that the wave packets were all riding on the troughs and shoulders of the internal tides, they were probably generated locally from the shelf break by the evolution of the internal tides due to nonlinear and dispersive effects.

A thin layer of phytoplankton observed in the Philippine Sea with a synthetic moored array of autonomous gliders

A synthetic moored array composed of five buoyancy‐propelled autonomous underwater gliders was used to characterize mesoscale variability and phytoplankton distribution in a 100 km × 100 km domain in the Philippine Sea east of Luzon Strait for 10 days in May 2004. The study area, located east of the Kuroshio near the subtropical front, is dominated by strong internal tides, by energetic westward‐propagating mesoscale eddies with azimuthal velocities exceeding 50 cm/s, and by a deep (130 m) maximum in chlorophyll fluorescence. Each glider in the array was instructed to maintain geographic position while repeatedly profiling to 200‐m depth. Good station‐keeping performance enabled the resulting series of vertical profiles to be interpreted in the same manner as a physically moored chain of instruments. Although organized primarily as a demonstration of glider capabilities, this field exercise provides a unique data set for examining biological‐physical interactions in the open ocean. Here we report on the evolution of a thin layer of phytoplankton observed near the deep chlorophyll maximum. Coincident observations of fine structure in temperature and salinity suggest that the thinning process of this layer was driven primarily by physical forcing, most probably vertical shear associated with energetic diurnal internal waves, as opposed to a biological mechanism, such as convergent swimming, grazing, or spatial variation in growth rate.

Observation of sub‐inertial internal tides in McMurdo Sound, Antarctica

McMurdo Sound, Antarctica, at 78°S is poleward of the critical latitudes for both diurnal and semi‐diurnal internal tides, which are, respectively, 30°S and 75°S. Yet fieldwork during April 2003 and November 2004 observed energy at both frequencies. High temporal resolution density profiles in April 2003 and mooring observations at 240 m depth in November 2004 show a diurnal internal tide with displacements of up to 12 m. The horizontal decay length scales for diurnal evanescent internal tides indicate that they were locally generated. The November 2004 mooring observations also show a semi‐diurnal tide with displacements up to 12 m, although some of the energy in these waves could be baroclinic inertial motion, which has a similar frequency at this latitude. The semi‐diurnal internal waves have a larger decay length scale than the diurnal internal waves, so are not necessarily locally generated.

Numerical simulation of the barotropic tides in the Tunisian Shelf and the Strait of Sicily

We have investigated the barotropic tides in the Tunisian shelf and the Strait of Sicily using the Regional Ocean Modelling System (ROMS) with very high-resolution. Model performance was evaluated with respect to tide gauge, satellite data, and current meter measurements. The model fields faithfully reproduced the major feature of the barotropic tidal currents and agreed well with existing tidal elevation and phase observations. General features for the various semidiurnal constituents are nearly similar to each other with maximum amplitude in the Gulf of Gabes. The larger tidal currents occur over the continental shelves. In the Adventure Bank, the current is essentially of diurnal type whereas in the Gulf of Gabes it is of semidiurnal type. Tidal energy lost, which is primarily due to bottom stress dissipation, is predominantly in the Gulf of Gabes (∼ 61%), the Strait of Sicily, and the Strait of Messina. The forcing function for internal tides shows for both M 2 and K 1 constituents, significant spatial variability in the Strait of Sicily. This suggests that some internal tides will be generated in these regions and could thus explain the observed strong diurnal internal waves in the Adventure Bank.

Baroclinic Energy Flux at the Hawaiian Ridge: Observations from the R/P FLIP

Abstract Estimates of baroclinic energy flux are made in the immediate “Nearfield” (September–October 2002) and 450 km offshore (“Farfield”; October–November 2001) of the Kaena Ridge, an active barotropic-to-baroclinic conversion site. The flux estimates are based on repeated profiles of velocity and density obtained from the Research Platform Floating Instrument Platform (FLIP) as an aspect of the Hawaii Ocean Mixing Experiment. Energetic beams associated with both semidiurnal and diurnal internal waves are observed in the Kauai Channel. Beam depths and orientations are consistent with generation along the upper flanks of the ridge. At the far-field site, the baroclinic energy flux is borne primarily by first-mode semidiurnal waves. The energy flux associated with the entire spectrum of internal waves is computed by cross-spectral analysis. Significant energy fluxes are found in the inertial, diurnal, semidiurnal, and twice-semidiurnal frequency bands. The semidiurnal energy flux strongly dominates the spectrum at both sites. The flux magnitude follows the spring–neap cycle of the semidiurnal barotropic tide. The averaged depth-integrated mode-1 semidiurnal energy flux (over the entire water column) in the Farfield is found to be 1.7 ± 0.3 kW m−1 away from the ridge, with peak values up to 4 kW m−1. Small fluxes toward the ridge are occasionally seen at neap tide. At both sites, energy fluxes in the diurnal frequency band represent 15%–20% of the semidiurnal energy flux. In the Farfield, the magnitude of the diurnal energy flux varies in accord with the fortnightly cycle of the barotropic semidiurnal tide, rather than with the diurnal forcing, suggesting that energy for those waves is supplied by a cross-frequency transfer from the low-vertical-mode M2 internal tide to higher-mode internal waves at frequencies ½M2. In the Nearfield, the diurnal flux varies with fluctuations in both diurnal and semidiurnal forcing.

Mode-1 internal tides in the western North Atlantic Ocean

Mode-1 internal tides were observed the western North Atlantic using an ocean acoustic tomography array deployed in 1991–1992 centered on 25°N, 66°W. The pentagonal array, 700-km across, acted as an antenna for mode-1 internal-tides. Coherent internal-tide waves with O(1 m) displacements were observed traveling in several directions. Although the internal tides of the region were relatively quiescent, they were essentially phase locked over the 200–300 day data record lengths. Both semidiurnal and diurnal internal waves were detected, with wavenumbers consistent with those calculated from hydrographic data. The M 2 internal-tide energy flux was estimated to be about 70 W m −1, suggesting that mode-1 waves radiate 0.2 GW of energy, with large uncertainty, from the Caribbean island chain at this frequency. A global tidal model (TPXO 5) suggested that 1–2 GW is lost from the M 2 barotropic tide over this region, but the precise value was uncertain because the complicated topography makes the calculation problematic. In any case, significant conversion of barotropic to baroclinic tidal energy does not occur in the western North Atlantic basin. It is apparent, however, that mode-1 internal tides have very weak decay and retain their coherence over great distances, so that ocean basins may be filled up with such waves. Observed diurnal amplitudes were an order of magnitude larger than expected. The amplitude and phase variations of the K 1 and O 1 constituents observed over the tomography array were consistent with the theoretical solutions for standing internal waves near their turning latitude. The energy densities of the resonant diurnal internal waves were roughly twice those of the barotropic tide at those frequencies.

Bispectral Analysis of Energy Transfer within the Two-Dimensional Oceanic Internal Wave Field

Abstract Bispectral analysis of the numerically reproduced spectral responses of the two-dimensional oceanic internal wave field to the incidence of the low-mode semidiurnal internal tide is performed. At latitudes just equatorward of 30°, the low-mode semidiurnal internal tide dominantly interacts with two high-vertical-wavenumber diurnal (near inertial) internal waves, forming resonant triads of parametric subharmonic instability (PSI) type. As the high-vertical-wavenumber near-inertial energy level is raised by this interaction, the energy cascade to small horizontal and vertical scales is enhanced. Bispectral analysis thus indicates that energy in the low-mode semidiurnal internal tide is not directly transferred to small scales but via the development of high-vertical-wavenumber near-inertial current shear. In contrast, no noticeable energy cascade to high vertical wavenumbers is recognized in the bispectra poleward of ∼30° as well as equatorward of ∼25°. A new finding is that, although PSI is possible equatorward of ∼30°, the efficiency drops sharply as the latitude falls below ∼25°. At all latitudes, another resonant interaction suggestive of induced diffusion is found to occur between the low-mode semidiurnal internal tide and two high-frequency internal waves, although bispectral analysis shows that this interaction plays only a minor role in cascading the low-mode semidiurnal internal tide energy.

Internal Tide and Nonlinear Internal Wave Behavior at the Continental Slope in the Northern South China Sea

A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were measured with 21 moorings in water of 350- to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.

Resonant diurnal internal tides in the North Atlantic

Using an acoustic tomography array located midway between Puerto Rico and Bermuda, enhanced diurnal tidal signals associated with the lowest internal-wave mode have been observed. These signals result from a diurnal internal wave with O(500-km) wavelength that is resonantly trapped between the shelf just north of Puerto Rico and the turning latitude, a distance of about 1100 km. The data obtained using the large acoustical array are consistent with the predicted Airy-function variation with latitude of diurnal internal waves near the turning latitude. The existence of this wave is a striking demonstration of the long spatial and temporal coherence of oceanic internal tides.

Resonant diurnal internal tides in the North Atlantic

Using a large acoustical array located midway between Puerto Rico and Bermuda, we have observed enhanced diurnal tidal signals associated with the lowest internal‐wave mode. These signals result from a diurnal internal wave resonantly trapped between the shelf just north of Puerto Rico and the turning latitude, a distance of about 1100 km. The data obtained using the large acoustical array are consistent with the predicted Airy‐function variation with latitude of diurnal internal waves near the turning latitude. The existence of this wave is a striking demonstration of the long spatial and temporal coherence of oceanic internal tides. In general, the energy radiated by such waves is a loss of energy from the earth‐moon system and a source of energy for mixing in the deep ocean.