Internal Wave Activity Research Articles

Horizontal coherence of low-frequency fixed-path sound in a continental shelf region with internal-wave activity

Sound at 85 to 450 Hz propagating in approximately 80-m depth water from fixed sources to a joint horizontal/vertical line array (HLA/VLA) is analyzed. The data are from a continental shelf area east of Delaware Bay (USA) populated with tidally generated long- and short-wavelength internal waves. Sound paths are 19 km in the along-shore (along internal-wave crest) direction and 30 km in the cross-shore direction. Spatial statistics of HLA arrivals are computed as functions of beam steering angle and time. These include array gain, horizontally lagged spatial correlation function, and coherent beam power. These quantities vary widely in magnitude, and vary over a broad range of time scales. For example, correlation scale can change rapidly from forty to five wavelengths, and correlation-scale behavior is anisotropic. In addition, the vertical array can be used to predict correlation expected for adiabatic propagation with cylindrical symmetry, forming a benchmark. Observed variations are in concert with internal-wave activity. Temporal variations of three coherence measures, horizontal correlation length, array gain, and ratio of actual correlation length to predicted adiabatic-mode correlation length, are very strong, varying by almost a factor of ten as internal waves pass.

Nesting a nonhydrostatic model in a hydrostatic model: The boundary interface

A method is developed for adjusting the values of the prognostic variables near the interface between a nonhydrostatic, high resolution model embedded in a hydrostatic, coarser resolution model. It incorporates a method of conditioning the outer domain lateral boundary values to enforce conservation of volume when the variables are interpolated onto the inner domain grid. This is accomplished by adjusting the baroclinic normal velocities at the open boundaries after interpolation. The method also includes a relaxation scheme which matches the values of the prognostic variables across a narrow zone near the open boundaries of the submesoscale inner domain. This prevents the development of discontinuities, reflections and perimeter currents at the periphery of the inner domain. Submesoscale hindcasts are conducted in areas of high Nonlinear Internal Wave (NLIW) activity. Since NLIWs have amplitudes, O(100 m), which are not small relative to their wavelengths, O(100 – 1000 m) these hindcasts require a nonhydrostatic model in the inner domain. Since the hydrostatic model lacks the physics and resolution to support NLIWs there will be discrepancies between the values of the hydrostatic and nonhydrostatic prognostic variables at the boundaries. It is shown that the application of these method for adjusting the values of the prognostic variables near the interface allows the computation of a nonhydrostatic, submesoscale hindcast forced by a nested hydrostatic forecasting system.

Determinants of pelagic metabolism in the Timor Sea during the inter-monsoon period

The Timor Sea is a major conduit of the Indonesian Throughflow characterised by large internal waves and tides. To ascertain whether these result in high pelagic productivity, we conducted experiments to determine the metabolic balance between net community production (NCP) and community respiration (CR) on the Sahul Shelf, the Sahul Shoals and the Yampi Shelf, an area of active hydrocarbon seeps. The barrier to vertical mixing of subthermocline nutrients represented by the halocline allowed new production to dominate in March 2004, whereas production in June 2005 depended on recycled nutrients. CR was correlated with dissolved organic carbon (DOC) in 2004, but with chlorophyll in 2005, suggesting that respiration was dominated by microheterotrophs in 2004 but by autotrophs in 2005. Overall, area-specific CR averaged 120 ± 92 (s.d.), 101 ± 52 and 61 ± 6 mmol O2 m–2 day–1, NCP averaged 109 ± 85 (s.d.), 32 ± 41 and 57 ± 10 mmol O2 m–2 day–1, and average gross primary production (= CR+NCP) : R ratios were 1.9, 1.4 and 1.9 on the shelf, at the Sahul Shoals and the Yampi Shelf, respectively. We suggest that differences in water column structure and internal wave activity drive intermittent high production events in a predominantly oligotrophic sea.

Low-frequency internal waves and their influence on transmission loss variability

Analysis of the time series data collected from a stationary location in the continental shelf of the southeastern Arabian Sea during different month indicated prominent internal wave (IW) activity. Time evolution of temperature, resolved using Morlet wavelet technique, revealed that maximum energy was concentrated in the diurnal band at the density interface, whereas within the interior of thermocline, the dominant energy concentration shifted to semi-diurnal band. Both these harmonics have maximum amplitude (>15 m) during the pre-monsoon and monsoon season when the water column was highly stratified (>0.05 kg/m4), but they were not discernable in the temperature record when the stratification was weak (i.e., especially during winter). An acoustic propagation model based on ray theory, Bellhop (http:/oalib.hlsresearch.com/modes/acoustictoolbox/at.zip) was utilized to compute the transmission loss (TL) associated with the passage of low-frequency IWs. The TL was computed using the model considering (1) range-dependent and range-independent environmental scenario and (2) for different source and receiver depth configurations. Intermittent fading of acoustic signals was observed in the presence of IW. It was also observed that fading of signals very much depends on the source–receiver configuration.

Investigation of Mode Filtering as a Preprocessing Method for Shallow-Water Acoustic Communications

Acoustical array data from the 2006 Shallow Water Experiment (SW06) was analyzed to show the feasibility of broadband mode decomposition as a preprocessing method to reduce the effective channel delay spread and concentrate received signal energy in a small number of independent channels. The data were collected by a vertical array, which spans the water column from 12-m depth to the bottom in shallow water 80 m in depth. Binary-sequence data were used to phase-shift-keyed (PSK) modulate signals with different carrier frequencies. No error correction coding was used. The received signals were processed by a system that does not use training or pilot signals. Signals received both during periods of ordinary internal wave activity and during a period with unusually strong internal wave solitons were processed and analyzed. Different broadband mode-filtering methods were analyzed and tested. Broadband mode filtering decomposed the received signal into a number of independent signals with a reduced delay spread. The analysis of signals from the output of mode filters shows that even a simple demodulator can achieve a low bit error rate (BER) at a distance 19.2 km.

Ocean environmental effects on low‐frequency sound propagation in shallow water with application to marine mammals.

In shallow ocean areas (less than 100 m), the water‐column and seafloor structure play a critical role in sound propagation and can influence many different applications of shallow water acoustics including source detection and localization. While the seafloor and subbottom effects on sound propagation are relately well known, water‐column variability, especially that of internal wave activity, thermohaline intrusions, and fronts, are only beginning to be understood, but should be considered when predicting sound propagation in shallow water. For example, termination of the mid water‐column duct by a shelfbreak front or the appearance of an internal wave duct can change acoustic channels which may hinder marine mammal communication or may be exploited by the animals. In this paper, we will examine the effects of the ocean environment on sound propagation in shallow water in the presence of these features in a continental shelf region. Specifically, we will use field data and numerical acoustic simulations to study low‐frequency sound propagation in a dynamic ocean and how it relates to marine mammal vocalization, communication, and localization. [Work supported by ONR.]

Seismic imaging of a thermohaline staircase in the western tropical North Atlantic

Abstract. Multichannel seismic data acquired in the Lesser Antilles in the western tropical North Atlantic indicate that the seismic reflection method has imaged an oceanic thermohaline staircase. Synthetic acoustic modeling using measured density and sound speed profiles corroborates inferences from the seismic data. In a small portion of the seismic image, laterally coherent, uniform layers are present at depths ranging from 550–700 m and have a separation of ~20 m, with thicknesses increasing with depth. The reflection coefficient, a measure of the acoustic impedance contrasts across these reflective interfaces, is one order of magnitude greater than background noise. Hydrography sampled in previous surveys suggests that the layers are a permanent feature of the region. Spectral analysis of layer horizons in the thermohaline staircase indicates that internal wave activity is anomalously low, suggesting weak internal wave-induced turbulence. Results from two independent measurements, the application of a finescale parameterization to observed high-resolution velocity profiles and direct measurements of turbulent dissipation rate, confirm these low levels of turbulence. The lack of internal wave-induced turbulence may allow for the maintenance of the staircase or may be due to suppression by the double-diffusive convection within the staircase. Our observations show the potential for seismic oceanography to contribute to an improved understanding of occurrence rates and the geographical distribution of thermohaline staircases, and should thereby improve estimates of vertical mixing rates ascribable to salt fingering in the global ocean.

Internal Waves and Mixing in the Marginal Ice Zone near the Yermak Plateau*

Abstract Observations were made of oceanic currents, hydrography, and microstructure in the southern Yermak Plateau in summer 2007. The location is in the marginal ice zone at the Arctic Front northwest of Svalbard, where the West Spitsbergen Current (WSC) carries the warm Atlantic Water into the Arctic Ocean. Time series of approximately 1-day duration from five stations (upper 520 m) and of an 8-day duration from a mooring are analyzed to describe the characteristics of internal waves and turbulent mixing. The spectral composition of the internal-wave field over the southern Yermak Plateau is 0.1–0.3 times the midlatitude levels and compares with the most energetic levels in the central Arctic. Dissipation rate and eddy diffusivity below the pycnocline increase from the noise level on the cold side of the front by one order of magnitude on the warm side, where 100-m-thick layers with average diffusivities of 5 × 10−5 m2 s−1 lead to heat loss from the Atlantic Water of 2–4 W m−2. Dissipation in the upper 150 m is well above the noise level at all stations, but strong stratification at the cold side of the front prohibits mixing across the pycnocline. Close to the shelf, at the core of the Svalbard branch of the WSC, diffusivity increases by another factor of 3–6. Here, near-bottom mixing removes 15 W m−2 of heat from the Atlantic layer. Internal-wave activity and mixing show variability related to topography and hydrography; thus, the path of the WSC will affect the cooling and freshening of the Atlantic inflow. When generalized to the Arctic Ocean, diapycnal mixing away from abyssal plains can be significant for the heat budget. Around the Yermak Plateau, it is of sufficient magnitude to influence heat anomaly pulses entering the Arctic Ocean; however, diapycnal mixing alone is unlikely to be significant for regional cooling of the WSC.

Observations of semidiurnal internal tidal currents off central Chile (36.6°S)

Observations of semidiurnal internal tidal currents from three moorings deployed on the continental shelf off central Chile during summer and winter of 2005 are reported. The spectra of the baroclinic currents showed large peaks at the semidiurnal band with a dominant counterclockwise rotation, which was consistent with internal wave activity. The amplitude of the barotropic tidal currents varied according to the spring–neap cycle following the sea level fluctuations. In contrast, the amplitudes of the internal tide showed high spatial-temporal variability not directly related to the spring–neap modulation. Near the middle of the continental shelf and near the coast (San Vicente Bay) the variance of the semidiurnal baroclinic current is larger than the variance of its barotropic counterpart. The vertical structure of the baroclinic tidal current fluctuations was similar to the structure of the first baroclinic internal wave mode. In general, in the three study sites the variance of the baroclinic current was larger near the surface and bottom and tended to show a minimum value at mid depths. Kinetic energy related to semidiurnal internal waves was larger in winter when stratification of the water column was stronger. During summer, upwelling and the decrease of freshwater input from nearby rivers reduced the vertical density stratification. The amplitude of the semidiurnal internal tide showed a tendency to be enhanced with increasing stratification as observed in other upwelling areas. The continental shelf break and submarine canyons, which limit the continental shelf in the alongshore direction, represent near-critical slopes for the semidiurnal period and are suggested to be the main internal tide generation sites in the study region.

Rapid dispersal of a hydrothermal plume by turbulent mixing

The water column imprint of the hydrothermal plume observed at the Nibelungen field (8°18 ′S 13°30 ′W) is highly variable in space and time. The off-axis location of the site, along the southern boundary of a non-transform ridge offset at the joint between two segments of the southern Mid-Atlantic Ridge, is characterized by complex, rugged topography, and thus favorable for the generation of internal tides, subsequent internal wave breaking, and associated vertical mixing in the water column. We have used towed transects and vertical profiles of stratification, turbidity, and direct current measurements to investigate the strength of turbulent mixing in the vicinity of the vent site and the adjacent rift valley, and its temporal and spatial variability in relation to the plume dispersal. Turbulent diffusivities K ρ were calculated from temperature inversions via Thorpe scales. Heightened mixing (compared to open ocean values) was observed in the whole rift valley within an order of K ρ around 10 −3 m 2 s −1. The mixing close to the vent site was even more elevated, with an average of K ρ = 4 × 10 − 2 m 2 s − 1 . The mixing, as well as the flow field, exhibited a strong tidal cycle, with strong currents and mixing at the non-buoyant plume level during ebb flow. Periods of strong mixing were associated with increased internal wave activity and frequent occurrence of turbulent overturns. Additional effects of mixing on plume dispersal include bifurcation of the particle plume, likely as a result of the interplay between the modulated mixing strength and current speed, as well as high frequency internal waves in the effluent plume layer, possibly triggered by the buoyant plume via nonlinear interaction with the elevated background turbulence or penetrative convection.

Meteorological and internal wave forcing of seiches along the Sri Lanka coast

Regularly observed seiches from tide gauge records around the Sri Lanka coast have been analyzed to determine the forcing mechanisms. The seiche periods range from a few 10s of min to 2 h. Fortnightly and seasonal variations of seiche amplitudes are clearly visible on the east coast. These seiche amplitudes are particularly large approximately 6–8 days after spring tides, with maximum seiche amplitudes observed during March–April and October–November, suggesting that the seiche variation could be related to stratification and internal wave activity. There is no visible fortnightly and seasonal variation of seiche amplitudes in west coast records. Instead, they show a daily pattern with relatively larger seiches around 1000 LT, suggesting that the seiches on the west coast could result from diurnal atmospheric forcing. Barotropic and two‐layer models have been developed to investigate the influence of atmospheric forcing and internal wave activity on the seiches. A barotropic model applied to the west coast shows that the daily seiche amplitude variation could be simulated with cyclic diurnal meteorological forcing. Two‐layer model runs for Trincomalee Bay and the adjacent east coast shelf suggest that seiche amplitudes are proportional to vertical stratification and mixed layer depths. Therefore, the observed seiches there could be excited by internal waves, which originate as far away as the Andaman Sea during spring tides and have a travel time of 6–8 days to the Sri Lankan east coast. However, further studies, including direct measurements of internal wave activity within the region, are required to confirm this hypothesis.

Boundary roughness and the effect of internal waves on signal coherence for shallow water transmission.

Broadband acoustic propagation experiments at three shallow sites allow for comparison of coherency of individual surface‐reflected bottom‐reflected modes of propagation. There appears to be a dependence of the correlation parameters of times and length on frequency and mode number that cannot be attributed to internal waves alone and likely depends on bottom and surface roughness. At low frequencies, f<100 Hz, during periods of quiescence internal waves, all modes of propagation have equally long coherence parameters. The coherency decrease equally for all modes as internal wave activity increases. For higher frequencies, 200 Hz <f<1000 Hz, the coherence parameters depend on mode number, with the lower order modes always more coherent than successive higher order modes. At still higher frequencies, f>1000 Hz, identifiable modes are not always observed; instead there is a continuum of arriving pulse energy with very low coherency even with minimal internal waves. Apparently, the randomizing effect of internal waves depends on bottom and surface roughness and frequency. At low frequencies, the boundaries appear flat and internal waves have a minimal effect. At the highest frequencies, phase coherence is already degraded by boundary roughness so that the slightest of internal wave activity completely randomizes the signals.

Adjoint study of water column variability and propagation in the transverse acoustic variability experiment (TAVEX 2008).

Internal wave and other water column activity in the conductivity, temperature, and depth (CTD) data collected in the East China Sea as a part of TAVEX 2008 are used as a basis for analyzing acoustic propagation at 500 Hz between a fixed source and fixed horizontal line array placed on the seafloor. Because the CTD data were not taken coincident with the acoustic propagation path (in general), adjoint modeling methods will be used to study acoustic data and prediction misfit. Usefulness of the adjoint method for these experimental conditions will be discussed. [Work supported by ONR.]

Short-term hydrographic variability in a stratified Arctic fjord

Abstract Fjords in the Arctic often have a more complex circulation pattern than the classical two-dimensional estuarine circulation. This is due to the effects of the Earth's rotation on stratified waters in wide fjords. Observations from a semi-enclosed fjord basin, Van Mijenfjorden on Spitsbergen, show that the hydrography and circulation vary considerably on short timescales (hours) in the summer season. The depth and distribution of the low salinity upper water layer respond quickly to changes in the wind field. The Coriolis effect has an essential impact on the circulation, inducing eddy-like flow patterns and strong cross-fjord gradients. Within the upper layer, the lowest salinity values and highest temperatures were found on the northern side of the fjord in calm wind periods. When the wind was strong from the west, the cross-fjord gradients were reversed. Internal wave activity contributes to large vertical displacement of water below the upper layer. Knowledge of such strongly variable hydrographic conditions in fjords are important for sampling strategy and interpretation of data, for instance of primary production and sedimentation processes, and for the understanding of fjords as depositional systems.

Observations of internal wave generation in the seasonally ice‐free Arctic

The Arctic is generally considered a low energy ocean. Using mooring data from the northern Chukchi Sea, we confirm that this is mainly because of sea‐ice impeding input of wind energy into the ocean. When sea‐ice is present, even strong storms do not induce significant oceanic response. However, during ice‐free seasons, local storms drive strong inertial currents (&gt;20 cm/s) that propagate throughout the water column and significantly deepen the surface mixed layer. The large vertical shear associated with summer inertial motions suggests a dominant role for localized and seasonal vertical mixing in Arctic Ocean dynamics. Our results imply that recent extensive summer sea‐ice retreat will lead to significantly increased internal wave generation especially over the shelves and also possibly over deep waters. This internal wave activity will likely dramatically increase upper‐layer mixing in large areas of the previously quiescent Arctic, with important ramifications for ecosystems and ocean dynamics.

Numerical studies of flow over a sill: sensitivity of the non-hydrostatic effects to the grid size

A non-hydrostatic terrain-following model in cross sectional form is applied to study the processes in the lee of a sill in an idealized stratified fjord during super-critical tidal inflow. A sequence of numerical studies with horizontal grid sizes in the range from 100 to 1.5625 m are performed. All experiments are repeated using both hydrostatic and non-hydrostatic versions of the model, allowing a systematic study of possible non-hydrostatic pressure effects and also of the sensitivity of these effects to the horizontal grid size. The length scales and periods of the internal waves in the lee of the sill are gradually reduced and the amplitudes of these waves are increased as the grid size is reduced from 100 down to 12.5 m. With a further reduction in grid size, more short time and space scale motions become superimposed on the internal waves. Associated with the internal wave activity, there is a deeper separation point that is fairly robust to all parameters investigated. Another separation point nearer to the top of the sill appears in the numerical results from the high-resolution studies with the non-hydrostatic model. Associated with this shallower separation point, an overturning vortex appears in the same set of numerical solutions. This vortex grows in strength with reduced grid size in the non-hydrostatic experiments. The effects of the non-hydrostatic pressure on the velocity and temperature fields grow with reduced grid size. In the experiments with horizontal grid sizes equal to 100 or 50 m, the non-hydrostatic pressure effects are small. For smaller grid sizes, the time mean velocity and temperature fields are also clearly affected by the non-hydrostatic pressure adjustments.

Near-bed particle deposition and resuspension in a cold-water coral mound area at the Southwest Rockall Trough margin, NE Atlantic

Cold-water coral reefs and mounds are observed mainly on slopes and topographic highs, in areas with high current speeds. Previous investigations of the near-bed hydrodynamic regime around cold-water coral mounds at the Southwest Rockall Trough margin have revealed the presence of internal waves with a diurnal tidal frequency. Hitherto only short-term measurements existed on the particle supply to the corals and data are lacking on the seasonal variability. Bottom landers equipped with sensors recording near-bottom current dynamics were deployed at two sites in a mound area on the Southwest Rockall Trough margin, one with a dense coral cover and one without coral cover. At both sites a similar seasonal variation in internal-wave activity was recorded with high activity during winter and summer months and less dynamic conditions in spring and autumn. Increased intensity of internal-wave activity, reflected in higher average near-bottom current speed and amplitude of daily temperature fluctuations, results in higher mass fluxes as recorded in the sediment traps. On the site without coral cover, mass fluxes are two times higher, compared to the site with dense coral cover. During periods of high mass fluxes a predominance of resuspended material was observed at both sites, as indicated by reduced 210Pb activity and low organic matter concentrations. The flux of resuspended material largely masked the primary pelagic signal. However, low δ 15N values in early spring and summer marked the arrival of fresh particles on both sites. A dense coral framework, baffling a large amount of particles settling between the coral branches, results in differences in particle flux, chemical composition and freshness of the trapped material. On the long term the presence of a coral framework plays a crucial role in the build-up of cold-water coral mounds.

Mid‐depth internal wave energy off the Iberian Peninsula estimated from seismic reflection data

Energy levels of internal waves are estimated from seismic reflection data. Three legacy seismic sections from 1993 and 1997 obtained off the Iberian Peninsula have been analyzed for acoustic reflections within the water column. The reflections are aligned continuously for up to several kilometers over large parts of the sections and in the depth interval from 200 to 2000 m. Depth variations of these reflections are thought to be caused by the background internal wave field. From the variations we derive horizontal wave number spectra of normalized internal wave displacement. The general slope of the power density spectra is remarkably consistent for all sections and agrees well with model spectra for internal waves. Significant differences within the sections can be found when sufficiently large subsections are averaged. The spatial variation of the energy level indicates increasing internal wave activity with shallower water depths as well as near a subsurface eddy.

Acoustic hindcasts of array performance using a combination of submesoscale hydrodyamic and acoustic models.

It is well known that submesoscale ocean processes such as nonlinear internal gravity waves can have a significant effect on both the amplitude and the phase of an acoustic field propagating through this type of ocean environment. We report here on hindcasts computed with a numerical model combination consisting of a submesoscale hydrodynamic solver to compute a set of three‐dimensional (3‐D) environmental (sound speed) volumes evolving in time and a 3‐D wide‐angle parabolic equation code for acoustic field computation within each environmental volume. The data set was chosen from the ASIAEX 2001 experiment in the South China Sea, during a period of strong internal wave activity. A nonlinear wave packet was simulated propagating up the shelf and passing through both the acoustic source and receiver positions. The hindcasts computed the time evolving beam response on a horizontal array, located approximately 19 km from a 300 Hz low‐frequency modulat source. Comparison of experimental and modeled beamformed...

Temperature finestructure in the Indonesian Seas

Twenty‐one years of eXpendable BathyThermograph temperature profile data are used to examine the spatial and temporal variability in finestructure throughout the Indonesian Seas region. Finestructure is the variation of vertical properties on scales from several meter to tens of meter thickness, and is associated, for example, with internal wave activity, an evolving system that can be a precursor to turbulent vertical mixing. As might be expected, on average, the highest temperature finestructure values are observed within 5 km of the shelf‐slope boundary, especially within straits, and the lowest values at distances greater than 35 km. However, the long‐period time variability in the temperature finestructure reveals significant, unexpected, depth‐dependent monsoonal and interannual modulations. In the upper 100 m, there is a 63% enhancement in the temperature finestructure during the windy Southeast Monsoon months of June, July, and August (in particular during La Niña years), relative to the low‐wind intermonsoon months of September, October, and November (in particular during El Niño years). Below 100 m, the monsoon influence is much reduced, but surprisingly significant interannual temperature finestructure variability is observed down to 600‐m depth with a signal opposite to that in the upper layer of the ocean: a 6‐month lagged 13% enhancement associated with El Niño relative to La Niña time periods between 100‐ and 600‐m depths. These results provide a background framework in which to interpret modeling and observational vertical mixing studies in the Indonesian Seas region.