Inverted Echo Sounders Research Articles

Using the Inverted Echo Sounder to Measure Vertical Profiles of Gulf Stream Temperature and Geostrophic Velocity

Abstract The methodology for converting the travel time measurement of the inverted echo sounder (IES) into an amplitude of the first baroclinic dynamical mode, A1, is presented. For a Gulf Stream IES record the so-generated A1(t) time series is used to compute a vertical profile of first mode temperature versus time by perturbing a basic state temperature profile. The basic state is constructed by averaging together historical CTD data collected near the IES site. Similarly the first mode amplitudes are used to perturb a basic state dynamic height profile, and, using neighboring IESs, a profile of alongstream geostrophic velocity is obtained at the same location. The resulting IES-derived temperatures and velocities compare favorably to independent current meter results, exhibiting most of the variability observed in both the current meter temperature and alongstream velocity.

Inverted echo sounder measurement of dynamic height through an ENSO cycle in the central equatorial Pacific

A four-year record from an inverted echo sounder deployed near Palmyra Island at 6 degrees N in the central Pacific Ocean is compared with a simultaneous record of subsurface pressure from this island lagoon. A factor m, converting round-trip acoustic travel time to surface dynamic height relative to a deep pressure level, was estimated from the ratio of the spectra of the two records in the energetic synoptic oscillation band. Year-to-year variation in m was not statistically significant. For the overall record, m was found to be -70+or-8 dynamic m/s, where the error bounds represent a 90% confidence interval. This is consistent with first-baroclinic-mode excitation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Waveguide propagation of ambient sound in the ocean-surface bubble layer

Measurements of the ambient sound generated by breaking waves over the range 40–20 000 Hz reveal well-defined spectral peaks, the frequency of which may remain generally consistent from one breaking event to the next, but which can change significantly over the course of a storm, or from one storm to another. A theory is proposed, based on the concept of trapping of a portion of the sound in the waveguide formed by the ocean-surface bubble layer. Simultaneous measurements of the bubble population and size distribution as a function of depth and time were obtained with a multifrequency inverted echo sounder, allowing calculation of the resulting (dispersive) sound-speed anomaly profile. Theoretical predictions of the spectral peaks, which are associated with modal cutoff frequencies, are in good agreement with the observations. It is suggested that this result might have application to the remote determination of ocean-surface bubble fields relevant to the study of wave breaking, turbulence, and the air–sea gas flux.

Sea surface topographic variability near the New England Seamounts: An intercomparison among in situ observations, numerical simulations, and Geosat altimetry from the regional energetics experiment

Intercomparisons are made among three sets of results for two regions of the Gulf Stream, upstream and downstream of the New England Seamount Chain (NESC): observational records from 12 inverted echo sounders with pressure gauges (IES/PGs), concurrent Geosat altimeter crossover point differences, and results from a primitive equation model of the region. Standard deviations of sea surface topography show generally good agreement for the three data sources. No significant difference between the upstream and downstream regions is observed in the actual data. IES/PG records and Geosat topographic variability have values of about 30 cm on both sides of the NESC, although model results show a discrepancy in the eastern region. Barotropic fluctuations account for about 30% of the total surface topographic variability and are only partially correlated with baroclinic fluctuations.

Transports, frontal motions and eddies at the Brazil-Malvinas currents confluence

Three inverted echo sounders were deployed to study the dynamics of the western South Atlantic Brazil-Malvinas Confluence region. From the original travel time data obtained with the sounders, the dynamic height of the surface relative to 800 m at the three locations was obtained and geostrophic velocities between sites calculated. The time-averaged geostrophic velocity of the Brazil Current (relative to 800 m) during the observed period is 35 cm s −1 (±2.2 cm s −1) and the mean transport relative to 800 m is 11 Sv. For the Malvinas Current return flow, the mean geostrophic velocity is 9 cm s −1 (±2.0 cm s −1) and the mean associated transport 3.5 Sv. Cold intrusions are observed in offshore records during the 17-month recorded period, some of them associated with cold cyclonic eddies with available potential energy of 6.3 × 10 15 J. Using a simple model the location of the thermal front is obtained from the time series of the depth of the thermocline. Results show that the main motion of the front is an east-west displacement for distances of about 100 km; the observed period occurs with a periodicity of 12 months and is related to a variability in the latitude of maximum northward penetration of the Malvinas Current. Motions of the front, with a period of 1–2 months, are probably related to a north-south variation of the latitude of return of the Brazil Current.

Ambient sound beneath breaking surface waves

Ambient sound measurement obtained beneath breaking surface waves over the frequency range 44–20 000 Hz are described. The observations are obtained from an instrument located 24–32 m beneath the surface during experiments both in open ocean and continental shelf environments. Individual breaking events are positively identified using underwater video photography, and it is confirmed that they radiate sound throughout the observed frequency range. A distinctive feature of the acoustic signature is the presence of spectral peaks, the frequency of which may remain generally consistent from one breaking event to the next, but which can change significantly over the course of a storm or from one storm to another. A theory is proposed based on the concept of trapping of a portion of the sound in the surface waveguide formed by the ocean surface bubble layer. Simultaneous measurements of the bubble population and size distribution obtained with a multifrequency inverted echo sounder allows calculation of the re...

Low‐frequency meandering of the Atlantic North Equatorial Countercurrent

Four 19‐month time series of indirect measurements of dynamic height were obtained in the tropical Atlantic along 28°W at 0°, 3°, 6°, and 9°N with three inverted echo sounders and one current meter mooring. The series were analyzed to study the time‐latitude variability of the North Equatorial Countercurrent (NECC). The eastward flow associated with the NECC was present at 28°W from 3° to 9°N during most of the observed period except in March‐April 1983 and April‐May 1984, periods that coincided with the onset of the wind at the equator. The amplitude of the NECC's annual cycle was maximum at 6°N and was larger in 1983 than in 1984. The analysis of the time‐latitude variability of differences in dynamic height shows a long‐period meridional shift of the NECC. The core of the current attains its northernmost location during August‐September in both years and its southernmost location during March‐April in 1983 and March in 1984. The location of the core is directly related to the position of the intertropical convergence zone. From the time series of dynamic height obtained from the indirect measurements, geostrophic velocities and transports were estimated and compared with direct observations of currents and values obtained from hydrographic casts.

Report on SEQUAL/FOCAL

In Eos (April 6, 1982), United States plans for a program to study the dynamic response of the equatorial Atlantic to seasonally varying surface winds were described. Now, 6 years later, we report on progress toward our goal “to describe accurately, and to model correctly” the changes in the currents and density field of the upper equatorial Atlantic Ocean during a 2‐year period. A major effort toward this goal was the field phase of SEQUAL (Seasonal Response of the Equatorial Atlantic) and the closely coordinated French program FOCAL (Français Océan et Climat dans l'Atlantique Equatorial).Between February 1983 and September 1984 changes in the surface winds and in oceanic conditions in the equatorial Atlantic were monitored continuously with a variety of instruments. Figure 1 shows key deployments and sections. The resulting data include six current meter moorings, 15 inverted echo sounders and island tide gauges, 57 near‐surface drifters, and 18 French and five U.S. cruises that made 1200 hydrographic stations and 800 current profiles. All these data, as well as nearly 10,000 expendable bathythermographs (XBTs) (from the cruises, from air‐dropped expendable bathythermograph (AXBT) programs, and from repeated ship of opportunity lines run between 1980 and 1985) were combined and documented by George Heimerdinger, of the National Oceanographic Data Center, and are available from NODC on request.

One-dimensional baroclinically unstable waves on the Gulf Stream potential vorticity gradient near Cape Hatteras

Application of linear baroclinic instability theory to the observed distributions of velocity, stratification, and potential vorticity in the Gulf Stream near 74° W is successful in predicting the time and length scales of the most rapidly growing disturbances. A continuously-stratified, one-dimensional model with realistic bottom slope predicts propagation speeds of 10–50 cm s −1 associated with two regimes of rapid temporal growth centered at periods of 28 days and 5–7 days. This prediction is consistent with observations of the propagation and growth of Gulf Stream meanders derived from inverted echo sounder measurements in this region. The instability model also predicts that for realistic bottom slopes the baroclinic energy transfer should be weakly negative (eddy-to-mean) in deep water, but for low-frequency waves should change to significant positive (mean-to-eddy) transfer above depths of ∼1500 m, consistent with observations.

Comparison of dynamic height measurements from an inverted echo sounder and an island tide gauge in the central Pacific

An inverted echo sounder (IES) and deep pressure sensor were deployed within 70 km of a shallow pressure sensor at Palmyra Island (6°N, 162°W) in the central Pacific. These instruments provided yearlong records of acoustic travel time, deep pressure and sea level. Two independent time series of dynamic height are derived from travel time and sea surface elevation, respectively. The spectra of these time series are similar, and at the spectral peaks the coherence between them exceeds 99.9% confidence levels, indicating that travel time can be used to record dynamic height fluctuations. This investigation provides a frequency dependent calibration for the IES in this region. At the energetic low frequencies (periods ∼ 1 month), this calibration agrees with a calibration by the standard method using conductivity‐temperature‐depth (CTD) casts. At higher frequencies (periods of ∼3 days), using the CTD‐derived calibration may underestimate the amplitude of some processes by as much as 30%.

The equatorial source of propagating variability along the Peru coast During the 1982–1983 El Niño

Using data obtained from tide gages in South America, current meters along the equator and the Peru coast, and an array of pressure gages and inverted echo sounders within and around the Galapagos archipelago, we have analyzed the equatorial origin of coastal trapped waves observed by Cornejo‐Rodriquez and Enfield (this issue) along the Peru coast during the intense 1982–1983 El Niño. The propagating fluctuations along the coast were much stronger at that time than either before or after the El Niño, and the variability was not locally forced by coastal winds. We find that the coastal variability was also more energetic during previous El Niño occurrences. At periods of 1–2 weeks the meridional component of currents on the equator is up to an order of magnitude more energetic than the zonal fluctuations and is consistently associated with sea level that fluctuates antisymmetrically between hemispheres. At periods longer than 2 weeks the zonal velocity component is more energetic, and the cross‐equatorial sea level variability is symmetric. The meridional and zonal phase structures of cross spectra involving the currents and sea level establish the 1‐ to 2‐week equatorial fluctuations as mixed Rossby‐gravity (Yanai) waves of low wave number with infinite phase speed (standing oscillations) in the middle of the band (10 days); the corresponding structures for longer periods are consistent with nondispersive Kelvin waves. Frequency domain EOF modes of the sea level and current data establish the mixed Rossby‐gravity waves as the principal source of the strong trapped wave variability in the 1‐ to 2‐week band along the Ecuador‐Peru coast during the 1982–1983 El Niño episode.

Satellite Thermal Infrared and Inverted Echo Sounder Determinations of the Gulf Stream Northern Edge

Abstract The northern edge of the Gulf Stream off Cape Hatteras, North Carolina was located in 155 AVHRR-derived maps of sea surface temperature (SST) using five different methods. One method was subjective location of the northern edge by an analyst; the other four involved objective location of the edge by computer using various statistics of the SST field. Specifically, the quantities considered were: maximum SST gradient (calculated over a 3 × 3 pixel box), maximum SST (on a pixel-by-pixel basis), maximum variance (calculated over a 7 × 7 pixel box), and change in the skewness of the SST distribution (calculated over a 5 × 5 pixel box). The resulting locations were compared with the location of the 15°C isotherm at 200 m (T15) determined from inverted echo sounders (IESs) moored on the sea floor. The best method, which yielded the smallest rms difference from the IES-derived T15, was the subjective one; the surface front was located 9.0 km shoreward of T15 with a rms difference of 14.3 km. The best ob...

Regional Characteristics for Interpreting Inverted Echo Sounder (IES) Observations

Changes in round-trip acoustc travel time (τ) measured between a bottom moored inverted echo sounder and the sea surface can be interpreted as changes in dynamic height (D) with suitable calibration information. The τ, D, and isotherm and isopycnal depths (Z) have been calculated using hydrographic (CTD) data from three regions: the Norwegian Current, the Sargasso Sea and the eastern Gulf Stream. Regressions of D and Z on τ were performed. The slope for the Norwegian Current is − 1.70 ± 0.01 [dyn cm (m s)−1], for the Sargasso Sea − 3.89 ± 0.16 [dyn cm (m s)−1] and for the Gulf Stream − 3.13 ± 0.07 [dyn cm (m s)−1]. The quasi-random scatter about regression curves is found to be primarily the result of variability in the seasonal thermocline, where present.

Forced oscillations on the equatorial Atlantic Basin during the seasonal response of the Equatorial Atlantic Program (1983–1984)

A subset of data from an array of eight inverted echo sounders and a meteorological station deployed and maintained for 15 months in the equatorial Atlantic as part of the Seasonal Response of the Equatorial Atlantic program are analyzed to determine the variability of forced oscillations of the basin. The analysis is done to study both low‐ and high‐frequency variability. At low frequencies the equatorial basin responds as a whole to the seasonally varying winds. The dominant signal is the annual cycle of the equatorial system as a composite of the response to the strong seasonal wind curl that is maximum in the western basin and the part of the zonal transfer to mass associated with the seasonal equatorial upwelling and downwelling system. Three bands of energy appear as dominant in the combined spectrum of the ocean and the atmosphere at higher frequencies: 20–30 days, 13–16 days, and the inertial gravity wave band. Each one of the bands is analyzed and discussed. Instability waves are detected on the echo sounder records. The waves have periods centered at 30.2 days and propagate westward with a wavelength of 1000 km and a phase speed of 0.35 m/s. The most energetic oscillation in the analyzed wind records is at periods centered around 14 days. The response of the ocean to this oscillation has a meridional distribution with maximum amplitude at 3°N. The most significant oceanic oscillations in the inertial gravity band are centered at 5.2 and 3.5 days. From the study of their meridional structure it is concluded that those oscillations correspond to forced inertial gravity waves with meridional structures corresponding to the meridional numbers n = 1 and n = 3, respectively.

Hindcasts of equatorial sea surface dynamic height in the Atlantic in 1982–1984

Two different wind data sets of the years 1982–1984 are used to force a linear multimode model of the tropical Atlantic ocean. We describe the variations of the equtorial slope in both cases and investigate the differences between the two runs. The main discrepancy occurs in 1983 for the amplitude of the seasonal signal all along the equator. Model results are also compared with observations (hydrocasts and inverted echo sounders) taken during the Programme Français Océan et Climat dans l'Atlantique Equatorial/Seasonal Response of the Equatorial Atlantic (FOCAL/SEQUAL) field experiment (July 1982 through October 1984). The contrast between 1983 and 1984 is well captured by the two runs, especially the abnormal flat topography of the sea surface along the equator in the first 3 months of 1984 due to an unusual basin wide relaxation of the equatorial wind stress. The major discrepancy between model results and observations is found during the up welling periods east of 10°W, but at that time dynamic height measurements are too sparse to give a good description of these events.

Inverted echo sounder observations of variability in the eastern equatorial Pacific during the 1982–1983 El Niño

Meridional and zonal baroclinic variability during the 1982–1983 El Niño was investigated in a ∼ 1000-km region around the Galapagos Islands with an array of three inverted echo sounders. The resulting dynamic height time series show two principal maxima separated by several months, in good agreement with the available CTD observations of this El Niño. Autospectra, dispersion characteristics and symmetry properties reveal that the first baroclinic mode wave field contains five regimes: (1) at long time scales (T ∼ 114 days), westward-propagating Rossby waves; (2) in the 38- to 16-day band, a barotropic instability mode with westward phase and group velocity; (3) near 12 days, eastward-propagating Kelvin waves; (4) from 10 to 4 days, clearly defined eastward-propagating Rossby-gravity waves; and (5) in the 6- to 2-day band, inertia-gravity waves of locally resonant second and possibly higher meridional modes. Strong 3.3-day peaks at two sites are not locally resonant inertia-gravity waves and instead are likely to be basin-wide third meridional mode inertia-gravity waves generated in the western or central Pacific.

Time‐space variability of the local dynamics of the Malvinas‐Brazil confluence as revealed by inverted echo sounders

Two inverted echo sounders have been deployed for approximately 8 months in the region of the confluence of western boundary currents in the South Atlantic (37°58.7′S, 51°56.4′W, and 37°29.3′S, 53°49.3′S). The confluence of the subtropical Brazil Current with the sub‐Antarctic Malvinas Current creates a strong, sharp thermohaline front. From the time series obtained with the sounders, two parameters are obtained and analyzed: dynamic height from the surface relative to 800 m and the position of the front. The time series of dynamic height indicates the presence of a cold intrusion during November, the southward extension of the Brazil Current and northward extension of the Malvinas Current during January, and a simultaneous warming of the area at both moored locations (0.04 dyn m/month) from March through June. From the dynamic height time series, mean geostrophic velocities are calculated. The mean value from mid‐December through mid‐June is −14 cm/s, representative of an average southward transport of 10 × 106 m3/s. The maximum southward transport, 23 × 106 m3/s, is during January. The front oscillates around its mean position with a period of about 1 month and an amplitude that varies from 10 to 50 km. The velocity of the displacement reaches values up to 10 km/d. Atmospherically forced oscillations are detected in the records. A 21‐day oscillation in the time series, obtained with the instrument located on the slope of the continental shelf, is associated with a coastally trapped wind forced response.

Seasonal response of the sea surface to the wind in the equatorial Atlantic

From February 1983 through September 1984, 18 inverted echo sounder deployments yielded time series from 10 locations as part of the Seasonal Response of the Equatorial Atlantic (SEQUAL) program. Using historical and in situ data, the sounders' travel time data are interpreted as dynamic height (0 relative to 500 dbar) and then related to representations of the synoptic wind field. The seasonal variations of the zonal pressure gradient along the equator and the height of the dynamic ridge along 3°N relative to the equator and 9°N are described. The pressure gradient between 10°W and 34°W has a minimum each April and a maximum each July. Its value ranges between 1.4 (not significantly different from zero) and 8.3 × 10−5 dyn g−1 during the observational period. The zonal pressure gradient is correlated with zonal wind stress from an in situ wind sensor at midfetch, and a 10‐day lag time is indicated. The North Equatorial Countercurrent trough attains a maximum in the late boreal summer and fall and a minimum in the spring. It ranges from +39 to −8 dyn cm deep at 38.5°W and from 18 to −1 dyn cm at 28°W. A two‐layer model calculation, using monthly mean winds derived by Servain et al. (1986) for the appropriate time period, generally reproduces the observed seasonal variation along the two meridians and the large zonal variation between them. Tracking the individual terms, month by month, illustrates the rate of the Sverdrup adjustment to the local wind stress with surface elevation rising as fast as 10 cm per month.

Pressure variations on the Atlantic equator

Records of more than a year's duration from three deep ocean pressure sensors and three island shallow pressure sensors, all close to the equator between 30°W and 7°E, are analyzed spectrally at frequencies below the diurnal tides. The records were taken as integral parts of the FOCAL‐SEQUAL experiments in the equatorial Atlantic, 1983–1984. The low‐frequency gravitational tides at 1, 2, 3 cycles per month formed an important, nearly simultaneous part of the signal and had to be removed. The seasonal variations associated with equatorial upwelling are prominent in the shallow but not the deep pressure signals. Practically all the non tidal deep pressure signals at 0.1–0.7 cycles per day (cpd) are highly coherent between 20°W and 6°E, some 2900 km, and show no sign of zonal propagation. A comparison of the coherent part of the signals between the equator and Ascension Island (8°S) suggests a southward component of propagation, at least in the region of the Mid‐Atlantic Ridge. These motions are evidently barotropic and are not trapped near the equator. A close relationship to the atmospherically forced barotropic waves at 4–6 days period in the Pacific, identified by Luther (1980), is suggested. Special consideration is given to a comparison of a 3.6‐km‐deep pressure record at 6°E with a shallow pressure record from Ilha São Tomé' (supplied by J. M. Verstraete). Their ordinary tidal components are practically identical, as befits purely barotropic motion, but the sub‐tidal continuum shows fairly low (though significant) coherence, with higher energy in the island record. This is attributed to the presence of internal wave modes of several meters internal amplitude. Theoretically, the contribution of internal waves should give a much stronger signal in the surface pressure than in the deep pressure signal. The relationships are demonstrated by calculations based on acoustic return times by two inverted echo sounder recorders placed close to deep pressure sites by E. J. Katz. Deduced coherences agree reasonably with observed values and are consistent with internal waves dominated by mode 1, uncorrelated with the barotropic motion, sensed in both deep and shallow pressure gauges.

Simulation of side‐scan sonar

Acoustic remote sensing is becoming widespread in oceanographic measurement programs. Acoustic systems for such measurements include seafloor subbottom profilers, inverted echo sounders, tomography systems, and side‐scan sonars. Numerical simulation of such systems can provide valuable information for system design and for analysis of data from an operating system. A long‐range side‐scan system currently under development will operate in deep water over a frequency range from 12–150 kHz and will provide side‐scan images, bathymetry, and seafloor reflectivity data. Simulations of the system have been performed. These simulations provide insight into the influence of various system parameters and dispalys on usefulness of the system as a tool for the seafloor geologist. This paper describes the models used in the simulations and illustrates their use in performance prediction for this scientific data acquisition system.