Antipodal acoustic thermometry: 1960, 2004

On 21 March 1960, sounds from three 300-lb depth charges deployed at 5.5-min intervals off Perth, Australia were recorded by the SOFAR station at Bermuda. The recorded travel time of these signals, about 13,375 s, is a historical measure of the ocean temperature averaged across several ocean basins. The 1960 travel time measurement has about 3-s precision. High-resolution global ocean state estimates for 2004 from the “Estimating the Circulation and Climate of the Ocean, Phase II” (ECCO2) project were combined with ray tracing to determine the paths followed by the acoustic signals. The acoustic paths are refracted geodesics that are slightly deflected by either small-scale topographic features in the Southern Ocean or the coast of Brazil. The refractive influences of intense, small-scale oceanographic features, such as Agulhas Rings or eddies in the Antarctic Circumpolar Current, greatly reduce the necessary topographic deflection and cause the acoustic paths to meander in time. The ECCO2 ocean state estimates, which are constrained by model dynamics and available data, were used to compute present-day travel times. Measured and computed arrival coda were in good agreement. Based on recent estimates of warming of the upper ocean, the travel-time change over the past half-century was nominally expected to be about −9 s, but little difference between measured (1960) and computed (2004) travel times was found. Taking into account uncertainties in the 1960 measurements, the 2004 ocean state estimates, and other approximations, the ocean temperature averaged along the sound channel axis over the antipodal paths has warmed at a rate less than about 4.6 m °C yr⁻¹ (95% confidence). At this time, the estimated uncertainties are comparable in size to the expected warming signal, however.     

The coupled oceanographic–tomographic analysis and prediction system: A practical means for utilizing high frequency tomography in the world's oceans

Details are presented of a methodology that utilizes acoustic travel time information in an ocean circulation model. Recent developments of this model-oriented tomography are discussed, representing some significant improvements over earlier formulations. More accurate means of determining the arrival times of specific ray paths are detailed, along with a means of estimating possible errors in the calculated travel times. The assimilation of the observed arrival time information into an ocean model is achieved using a Kalman gain, and more advanced expressions for calculating the Kalman gain are presented. A formulation to account for errors in the stated positions of a source and receiver is also presented. It is shown that the methodology performs fairly well in reproducing observed travel time anomalies. However, the model-predicted anomalies along a specific ray path may not always track the observed anomalies for that path when assimilating multiple ray path data. Results indicate that additional work is required to determine a means of handling observed arrival time data without having prior knowledge of the magnitude of errors in the observations. Results from simulation experiments provide estimates of: (1) potential errors when the travel times for ray paths are only sampled at discreet intervals as opposed to continuously and (2) to what degree acoustic data can be expected to “correct” model-predicted fields.     

Acoustic Ray-Trace Equations for Seafloor Geodesy

One goal of seafloor geodesy is to measure horizontal deformation of the seafloor with millimeter resolution. A common technique precisely times an acoustic signal propagating between two points to estimate distance and then repeats the measurement over time. The accuracy of the distance estimate depends upon the travel time resolution, sound speed uncertainty, and the degree to which the path computed from propagation equations replicates the actual path traveled by the signal. In this paper, we address the error from ray propagation equations by comparing three approximations to Snell's Law with ellipsoidal geometry.     

Simulating ocean acoustic tomography measurements with Hamiltonian ray tracing

Tomographers map mesoscale ocean structure by inverting acoustic travel-time measurements through networks of underwater paths. To know where to deploy sensors and how to interpret their measurements, one must first understand the "forward problem," that is, how the sound channel and mesoscale features refract sound in three dimensions, and how such refraction alters the pulse-arrival sequence. We use a Hamiltonian ray-tracing program called HARPO to compute the refraction by continuous three-dimensional ocean models and to display the results in ways that add insight about refractive effects. We first simulate propagation in a simple range-independent sound channel, showing how pulse-arrival sequence depends on channel parameters and sensor placement. Next, we add linear range dependence and show that it is hard to extract range information from pulse measurements at one range. Finally, we add a simple model of a mesoscale eddy including its currents and show that deflection and splitting of the sound channel significantly alter the pulse-arrival sequence. Two diagrams that have not been widely used before are useful ways to display the arrival-time and ray-focusing perturbations caused by changes in ocean structure: they are plots of range versus launch angle and range versus travel time. Examples of azimuthal deflection, three-dimensional eigenrays, and reciprocal propagation through eddy currents are shown, and simplified methods for estimating the travel time of three-dimensional eigenrays are evaluated.     

Long-period acoustic and seismic measurements and ocean floorcurrents

Pressure fluctuations caused by a strong ocean floor current are evident during most of an eighty-day-long record of very-low-frequency acoustic ambient noise measured by an instrument on the seafloor in the western Atlantic in the framework of the HEBBLE (High Energy Benthic Boundary Layer Experiment). The differential pressure gauges on the instrument produce useful measurements over a wide frequency band extending from 0.0005 to 16 Hz. The spectrum of current-induced pressure fluctuations is red with a power-law dependence on frequency with an exponent of -1.5. Turbulence in the ocean floor boundary layer is the source of these pressure fluctuations rather than the effects of flow around the transducers. This record of boundary-layer pressure fluctuations is used to predict the effect of seafloor currents on long-period seismograph measurements from the seafloor and from under the seafloor in boreholes     

Studies of the sea using HF radio scatter

Radio signals of decameter wavelength resonantly scattered from waves on the sea surface are used to measure precisely the wavelength, frequency, and direction of travel of those waves. These measurements are not only important in themselves, but are also used to deduce currents, winds, and perhaps wind stress at the sea surface. Techniques for obtaining these measurements, as well as experiments to evaluate these techniques are discussed. Finally, scatter has been used to produce the first high-resolution measurements of the directional distribution of large ocean waves, measurements of ocean surface currents at ranges of 20 km, and of surface winds at ranges of 3000 km.     

海洋声层析的基本原理和应用

从射线声学和简正波声学的角度，概述了海洋声层析的基本理论，包括射线走时反演、简正波走时反演、简正波相位反演和简正波水平折射层析。海洋声层析以反演海水温度和流速为基础。还总结了声层析在海洋学研究中的应用。     

Monitoring the Kuroshio Extension with Dynamically Constrained Synthesis of the Acoustic Tomography, Satellite Altimeter and in situ Data

A finite-difference quasigeostrophic (QG) model of an open ocean region has been employed to produce a dynamically constrained synthesis of acoustic tomography and satellite altimetry data with in situ observations. The assimilation algorithm is based upon the 4D variational data interpolation scheme controlled by the model's initial and boundary conditions. The data sets analyzed include direct and differential travel times measured at the array of five acoustic transceivers deployed by JAMSTEC in the region of the Kuroshio Extension in 1997, Topex/Poseidon altimetry, CTD soundings, and ADCP velocity profiles. The region monitored is located within the area 27.5°–36.5°N, 143°–155°. The results of assimilation show that mesoscale variability can be effectively reconstructed by five transceivers measuring direct and reciprocal travel times supported by relatively sparse in situ measurements. The misfits between model and data lie within the observational error bars for all the data types used in assimilation. We have compared the results of assimilation with the statistical inversion of travel time data and analyzed energy balances of the optimized model solution. Energy exchange between the depth-averaged and shear components of the observed currents reveals a weak decay of the barotropic mode at the rate of 0.2 ± 0.7⋅10⁻⁵ cm²/s³ due to topographic interaction. Mean currents in the region are unstable with an estimate of the available potential energy flux from the mean current to the eddies of 4.7 ± 2.3⋅10⁻⁵ cm²/s³. Kinetic energy transition has the same sign and is estimated as 2.8 ± 2.5⋅10⁻⁵ cm²/s³. Potential enstrophy is transferred to the mesoscale at a rate of 5.5 ± 2.7⋅10⁻¹⁸ s⁻³. These figures provide observational evidence of the properties of free geostrophic turbulence which were predicted by theory and observed in numerical experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Multimegameter-range acoustic data obtained by bottom-mountedhydrophone arrays for measurement of ocean temperature

Acoustic signals transmitted from the ATOC source on Pioneer Seamount off the coast of California have been received at various sites around the Pacific Basin since January 1996. We describe data obtained using bottom-mounted receivers, including US Navy Sound Surveillance System arrays, at ranges up to 5 Mm from the Pioneer Seamount source. Stable identifiable ray arrivals are observed in several cases, but some receiving arrays are not well suited to detecting the direct ray arrivals. At 5-Mm range, travel-time variations at tidal frequencies (about 50 ms peak to peak) agree well with predicted values, providing verification of the acoustic measurements as well as the tidal model. On the longest and northernmost acoustic paths, the time series of resolved ray travel times show an annual cycle peak-to-peak variation of about 1 s and other fluctuations caused by natural oceanic variability. An annual cycle is not evident in travel times from shorter acoustic paths in the eastern Pacific, though only one realization of the annual cycle is available. The low-pass-filtered travel times are estimated to an accuracy of about 10 ms. This travel-time uncertainty corresponds to errors in range- and depth-averaged temperature of only a few millidegrees, while the annual peak-to-peak variation in temperature averaged horizontally over the acoustic path and vertically over the upper 1 km of ocean is up to 0.5°C     

Ocean acoustic tomography: estimating the acoustic travel time withphase

Continuous acoustic transmission (133 Hz, 60-ms resolution) between a bottom-mounted source near Oahu, Hawaii, and a bottom-mounted receiver at 4000-km range near the coast of northern California was recorded to learn how to measure precisely the travel time so that basin-scale fluctuations in the Pacific can be detected. Daily incoherent averages of some of the multipaths exhibited stability during this period. The standard deviation of the travel time of the resolved peaks in the daily incoherent averages is about 30 ms. An acoustic method, based on cross-correlation, is derived to estimate the change in the average acoustic phase (travel time) to a precision of about 0.018 cycles (135 μs) every 2 min. Travel-time estimates based on the cross-correlator reduce the aberrations due to internal waves by about 19 dB in comparison with CW transmissions. The new travel-time estimator is applied to the measurements to examine some of the fluctuations of the Pacific     

海洋声层析观测技术和方法

叙述海洋声层析观测系统,以声线传播时间层析为重点概括了海洋声层析的基本原理和其他主要方法,共6个方法。对运用海洋声层析观测来反演海洋状态问题的建立、求解及其误差来源作了分析和讨论。以测量声线传播时间为例介绍了海洋声层析观测系统主要设计技术。     

A comparison of multifrequency HF radar and ADCP measurements ofnear-surface currents during COPE-3

A high-frequency multifrequency coastal radar operating at four frequencies between 4.8 and 21.8 MHz was used as part of the third Chesapeake Bay Outflow Plume Experiment (COPE-3) during October and November, 1997. The radar system surveyed the open ocean east of the coast and just south of the mouth of Chesapeake Bay from two sites separated by about 20 km. Measurements were taken once an hour, and the eastward and northward components of ocean currents were estimated at four depths ranging from about 0.5 m to 2.5 m below the surface for each location on a 2 by 2 km grid. Direction of arrival of the signals was estimated using the MUSIC algorithm. The radar measurements were compared to currents measured by several moored acoustic Doppler current profilers (ADCPs) with range bins 2-14 m below the water surface. The vertical structure of the current was examined by utilizing four different radar wavelengths, which respond to ocean currents at different depths, and by using several ADCP range bins separated by 1-m intervals. The radar and ADCP current estimates were highly correlated and showed similar depth behavior, and there was significant correlation between radar current estimates at different wavelengths and wind speed     

Spatial diversity processing for underwater acoustic telemetry

A large increase in the reliability of shipboard or stationary underwater acoustic telemetry systems is achievable by using spatially distributed receivers with aperture sizes from 0.35 to 20 m. Output from each receiver is assigned a quality measure based on the estimated error rate, and the data, weighted by the quality measure, are combined and decoded. The quality measure is derived from a Viterbi error-correction decoder operating on each receiver and is shown to perform reliability in a variety of non-Gaussian noise and jamming environments and reduce to the traditional optimal diversity system in a Gaussian environment. The dynamics of the quality estimator allow operation in the presence of high-power impulsive interference by exploiting the signal and noise differential travel times to individual sensors. The spatial coherence structure of the shallow water acoustic channel shows relatively low signal coherence at separations as short as 0.35 m. Increasing receiver spacing beyond 5 m offers additional benefits in the presence of impulsive noise and larger-scale inhomogeneities in the acoustic field. A number of data transmission experiments were carried out to demonstrate system performance in realistic underwater environments     

Acoustic tomography at basin scales and clock errors

A basin-scale acoustic tomography simulation is carried out for the northeast Pacific Ocean to determine the accuracy with which time must be kept at the sources when clocks at the receivers are accurate. A sequential Kalman filter is used to estimate sound-speed fluctuations and clock errors. Sound-speed fluctuations in the simulated ocean are estimated from an eddy-resolving hydrodynamic model of the Pacific forced by realistic wind fields at daily resolution from 1981-1993. The model output resembles features associated with El Nino and the Southern Oscillation, as well as many other features of the ocean's circulation. Using a Rossby-wave resolving acoustic array of four fixed sources and twenty drifting receivers, the authors find that the percentage of the modeled ocean's sound-speed variance accounted for with tomography is 92% at 400-km resolution, regardless of the accuracy of the clocks. Clocks which drift up to hundreds of seconds of error or more for a year do not degrade tomographic images of the model ocean. Tomographic reconstructions of the sound-speed field are insensitive to clock error primarily because of the wide variety of distances between the receivers from each source. Every receiver “sees” the same clock error from each source, regardless of section length, but the sound-speed fluctuations in the modeled ocean cannot yield travel times which lead to systematic changes in travel time that are independent of section length. The Kalman filter is thus able to map the sound-speed field accurately in the presence of large errors at the source's clocks     

Coastal acoustic tomography system and its field application

The coastal acoustic tomography system (CATS), composed of five moored acoustic stations, has been constructed to measure current fields. The system is developed with special considerations in mind, including the use of Global Positioning System clock signals in the synchronization of the system clock timing among the multiple acoustic stations, and the use of the differently coded Gold sequences to identify the acoustic signals corresponding to individual stations from a received signal. The CATS was successfully applied to map the structure of strongly nonlinear tidal currents in the coastal sea. In spite of the limited spatial resolution caused by inadequate sound transmission data, the two-dimensional tidal vortices features of growth, translation, and decay processes are reconstructed through an inverse analysis of the acoustic travel time obtained among the station pairs. It is evident that the CATS is a powerful tool for measuring variable current fields generated in the coastal seas     

A high-resolution pulse-Doppler underwater acoustic navigation system

A high-resolution underwater acoustic pulse-Doppler navigation system has been developed and tested at sea. The system provides continuous, highly accurate tracking of underwater and ocean-surface platforms in a fixed 50-km²navigation net. Three reference buoys, moored 20 m from the ocean bottom, provide the navigation net used by shipboard processing equipment. Each reference buoy contains an acoustic transponder, used to obtain the acoustic travel times from the transponder to the platform, and a continuous-tone beacon, used to obtain the Doppler shift due to platform motion. The system is capable of determining the position of a platform with respect to the reference net with an error of 2-3 m. The relative position of the platform on a fix-to-fix basis can be determined within several centimeters over short time intervals (approx 10min).     

Transient sea-ice polynya forced by oceanic flow variability

In the Weddell Sea during the winters of 1974–1976 a significant opening in the sea-ice cover occurred in the vicinity of a large bathymetric feature — the Maud Rise seamount. The event is commonly referred to as the Weddell Polynya. Aside from such a large-scale, relatively persistent polynya in the Weddell Sea, transient, small-scale polynya can also appear in the sea-ice cover at various times throughout the winter and at various locations with respect to the Maud Rise. The underlying causes for the occurrence of such transient polynya have not been unambiguously identified. We hypothesize that variations in the mean ocean currents are one major contributor to such variability in the sea-ice cover. Analysis of the sea-ice equations with certain idealized patterns of ocean currents serving as forcing is shown to lead to Ekman transports of sea ice favorable to the initiation of transient polynya. Aside from the actual spatial pattern of the idealized ocean currents, many other factors need also be taken into account when looking at such transient polynya. Two other such factors discussed are variations in the sea-ice thickness field and the treatment of the sea-ice rheology. Simulations of a sea-ice model coupled to a dynamical ocean model show that the interaction of (dynamical) oceanic currents with large-scale topographic features, such as the Maud Rise, does lead to the formation of transient polynya, again through Ekman transport effects. This occurs because the seamount has a dynamic impact on the three-dimensional oceanic flow field all the way up through the water column, and hence on the near surface ocean currents that are in physical contact with the sea ice. Further simulations of a sea-ice model coupled to a dynamic ocean model and forced with atmospheric buoyancy fluxes show that transient polynya can be enhanced when atmospheric cooling provides a positive feedback mechanism allowing preferential open-ocean convection to occur. The convection, which takes hold at sites where transient polynya have been initiated by sea-ice–ocean stress interaction, has an enhancing effect arising from the convective access to warmer, deeper waters. To investigate all of these effects in a hierarchical manner we use a primitive equation coupled sea-ice–ocean numerical model configured in a periodic channel domain with specified atmospheric conditions. We show that oceanic flow variability can account for temporal variability in small-scale, transient polynya and thus point to a plausible mechanism for the initiation of large-scale, sustained polynya such as the Weddell Polynya event of the mid 1970s.     

Absolute geostrophic currents in the Drake Passage based on observations in 2003 and 2005

Geostrophic currents in the Drake Passage are studied using the data of two hydrographic sections across the passage occupied in December 2003 and November 2005 along with satellite altimetry data. A conclusion is reached that the altimetry correction of the geostrophic currents has advantages compared to the correction made on the basis of the lowered acoustic Doppler current profiler data. A number of new results about the structure and intensity of the ocean currents in the Drake Passage are obtained; the main one is the distinguishing of several abyssal currents of westward direction confined to deep passages in the bottom topography.     

Very high-frequency radar mapping of surface currents

An ocean surface current radar (OSCR) in the very high frequency (VHF) mode was deployed in South Florida Ocean Measurement Center (SFOMC) during the summer of 1999. During this period, a 29-d continuous time series of vector surface currents was acquired starting on 9 July 1999 and ending 7 August 1999. Over a 20-min sample interval, the VHF radar mapped coastal ocean currents over a 7.5 km × 8 km domain with a horizontal resolution of 250 m at 700 grid points. A total of 2078 snapshots of the two-dimensional current vectors were acquired during this time series and of these samples, only 69 samples (3.3%) were missing from the time series. During this period, complex surface circulation patterns were observed that included coherent, submesoscale vortices with diameters of 2 to 3 km inshore of the Florida Current. Comparisons to subsurface measurements from moored and ship-board acoustic Doppler current profiles revealed regression slopes of close to unity with biases ranging from 4 to 8 cm s^-1 between surface and subsurface measurements at 3 to 4 m beneath the surface. Correlation coefficients were 0.8 or above with phases of - 10 to - 20° suggestive of an anticyclonic veering of current with depth relative to the surface current. The radar-derived surface current field provided spatial context for an observational network using mooring-, ship- and autonomous underwater vehicle-sensor packages that were deployed at the SFOMC     

Rapid swing and spin of [deep] taut-wire-moored instruments

Rapid ‘swing’, compass variations O(10°) in O(10 s), and ‘spin’, complete rotations around the vertical axis within a few minutes, are a concern of acoustic current meters moored in-line. Observations are used from fast sampling, at once per 1 and 30 s, instrumentation on deep-ocean moorings mainly outside surface wave and bottom boundary influences. Such instruments do not require a vane common to some historic mechanical current meters and they are often moored in a much easier to handle sub-surface buoy or mounting rack, without vanes. In their mountings they are nearly symmetric, so that they can spin freely in (turbulent; shear) flows. A comparison is made between noise levels of such free spinning instrumentation with those of instruments mounted in a fixed bottom-frame and with those of instruments equipped with a vane to one side. Typical spinning has a single rotation varying between 40 and 200 s. Spinning is shown to be highly binary: on or off. Its effects are found negligible on estimates of ocean currents, provided compass updates are adequate as in existing instrumentation. Acoustic noise is O(10) times larger than noise due to spinning. Some effects of spinning are noticed in the acoustic echo amplitude showing higher noise at frequencies >100 cpd, cycles per day. The character of this noise changes dramatically due to spinning. However, it is mainly in the ocean turbulence range and does not affect measurements of internal waves or periodic zooplankton motions.