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Morris, M, Roemmich D, Cornuelle B.  1996.  Observations of variability in the South Pacific subtropical gyre. Journal of Physical Oceanography. 26:2359-2380.   10.1175/1520-0485(1996)026<2359:oovits>;2   AbstractWebsite

Variability of the subtropical gyre in the South Pacific Ocean was investigated using high-resolution expendable bathythermograph sections along a repeated track between New Zealand and Hawaii. The southern part of the section sampled most of the zonal flow in the subtropical gyre with the eastward flowing branch between New Zealand and Fiji and the westward branch extending north of Fiji to approximately 10 degrees S. The time series began in September 1987 and extended through 1994, averaging four cruises every year. The geostrophic shear field was calculated, relative to 800 m, with the aid of a mean T-S relationship. Variability was present at a broad range of spatial and temporal scales but annual fluctuations were particularly prominent. The authors conclude that 30 snapshots of temperature, measured over a period of seven years, are sufficient to resolve the annual cycle of the gyre scale circulation along the transect. The shape and intensity of the gyre varied seasonally throughout the water column (0-800 m). Geostrophic transport was most intense (15 Sv, where Sv=10(6)m(3)s(-1)) in November. At this time, the northern edges of eastward dow at the surface and in the thermocline were closest together and the ratio of thermocline to surface transport was highest. Most intense flow occurred approximately two to three months after the basinwide seasonal peak in Ekman pumping. Transport was weakest(ll Sv) in May and was associated with an increase in the poleward slant of the gyre center with depth and a decrease in the ratio of thermocline to surface transport. Seasonal wind forcing was considered as a possible mechanism for the observed annual intensification of the gyre-scale circulation. A simple linear model of thermocline response to local changes in wind stress curl explained a significant fraction of the observed annual variability. Conservation of potential vorticity q yielded an estimate for the absolute mean how (-1 cm s(-1) at 800 m), consistent with direct measurements in the region. Interannual variability, possibly related to the El Nino-Southern Oscillation cycle, was observed. The cold event of 1988/89 appeared to be associated with relatively weak gyre-scale transport. After 1991, gyre-scale transport was more intense and a prominent change in the small-scale circulation occurred, with a shift in the alongtrack wavenumber spectral energy to higher wavenumbers.

Roemmich, D, Cornuelle B.  1990.  Observing the Fluctuations of Gyre-Scale Ocean Circulation - a Study of the Subtropical South-Pacific. Journal of Physical Oceanography. 20:1919-1934.   10.1175/1520-0485(1990)020<1919:otfogs>;2   AbstractWebsite

Seasonal and interannual variability of the subtropical gyre in the South Pacific Ocean are investigated by means of a time series of expendable bathythermograph (XBT) sections between New Zealand (36-degrees-S, 175-degrees-E) and Fiji (18-degrees-S, 178-degrees-E). The experiment spans much of the subtropical gyre and is a protype for future basin-scale observations. Eddy-resolving transects along the precisely repeating ship track, spanning four years, are used to estimate the mean field and fluctuations of temperature and geostrophic velocity. The mean field dominates on very large spatial scales while the fluctuations dominate on small scales. Mean and fluctuations have equal energy at a horizontal wavelength of about 2000 km. The study region contains three recurring small-scale features. These are the East Auckland Current, flowing eastward along the New Zealand continental slope, a front at about 29-degrees-S which is likely an extension of the Tasman Front, and a weaker feature, the Tropical Convergence at about 22-degrees-S. At lower latitudes in the study region, the entire thermocline migrates vertically at annual period. This annual oscillation ends near the front at 29-degrees-S. Farther poleward, the only substantial subsurface annual variation is in the strength of the East Auckland Current. Interannual variability of circulation during 1986-90 consisted of rapid transitions between two rather steady states. In one state, which persisted through 1987-88 and from mid-1989 to the present (mid-1990), the eastward flowing limb of the gyre was relatively strong and narrow, with a reversal in velocity at the ocean surface south of Fiji.

Cornuelle, BD, Worcester PF, Hildebrand JA, Hodgkiss WS, Duda TF, Boyd J, Howe BM, Mercer JA, Spindel RC.  1993.  Ocean Acoustic Tomography at 1000-Km Range Using Wave-Fronts Measured with a Large-Aperture Vertical Array. Journal of Geophysical Research-Oceans. 98:16365-16377.   10.1029/93jc01246   AbstractWebsite

Broadband acoustic signals transmitted from a moored 250-Hz source to a 3-km-long vertical line array of hydrophones 1000 km distant in the north central Pacific Ocean were used to determine the amount of information available from tomographic techniques used in the vertical plane connecting a source-receiver pair. A range-independent, pure acoustic inverse to obtain the sound speed field using travel time data from the array is shown to be possible by iterating from climatological data without using any information from concurrent environmental measurements. Range-dependent inversions indicate resolution of components of oceanic variability with horizontal wavelengths shorter than 50 km, although the limited spatial resolution of concurrent direct measurements does not provide a strong cross-validation, since the typical cast spacing of 20-25 km gives a Nyquist wavelength of 40-50 km. The small travel time signals associated with high-wavenumber ocean variability place stringent but achievable requirements on travel time measurement precision. The forward problem for the high-wavenumber components of the model is found to be subject to relatively large linearization errors, however, unless the sound speed field at wavelengths greater than about 50 km is known from other measurements or from a two-dimensional tomographic array. The high-ocean-wavenumber resolution that is in principle available from tomographic measurements is therefore achievable only under restricted conditions.

Cornuelle, B, Munk W, Worcester P.  1989.  Ocean Acoustic Tomography from Moving Ships. Journal of Geophysical Research-Oceans. 94:6232-6250.   10.1029/JC094iC05p06232   AbstractWebsite

Mesoscale mapping of the ocean sound speed field in a 1000×1000 km area by means of ocean acoustic tomography is greatly enhanced by augmenting a few acoustic moorings with a movable ship-based receiver. Computer simulations based on realistic noise levels in the measured acoustic travel times give 5% (1%) residual variance in ΔC(x;y,z) for four (six) acoustic source moorings in an ocean perturbed in the gravest baroclinic mode. For comparison, objective mapping based on traditional vertical profiles requires 3 times the steaming distance to yield equivalent residual error. Detailed results depend on many parameters: the assumed mesoscale spectrum and vertical mode structure, the number of observed multipaths, the mooring configuration, the number of ship stations and the travel time signal level (due to mesoscale eddies) and noise level (due to internal waves and position-keeping errors). These parameters have critical values, below which there is distinct deterioration and beyond which there is little gain. We believe that the critical values can be attained in practice so the ultimate limit on mesoscale mapping is imposed by the internal wave-induced travel time error. This assumes that position keeping of the submerged acoustic sources and receiver by a combination of satellite navigation and high-frequency acoustics can be achieved with ±10-m accuracy. The present study assumes a stationary ocean; a second paper will deal with reciprocal transmissions yielding currents and hence the barotropic mode. This is required in a dynamic ocean model for estimating ΔC(x,y,z;t). All this is preparatory to a tomography experiment in the Greenland Sea in 1988–1989.

Baggeroer, AB, Birdsall TG, Clark C, Colosi JA, Cornuelle BD, Costa D, Dushaw BD, Dzieciuch M, Forbes AMG, Hill C, Howe BM, Marshall J, Menemenlis D, Mercer JA, Metzger K, Munk W, Spindel RC, Stammer D, Worcester PF, Wunsch C.  1998.  Ocean climate change; comparison of acoustic tomography, satellite altimetry, and modeling. Science. 281:1327-1332., Washington, DC, United States (USA): American Association for the Advancement of Science, Washington, DC   10.1126/science.281.5381.1327   AbstractWebsite

Comparisons of gyre-scale acoustic and direct thermal measurements of heat content in the Pacific Ocean, satellite altimeter measurements of sea surface height, and results from a general circulation model show that only about half of the seasonal and year-to-year changes in sea level are attributable to thermal expansion. Interpreting climate change signals from fluctuations in sea level is therefore complicated. The annual cycle of heat flux is 150 ± 25 watts per square meter (peak-to-peak, corresponding to a 0.2°C vertically averaged temperature cycle); an interannual change of similar magnitude is also detected. Meteorological estimates of surface heat flux, if accurate, require a large seasonal cycle in the advective heat flux.

Haidvogel, DB, Arango H, Budgell WP, Cornuelle BD, Curchitser E, Di Lorenzo E, Fennel K, Geyer WR, Hermann AJ, Lanerolle L, Levin J, McWilliams JC, Miller AJ, Moore AM, Powell TM, Shchepetkin AF, Sherwood CR, Signell RP, Warner JC, Wilkin J.  2008.  Ocean forecasting in terrain-following coordinates: Formulation and skill assessment of the Regional Ocean Modeling System. Journal of Computational Physics. 227:3595-3624.   10.1016/   AbstractWebsite

Systematic improvements in algorithmic design of regional ocean circulation models have led to significant enhancement in simulation ability across a wide range of space/time scales and marine system types. As an example, we briefly review the Regional Ocean Modeling System, a member of a general class of three-dimensional, free-surface, terrain-following numerical models. Noteworthy characteristics of the ROMS computational kernel include: consistent temporal averaging of the barotropic mode to guarantee both exact conservation and constancy preservation properties for tracers; redefined barotropic pressure-gradient terms to account for local variations in the density field; vertical interpolation performed using conservative parabolic splines; and higher-order, quasi-monotone advection algorithms. Examples of quantitative skill assessment are shown for a tidally driven estuary, an ice-covered high-latitude sea, a wind- and buoyancy-forced continental shelf, and a mid-latitude ocean basin. The combination of moderate-order spatial approximations, enhanced conservation properties, and quasi-monotone advection produces both more robust and accurate, and less diffusive, solutions than those produced in earlier terrain-following ocean models. Together with advanced methods of data assimilation and novel observing system technologies, these capabilities constitute the necessary ingredients for multi-purpose regional ocean prediction systems. (c) 2007 Elsevier Inc. All rights reserved.

Sutton, PJ, Worcester PF, Masters G, Cornuelle BD, Lynch JF.  1993.  Ocean Mixed Layers and Acoustic Pulse-Propagation in the Greenland Sea. Journal of the Acoustical Society of America. 94:1517-1526.   10.1121/1.408128   AbstractWebsite

A simple one-dimensional ocean mixed layer model is used to study the effect of the transition between summer and winter conditions in the Greenland Sea on range-independent acoustic propagation. Acoustic normal modes propagated through the evolving sound-speed profile simulate broadband acoustic receptions from the Greenland Sea Tomography Experiment. The resulting changes in arrival structure and travel time are compared with data recorded between two of the tomographic moorings. The starting state for the model is the average of measured summer temperature and salinity profiles. At each time step the surface layer is modified by the removal of heat (modeling heat loss to the atmosphere) and the removal of fresh water (modeling evaporation minus precipitation). When necessary, static stability is maintained by mixing the surface layer into deeper layers. The acoustic normal modes exhibit large changes in behavior as the profile changes. In both summer (seasonal thermocline) and winter (adiabatic sound-speed profile) individual modes show minimal frequency dispersion. Intermediate profiles with a shallow surface mixed layer give highly dispersive modes, delaying the final acoustic energy cutoff by several hundred milliseconds relative to the summer and winter cases. This is the largest travel time signal observed in the data. The largest peak in the late continuous acoustic energy is due to minimally dispersed modes and corresponds to ray arrivals with near horizontal receiver angles. The amplitude of the arrival is low when significant dispersion is present.

Pinkel, R, Munk W, Worcester P, Cornuelle BD, Rudnick D, Sherman J, Filloux JH, Dushaw BD, Howe BM, Sanford TB, Lee CM, Kunze E, Gregg MC, Miller JB, Merrifield MA, Luther DS, Firing E, Brainard R, Flament PJ, Chave AD, Moum JM, Caldwell DR, Levine MD, Boyd T, Egbert GD.  2000.  Ocean mixing studied near Hawaiian Ridge. Eos, Transactions American Geophysical Union. 81:545-553.   10.1029/EO081i046p00545-02   AbstractWebsite

The Hawaii Ocean Mixing Experiment (HOME) is a grassroots program to study turbulent mixing processes near the Hawaiian Ridge. The HOME is motivated by the desire to understand diffusive aspects of the advective-diffusive balance that mediates the general circulation of the oceans. HOME is focused on tidally driven mixing, given the ubiquity of the tide as a deep-sea energy source. As the sea surface cools at high latitude, surface waters sink. Subsidence rate is sufficient to fill the worlds ocean with cold bottom water in approximately 3,000 years. Diffusive processes that transfer heat into the abyssal ocean are required to maintain a steady-state thermal structure. An effective eddy diffusivity of order Kp=10−4 m2 s−1, 700 times the molecular diffusivity of heat, is necessary [Munk, 1966]. Such a diffusivity might be supported by either mechanical mixing (turbulent transport) or thermodynamic (so-called doubly diffusive) processes.

Dushaw, BD, Worcester PF, Cornuelle BD, Howe BM.  1993.  On Equations for the Speed of Sound in Seawater. Journal of the Acoustical Society of America. 93:255-275.   10.1121/1.405660   AbstractWebsite

Long-range acoustic transmissions made in conjunction with extensive environmental measurements and accurate mooring position determinations have been used to test the accuracy of equations used to calculate sound speed from pressure, temperature, and salinity. The sound-speed fields computed using the Del Grosso equation [ V. A. Del Grosso, J. Acoust. Soc. Am. 56, 1084-1091 (1974)] give predictions of acoustic arrival patterns which agree significantly better with the long-range measurements than those computed using the Chen and Millero equation [ C. Chen and F. J. Millero, J. Acoust. Soc. Am. 62, 1129-1135 (1977) The predicted ray travel times and travel time error have been calculated using objectively mapped sound-speed fields computed from conductivity, temperature, depth (CTD) and expendable bathythermograph (XBT) data. Using the measured and predicted ray travel times, a negligible correction to Del Grosso's equation of + 0.05 +/- 0.05 m/s at 4000-m depth is calculated.

Qiu, B, Rudnick DL, Cerovecki I, Cornuelle BD, Chen S, Schonau MC, McClean JL, Gopalakrishnan G.  2015.  The Pacific North Equatorial Current: New insights from the Origins of the Kuroshio and Mindanao Currents (OKMC) Project. Oceanography. 28:24-33.   10.5670/oceanog.2015.78   AbstractWebsite

Located at the crossroads of the tropical and subtropical circulations, the westward-flowing North Equatorial Current (NEC) and its subsequent bifurcation off the Philippine coast near 13 degrees N serve as important pathways for heat and water mass exchanges between the mid- and low-latitude North Pacific Ocean. Because the western Pacific warm pool, with sea surface temperatures > 28 degrees C, extends poleward of 17 degrees N in the western North Pacific, the bifurcation and transport partitioning of the NEC into the Kuroshio and Mindanao Currents are likely to affect the temporal evolution of the warm pool through lateral advection. In addition to its influence on physical conditions, NEC variability is also important to the regional biological properties and the fisheries along the Philippine coast and in the western Pacific Ocean. This article synthesizes our current understandings of the NEC, especially those garnered through the recent Origins of the Kuroshio and Mindanao Current (OKMC) project.

Todd, RE, Rudnick DL, Mazloff MR, Davis RE, Cornuelle BD.  2011.  Poleward flows in the southern California Current System: Glider observations and numerical simulation. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006536   AbstractWebsite

Three years of continuous Spray glider observations in the southern California Current System (CCS) are combined with a numerical simulation to describe the mean and variability of poleward flows in the southern CCS. Gliders provide upper ocean observations with good across-shore and temporal resolution along two across-shore survey lines while the numerical simulation provides a dynamically consistent estimate of the ocean state. Persistent poleward flows are observed in three areas: within 100 km of the coast at Point Conception, within the Southern California Bight (SCB), and offshore of the SCB and the Santa Rosa Ridge (SRR). Poleward transport by the flows within the SCB and offshore of the SRR exceeds the poleward transport off Point Conception, suggesting that the poleward flows are not continuous over the 225 km between observation lines. The numerical simulation shows offshore transport between the survey lines that is consistent with some of the poleward flow turning offshore before reaching Point Conception. The poleward current offshore of the SRR is unique in that it is strongest at depths greater than 350 m and it is observed to migrate westward away from the coast. This westward propagation is tied to westward propagating density anomalies originating in the SCB during the spring-summer upwelling season when wind stress curl is most strongly positive. The across-shore wave number, frequency, and phase speed of the westward propagation and the lack of across-shore transport of salinity along isopycnals are consistent with first-mode baroclinic Rossby dynamics.

Kim, SY, Cornuelle BD, Terrill EJ, Jones B, Washburn L, Moline MA, Paduan JD, Garfield N, Largier JL, Crawford G, Kosro PM.  2013.  Poleward propagating subinertial alongshore surface currents off the US West Coast. Journal of Geophysical Research-Oceans. 118:6791-6806.   10.1002/jgrc.20400   AbstractWebsite

The network comprising 61 high-frequency radar systems along the U.S. West Coast (USWC) provides a unique, high resolution, and broad scale view of ocean surface circulation. Subinertial alongshore surface currents show poleward propagating signals with phase speeds of O(10) and O(100-300) kmd-1 that are consistent with historical in situ observations off the USWC and that can be possibly interpreted as coastally trapped waves (CTWs). The propagating signals in the slow mode are partly observed in southern California, which may result from scattering and reflection of higher-mode CTWs due to curvature of shoreline and bathymetry near Point Conception, California. On the other hand, considering the order of the phase speed in the slow mode, the poleward propagating signals may be attributed to alongshore advection or pressure-driven flows. A statistical regression of coastal winds at National Data Buoy Center buoys on the observed surface currents partitions locally and remotely wind-forced components, isolates footprints of the equatorward propagating storm events in winter off the USWC, and shows the poleward propagating signals year round.

Raghukumar, K, Cornuelle BD, Hodgkiss WS, Kuperman WA.  2008.  Pressure sensitivity kernels applied to time-reversal acoustics. Journal of the Acoustical Society of America. 124:98-112.   10.1121/1.2924130   AbstractWebsite

Sensitivity kernels for receptions of broadband sound transmissions are used to study the effect of the transmitted signal on the sensitivity of the reception to environmental perturbations. A first-order Born approximation is used to obtain the pressure sensitivity of the received signal to small changes in medium sound speed. The pressure perturbation to the received signal caused by medium sound speed changes is expressed as a linear combination of single-frequency sensitivity kernels weighted by the signal in the frequency domain. This formulation can be used to predict the response of a source transmission to sound speed perturbations. The stability of time-reversal is studied and compared to that of a one-way transmission using sensitivity kernels. In the absence of multipath, a reduction in pressure sensitivity using time reversal is only obtained with multiple sources. This can be attributed both to the presence of independent paths and to cancellations that occur due to the overlap of sensitivity kernels for different source-receiver paths. The sensitivity kernel is then optimized to give a new source transmission scheme that takes into account knowledge of the medium statistics and is related to the regularized inverse filter. (c) 2008 Acoustical Society of America.

Heimbach, P, Fukumori I, Hills CN, Ponte RM, Stammer D, Wunsch C, Campin JM, Cornuelle B, Fenty I, Forget G, Kohl A, Mazloff M, Menemenlis D, Nguyen AT, Piecuch C, Trossman D, Verdy A, Wang O, Zhang H.  2019.  Putting it all together: Adding value to the global ocean and climate observing systems with complete self-consistent ocean state and parameter estimates. Frontiers in Marine Science. 6   10.3389/fmars.2019.00055   AbstractWebsite

In 1999, the consortium on Estimating the Circulation and Climate of the Ocean (ECCO) set out to synthesize the hydrographic data collected by the World Ocean Circulation Experiment (WOCE) and the satellite sea surface height measurements into a complete and coherent description of the ocean, afforded by an ocean general circulation model. Twenty years later, the versatility of ECCO's estimation framework enables the production of global and regional ocean and sea-ice state estimates, that incorporate not only the initial suite of data and its successors, but nearly all data streams available today. New observations include measurements from Argo floats, marine mammal-based hydrography, satellite retrievals of ocean bottom pressure and sea surface salinity, as well as ice-tethered profiled data in polar regions. The framework also produces improved estimates of uncertain inputs, including initial conditions, surface atmospheric state variables, and mixing parameters. The freely available state estimates and related efforts are property-conserving, allowing closed budget calculations that are a requisite to detect, quantify, and understand the evolution of climate-relevant signals, as mandated by the Coupled Model Intercomparison Project Phase 6 (CMIP6) protocol. The solutions can be reproduced by users through provision of the underlying modeling and assimilation machinery. Regional efforts have spun off that offer increased spatial resolution to better resolve relevant processes. Emerging foci of ECCO are on a global sea level changes, in particular contributions from polar ice sheets, and the increased use of biogeochemical and ecosystem data to constrain global cycles of carbon, nitrogen and oxygen. Challenges in the coming decade include provision of uncertainties, informing observing system design, globally increased resolution, and moving toward a coupled Earth system estimation with consistent momentum, heat and freshwater fluxes between the ocean, atmosphere, cryosphere and land.

Edwards, CA, Moore AM, Hoteit I, Cornuelle BD.  2015.  Regional ocean data assimilation. Annual Review of Marine Science, Vol 7. 7:21-42.   10.1146/annurev-marine-010814-015821   AbstractWebsite

This article reviews the past 15 years of developments in regional ocean data assimilation. A variety of scientific, management, and safety-related objectives motivate marine scientists to characterize many ocean environments, including coastal regions. As in weather prediction, the accurate representation of physical, chemical, and/or biological properties in the ocean is challenging. Models and observations alone provide imperfect representations of the ocean state, but together they can offer improved estimates. Variational and sequential methods are among the most widely used in regional ocean systems, and there have been exciting recent advances in ensemble and four-dimensional variational approaches. These techniques are increasingly being tested and adapted for biogeochemical applications.

Gilson, J, Roemmich D, Cornuelle B, Fu LL.  1998.  Relationship of TOPEX/Poseidon altimetric height to steric height and circulation in the North Pacific. Journal of Geophysical Research-Oceans. 103:27947-27965.   10.1029/98jc01680   AbstractWebsite

TOPEX/Poseidon altimetric height is compared with 20 transpacific eddy-resolving realizations of steric height. The latter are calculated from temperature (expendable bathythermograph (XBT)) and salinity (expendable conductivity and temperature profiler (XCTD)) profiles along a precisely repeating ship track over a period of 5 years. The overall difference between steric height and altimetric height is 5.2 cm RMS. On long wavelengths (lambda < 500 km), the 3.5 cm RMS difference is due mainly to altimetric measurement errors but also has a component from steric variability deeper than the 800 m limit of the XBT. The data sets are very coherent in the long wavelength band, with coherence amplitude of 0.89. This band contains 65% of the total variance in steric height. On short wavelengths (lambda > 500 km), containing 17% of the steric height variance, the 3.0 cm RMS difference and lowered coherence are due to the sparse distribution of altimeter ground tracks along the XBT section. The 2.4 cm RMS difference in the basin-wide spatial mean appears to be due to fluctuations in bottom pressure. Differences between steric height and altimetric height increase near the western boundary, but data variance increases even more, and so the signal-to-noise ratio is highest in the western quarter of the transect. Basin-wide integrals of surface geostrophic transport from steric height and altimetric height are in reasonable agreement. The 1.9 x 10(4) m(2) s(-1) RMS difference is mainly because the interpolated altimetric height lacks spatial resolution across the narrow western boundary current. A linear regression is used to demonstrate the estimation of subsurface temperature from altimetric data. Errors diminish from 0.8 degrees C at 200 m to 0.3 degrees C at 400 m. Geostrophic volume transport, 0-800 m, shows agreement that is similar to surface transport, with 4.8 Sverdrup (Sv) (10(6) m(3) s(-1)) RMS difference. The combination of altimetric height with subsurface temperature and salinity profiling is a powerful tool for observing variability in circulation and transport of the upper ocean. The continuing need for appropriate subsurface data for verification and for statistical estimation is emphasized. This includes salinity measurements, which significantly reduce errors in specific volume and steric height.

Sagen, H, Worcester PF, Dzieciuch MA, Geyer F, Sandven S, Babiker M, Beszczynska-Moller A, Dushaw BD, Cornuelle B.  2017.  Resolution, identification, and stability of broadband acoustic arrivals in Fram Strait. Journal of the Acoustical Society of America. 141:2055-2068.   10.1121/1.4978780   AbstractWebsite

An ocean acoustic tomography system consisting of three moorings with low frequency, broad-band transceivers and a moored receiver located approximately in the center of the triangle formed by the transceivers was installed in the central, deep-water part of Fram Strait during 2010-2012. Comparisons of the acoustic receptions with predictions based on hydrographic sections show that the oceanographic conditions in Fram Strait result in complex arrival patterns in which it is difficult to resolve and identify individual arrivals. In addition, the early arrivals are unstable, with the arrival structures changing significantly over time. The stability parameter a suggests that the instability is likely not due to small-scale variability, but rather points toward strong mesoscale variability in the presence of a relatively weak sound channel as being largely responsible. The estimator-correlator [Dzieciuch, J. Acoust. Soc. Am. 136, 2512-2522 (2014)] is shown to provide an objective formalism for generating travel-time series given the complex propagation conditions. Because travel times obtained from the estimator-correlator are not associated with resolved, identified ray arrivals, inverse methods are needed that do not use sampling kernels constructed from geometric ray paths. One possible approach would be to use travel-time sensitivity kernels constructed for the estimator-correlator outputs. (C) 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.

Worcester, PF, Cornuelle BD, Spindel RC.  1991.  A Review of Ocean Acoustic Tomography - 1987-1990. Reviews of Geophysics. 29:557-570. AbstractWebsite

Research in ocean acoustic tomography during the last quadrennium has resulted in substantial progress in understanding the capabilities and limitations of the technique. Theoretical studies and numerical simulations have led to greater understanding of the oceanographic information available in a vertical slice from acoustic transmissions between a single pair of instruments, of the horizontal geometries required to map the ocean mesoscale field with specified precision, and of the properties of tomographic reconstructions of the two-dimensional vector field of current. Simultaneously, the instrumentation used in tomographic experiments has been significantly improved, increasing the precision of the measurements and making gyre and basin scale experiments feasible between moored instruments. Experimental efforts to test the accuracy with which the ocean temperature and current fields can be measured acoustically have now demonstrated that tomographic techniques provide measurements with oceanographically useful precision up to ranges of about 1000 km. Such demonstrations are difficult due to the incompatibility between point measurements and the spatial averages provided by tomographic techniques. The experiments have also yielded significant information on the character of acoustic propagation at long range in the ocean. Experimental precision is now adequate to distinguish between competing algorithms for sound speed as a function of temperature, salinity, and depth. Finally, and most importantly, emphasis has shifted to use of the technique for studying the ocean, rather than on understanding the capabilities and limitations of the technique. Two major experiments, the Greenland Sea Tomography Experiment and the Gulf Stream Extension Tomography Experiment, both conducted during 1988-89, were devoted to improving our understanding of ocean dynamics, although results are not yet available. There is increased emphasis on exploiting the integrating nature of acoustic transmissions to study gyre and global scale temperature variability, phenomena difficult to study in any other way.

Colosi, JA, Grp A.  1999.  A review of recent results on ocean acoustic wave propagation in random media: Basin scales. Ieee Journal of Oceanic Engineering. 24:138-155.   10.1109/48.757267   AbstractWebsite

Measurements of basin-scale acoustic transmissions made during the last four years by the Acoustic Thermometry of Ocean Climate (ATOC) program have allowed for the study of acoustic fluctuations of low-frequency pulse propagation at ranges of 1000 to 5000 km, Analysis of data from the ATOC Acoustic Engineering Test conducted in November 1994 has revealed new and unexpected results for the physics of ocean acoustic wave propagation in random media, In particular, use of traditional Lambda, Phi methods (using the Garrett-Munk (GM) internal wave model) to identify the wave propagation regime for early identifiable wavefronts predict the saturated regime, whereas observations of intensity probability density functions, intensity variance, and pulse time spread and wander suggest that the propagation is more likely near the border between the unsaturated and partially saturated regimes. Calculations of the diffraction parameter Lambda are very sensitive to the broad-band nature of the transmitted pulse, with CW calculations differing from a simplistic broad-band calculation by 10(3)! A simple model of pulse propagation using the Born approximation shows that CW and broad-band cases are sensitive to a random medium very differently and a theoretical description of broad-band effects for pulse propagation through a random media remains a fundamental unsolved problem in ocean acoustics. The observations show that, at 75-Hz center frequency, acoustic normal mode propagation is strongly nonadiabatic due to random media effects caused by internal waves. Simulations at a lower frequency of 28 Hz suggest that the first few modes might be treated adiabatically even in a random ocean. This raises the possibility of using modal techniques for ocean acoustic tomography, thereby increasing the vertical resolution of thermometry. Finally, the observation of unsaturated or partially saturated propagation for 75-Hz broad-band transmissions, like those of ATOC, suggests that ray-based tomography will be robust at basin-scales. This opens up the possibility of ray-based internal wave tomography using the observables of travel time variance, and vertical and temporal coherence, Using geometrical optics and the GM internal wave spectrum, internal wave tomography for an assortment of parameters of the GM model can be formulated in terms of a mixed linear/nonlinear inverse, This is a significant improvement upon a Monte Carlo approach presented in this paper which is used to infer average internal wave energies as a function of depth for the SLICE89 experiment. However, this Monte Carlo approach demonstrated, for the SLICE89 experiment, that the GM model failed to render a consistent inverse for acoustic energy which sampled the upper 100 m of the ocean, Until a new theory for the forward problem is advanced, internal wave tomography utilizing the signal from strong mode coupling can only be carried out using time-consuming Monte Carlo methods.

Skarsoulis, EK, Cornuelle BD, Dzieciuch MA.  2011.  Second-Order Sensitivity of Acoustic Travel Times to Sound Speed Perturbations. Acta Acustica United with Acustica. 97:533-543.   10.3813/aaa.918434   AbstractWebsite

The second-order sensitivity of finite-frequency acoustic travel times to sound speed perturbations in range-independent environments is studied. Using the notion of peak arrivals and the normal-mode representation of the Green's function first- and second-order perturbation expressions are derived for the travel times in terms of the underlying perturbations in the Green's function and finally in the sound speed profile. The resulting theoretical expressions are numerically validated. Assuming small and local perturbations the non-linear effects appear to be strongest for sound speed perturbations taking place close to the lower turning depths of the corresponding eigenrays. At the upper turning depths - in the case of temperate propagation conditions - the effects are much weaker due to the larger sound speed gradients. The magnitude of the second-order sensitivity of travel times relative to the first-order sensitivity can be used to obtain an estimate for the limits of linearity.

McDonald, MA, Webb SC, Hildebrand JA, Cornuelle BD, Fox CG.  1994.  Seismic Structure and Anisotropy of the Juan-De-Fuca Ridge at 45-Degrees-N. Journal of Geophysical Research-Solid Earth. 99:4857-4873.   10.1029/93jb02801   AbstractWebsite

A seismic refraction experiment was conducted with air guns and ocean bottom seismometers on the Juan de Fuca Ridge at 45-degrees-N, at the northern Cleft segment and at the overlapping rift zone between the Cleft and Vance segments. These data determine the average velocity structure of the upper crust and map the thickness variability of the shallow low-velocity layer, which we interpret as the extrusive volcanic layer. The experiment is unique because a large number of travel times were measured along ray paths oriented at all azimuths within a small (20 km by 35 km) area. These travel times provide evidence for compressional velocity anisotropy in the upper several hundred meters of oceanic crust, presumed to be caused by ridge-parallel fracturing. Compressional velocities are 3.35 km/s in the ridge strike direction and 2.25 km/s across strike. Travel time residuals are simultaneously inverted for anisotropy as well as lateral thickness variations in the low-velocity layer. Extrusive layer thickness ranges from approximately 200 m to 550 m with an average of 350 m. The zone of the thinnest low-velocity layer is within the northern Cleft segment axial valley, in a region of significant hydrothermal activity. Layer thickness variability is greatest near the Cleft-Vance overlapping rift zone, where changes of 300 m occur over as little as several kilometers laterally. These low-velocity layer thickness changes may correspond to fault block rotations in an episodic spreading system, where the low side of each fault block accumulates more extrusive volcanics.

Hildebrand, JA, Dorman LM, Hammer PTC, Schreiner AE, Cornuelle BD.  1989.  Seismic tomography of jasper seamount. Geophysical Research Letters. 16:1355-1358.   10.1029/GL016i012p01355   AbstractWebsite

A vertical section of the interior structure of Jasper Seamount was modeled using a spectral tomographic inversion of P wave travel times. An array of ocean bottom seismographs (OBSs) deployed over the seamount detected the arrivals from a series of ocean bottom shots. A reference velocity model reveals that average compressional velocities within the seamount are similar to those found within Kilauea and are consistently slower than velocities at equivalent depths in typical oceanic crust. This suggests Jasper Seamount has a high average porosity. Perturbations from the reference model were imaged by tomographic inversion. A high velocity zone within the northwest flank of the seamount may result from dikes associated with a radial rift or from a shallow solidified magma reservoir. A low velocity summit may result from shallow, explosive eruptions. The tomographic model is consistent with the results of gravity, magnetic and dredging analyses.

Sabra, KG, Cornuelle B, Kuperman WA.  2016.  Sensing deep-ocean temperatures. Physics Today. 69:32-38. AbstractWebsite

Though not yet widely implemented, the technique of monitoring the ocean's warming via changes in the speed of sound through the water is a powerful complement to the more common tools available: free-floating thermometers and altimetry satellites.

Sarkar, J, Marandet C, Roux P, Walker S, Cornuelle BD, Kuperman WA.  2012.  Sensitivity kernel for surface scattering in a waveguide. Journal of the Acoustical Society of America. 131:111-118.   10.1121/1.3665999   AbstractWebsite

Using the Born approximation, a linearized sensitivity kernel is derived to describe the relationship between a local change at the free surface and its effect on the acoustic propagation in the water column. The structure of the surface scattering kernel is investigated numerically and experimentally for the case of a waveguide at the ultrasonic scale. To better demonstrate the sensitivity of the multipath propagation to the introduction of a localized perturbation at the air-water interface, the kernel is formulated both in terms of point-to-point and beam-to-beam representations. Agreement between theory and experiment suggests applications to sensitivity analysis of the wavefield for sea surface perturbations. (C) 2012 Acoustical Society of America. [DOI: 10.1121/1.3665999]

Zhang, XB, Cornuelle B, Roemmich D.  2012.  Sensitivity of Western Boundary Transport at the Mean North Equatorial Current Bifurcation Latitude to Wind Forcing. Journal of Physical Oceanography. 42:2056-2072.   10.1175/jpo-d-11-0229.1   AbstractWebsite

The bifurcation of the North Equatorial Current (NEC) plays an important role in the heat and water mass exchanges between the tropical and subtropical gyres in the Pacific Ocean. The variability of western boundary transport (WBT) east of the Philippine coast at the mean NEC bifurcation latitude (12 degrees N) is examined here. A tropical Pacific regional model is set up based on the Massachusetts Institute of Technology general circulation model and its adjoint, which calculates the sensitivities of a defined meridional transport to atmospheric forcing fields and ocean state going backward in time. The adjoint-derived sensitivity of the WBT at the mean NEC bifurcation latitude to surface wind stress is dominated by curl-like patterns that are located farther eastward and southward with increasing time lag. The temporal evolution of the adjoint sensitivity of the WBT to wind stress resembles wind-forced Rossby wave dynamics but propagating with speeds determined by the background stratification and current, suggesting that wind-forced Rossby waves are the underlying mechanism. Interannual-to-decadal variations of the WBT can be hindcast well by multiplying the adjoint sensitivity and the time-lagged wind stress over the whole model domain and summing over time lags. The analysis agrees with previous findings that surface wind stress (especially zonal wind stress in the western subtropical Pacific) largely determines the WBT east of the Philippines, and with a time lag based on Rossby wave propagation. This adjoint sensitivity study quantifies the contribution of wind stress at all latitudes and longitudes and provides a novel perspective to understand the relationship between the WBT and wind forcing over the Pacific Ocean.