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Colosi, JA, Baggeroer AB, Cornuelle BD, Dzieciuch MA, Munk WH, Worcester PF, Dushaw BD, Howe BM, Mercer JA, Spindel RC, Birdsall TG, Metzger K, Forbes AMG.  2005.  Analysis of multipath acoustic, field variability and coherence in the finale of broadband basin-scale transmissions in the North Pacific Ocean. Journal of the Acoustical Society of America. 117:1538-1564.   10.1121/1.1854615   AbstractWebsite

The statistics of low-frequency, long-range acoustic transmissions in the North Pacific Ocean are presented. Broadband signals at center frequencies of 28, 75, and 84 Hz are analyzed at propagation ranges of 3252 to 5171 km, and transmissions were received on 700 and 1400 in long vertical receiver arrays with 35 in hydrophone spacing. In the analysis we focus on the energetic "finale" region of the broadband time front arrival pattern, where a multipath interference pattern exists. A Fourier analysis of 1 s regions in the finale provide narrowband data for examination as well. Two-dimensional (depth and time) phase unwrapping is employed to study separately the complex field phase and intensity. Because data sampling occured in 20 or 40 min intervals followed by long gaps, the acoustic fields are analyzed. in terms of these 20 and 40 min and multiday observation times. An analysis of phase, intensity, and complex envelope variability as a function of depth and time is presented in terms of mean fields, variances, probability density functions (PDFs), covariance, spectra, and coherence. Observations are compared to a random multipath model of frequency and vertical wave number spectra for phase and log intensity, and the observations are compared to a broadband multipath model of scintillation index and coherence. 2005 Acoustical Society of America.

Worcester, PF, Cornuelle BD, Hildebrand JA, Hodgkiss WS, Duda TF, Boyd J, Howe BM, Mercer JA, Spindel RC.  1994.  A Comparison of Measured and Predicted Broad-Band Acoustic Arrival Patterns in Travel Time-Depth Coordinates at 1000-Km Range. Journal of the Acoustical Society of America. 95:3118-3128.   10.1121/1.409977   AbstractWebsite

Broadband acoustic signals were transmitted from a moored 250-Hz source to a 3-km-long vertical line array of hydrophones 1000 km distant in the eastern North Pacific Ocean during July 1989. The sound-speed field along the great circle path connecting the source and receiver was measured directly by nearly 300 expendable bathythermograph (XBT), conductivity-temperature-depth (CTD), and air-launched expendable bathythermograph (AXBT) casts while the transmissions were in progress. This experiment is unique in combining a vertical receiving array that extends over much of the water column, extensive concurrent environmental measurements, and broadband signals designed to measure acoustic travel times with 1-ms precision. The time-mean travel times of the early raylike arrivals, which are evident as wave fronts sweeping across the receiving array, and the time-mean of the times at which the acoustic reception ends (the final cutoffs) for hydrophones near the sound channel axis, are consistent with ray predictions based on the direct measurements of temperature and salinity, within measurement uncertainty. The comparisons show that subinertial oceanic variability with horizontal wavelengths shorter than 50 km, which is not resolved by the direct measurements, significantly (25 ms peak-to-peak) affects the time-mean ray travel times. The final cutoffs occur significantly later than predicted using ray theory for hydrophones more than 100-200 m off the sound channel axis. Nongeometric effects, such as diffraction at caustics, partially account for this observation.

Colosi, JA, Scheer EK, Flatte SM, Cornuelle BD, Dzieciuch MA, Munk WH, Worcester PF, Howe BM, Mercer JA, Spindel RC, Metzger K, Birdsall TG, Baggeroer AB.  1999.  Comparisons of measured and predicted acoustic fluctuations for a 3250-km propagation experiment in the eastern North Pacific Ocean. Journal of the Acoustical Society of America. 105:3202-3218.   10.1121/1.424650   AbstractWebsite

During the Acoustic Engineering Test (AET) of the Acoustic Thermometry of Ocean Climate (ATOC) program, acoustic signals were transmitted from a broadband source with 75-Hz center frequency to a 700-m-long vertical array of 20 hydrophones at a distance of 3252 km receptions occurred over a period of-six days. Each received pulse showed early identifiable timefronts, followed by about 2 s of highly variable energy. For the identifiable timefronts, observations of travel-time variance, average pulse shape, and the probability density function (PDF) of intensity are presented, and calculations of internal-wave contributions to those fluctuations are compared to the observations. Individual timefronts have rms travel time fluctuations of 11 to 19 ms, with time scales of less than 2 h. The pulse time spreads are between 0 and 5.3 ms rms, which suggest that internal-wave-induced travel-time biases are of the same magnitude. The PDFs of intensity for individual ray arrivals are compared to log-normal and exponential distributions. The observed PDFs are closer to the log-normal distribution, and variances of log intensity are between (3.1 dB)(2) (with a scintillation index of 0.74) for late-arriving timefronts and (2.0 dB)(2) (with a scintillation index of 0.2) for the earliest timefronts. Fluctuations of the pulse termination time of the transmissions are observed to be 22 ms rms. The intensity PDF of nonidentified peaks in the pulse crescendo are closer to a log-normal distribution than an exponential distribution, but a Kolmogorov-Smimov test rejects both distributions. The variance of the nonidentified peaks is (3.5 dB)(2) land the-scintillation index is 0.92. As a group, the observations suggest that the propagation is on the border of the unsaturated and partially saturated regimes. After improving the specification of the. ray weighting function, predictions of travel-time variance using the Garrett-Munk (GM) internal-wave spectrum at one-half the reference energy are in good agreement with the observations, and the one-half GM energy level compares well with XBT data taken along the transmission path. Predictions of pulse spread and wave propagation regime are in strong disagreement with the observations. Pulse time spread estimates are nearly two orders of magnitude too large, and Lambda-Phi methods for predicting the wave propagation regime predict full saturation. (C) 1999 Acoustical Society of America. [S0001-4966(99)04606-8].

Dushaw, BD, Worcester PF, Munk WH, Spindel RC, Mercer JA, Howe BM, Metzger K, Birdsall TG, Andrew RK, Dzieciuch MA, Cornuelle BD, Menemenlis D.  2009.  A decade of acoustic thermometry in the North Pacific Ocean. Journal of Geophysical Research-Oceans. 114   10.1029/2008jc005124   AbstractWebsite

Over the decade 1996-2006, acoustic sources located off central California (1996 1999) and north of Kauai (1997-1999, 2002-2006) transmitted to receivers distributed throughout the northeast and north central Pacific. The acoustic travel times are inherently spatially integrating, which suppresses mesoscale variability and provides a precise measure of ray-averaged temperature. Daily average travel times at 4-day intervals provide excellent temporal resolution of the large-scale thermal field. The interannual, seasonal, and shorter-period variability is large, with substantial changes sometimes occurring in only a few weeks. Linear trends estimated over the decade are small compared to the interannual variability and inconsistent from path to path, with some acoustic paths warming slightly and others cooling slightly. The measured travel times are compared with travel times derived from four independent estimates of the North Pacific: (1) climatology, as represented by the World Ocean Atlas 2005 (WOA05); (2) objective analysis of the upper-ocean temperature field derived from satellite altimetry and in situ profiles; (3) an analysis provided by the Estimating the Circulation and Climate of the Ocean project, as implemented at the Jet Propulsion Laboratory (JPL-ECCO); and (4) simulation results from a high-resolution configuration of the Parallel Ocean Program (POP) model. The acoustic data show that WOA05 is a better estimate of the time mean hydrography than either the JPL-ECCO or the POP estimates, both of which proved incapable of reproducing the observed acoustic arrival patterns. The comparisons of time series provide a stringent test of the large-scale temperature variability in the models. The differences are sometimes substantial, indicating that acoustic thermometry data can provide significant additional constraints for numerical ocean models.

Worcester, P, Dushaw BD, Andrew RK, Howe BM, Mercer JA, Spindel RC, Cornuelle B, Dzieciuch M, Birdsall TG, Metzger K, Menemenlis D.  2008.  A decade of acoustic thermometry in the North Pacific Ocean: Using long-range acoustic travel times to test gyre-scale temperature variability derived from other observations and ocean models. Journal of the Acoustical Society of America. 123 AbstractWebsite

Large-scale, range- and depth-averaged temperatures in the North Pacific Ocean were measured by long-range acoustic transmissions over the decade 1996-2006. Acoustic sources off central California and north of Kauai transmitted to receivers throughout the North Pacific. Even though acoustic travel times are spatially integrating, suppressing mesoscale variability and providing a precise measure of large-scale temperature, the travel times sometimes vary significantly on time scales of only a few weeks. The interannual variability is large, with no consistent warming or cooling trends. Comparison of the measured travel times with travel times derived from (i) the World Ocean Atlas 2005 (WOA05), (ii) an upper ocean temperature estimate derived from satellite altimetry and in situ profiles, (iii) an analysis provided by the Estimating the Circulation and Climate of the Ocean (ECCO) project, and (iv) simulation results from a high-resolution configuration of the Parallel Ocean Program (POP) show similarities, but also reveal substantial differences. The differences suggest that the data can provide significant additional constraints for numerical ocean simulations. The acoustic data show that WOA05 is a much better estimate of the time-mean hydrography than either the ECCO or POP estimates and provide significantly better time resolution for large-scale ocean variability than can be derived from satellite altimetry and in situ profiles.

Behringer, D, Birdsall T, Brown M, Cornuelle B, Heinmiller R, Knox R, Metzger K, Munk W, Spiesberger J, Spindel R, Webb D, Worcester P, Wunsch C.  1982.  A demonstration of ocean acoustic tomography. Nature. 299:121-125.   10.1038/299121a0   AbstractWebsite

Over the past decade oceanographers have become increasingly aware of an intense and compact ocean ‘mesoscale’ eddy structure (the ocean weather) that is superimposed on a generally sluggish large-scale circulation (the ocean climate). Traditional ship-based observing systems are not adequate for monitoring the ocean at mesoscale resolution. A 1981 experiment mapped the waters within a 300 × 300 km square south-west of Bermuda, using a peripheral array of moored midwater acoustic sources and receivers. The variable acoustic travel times between all source–receiver pairs were used to construct the three-dimensional (time-variable) eddy fields, using inverse theory. Preliminary results from inversions are consistent with the shipborne and airborne surveys.

Fujii, Y, Cummings J, Xue Y, Schiller A, Lee T, Balmaseda MA, Remy E, Masuda S, Brassington G, Alves O, Cornuelle B, Martin M, Oke P, Smith G, Yang XS.  2015.  Evaluation of the Tropical Pacific Observing System from the ocean data assimilation perspective. Quarterly Journal of the Royal Meteorological Society. 141:2481-2496.   10.1002/qj.2579   AbstractWebsite

The drastic reduction in the number of observation data from the Tropical Atmospheric Ocean (TAO)/Triangle Trans-Ocean Buoy Network (TRITON) array since 2012 has given rise to a need to assess the impact of those data in ocean data assimilation (DA) systems. This article provides a review of existing studies evaluating the impacts of data from the TAO/TRITON array and other components of the Tropical Pacific Observing System (TPOS) on current ocean DA systems used for a variety of operational and research applications. It can be considered as background information that can guide the evaluation exercise of TPOS. Temperature data from TAO/TRITON array are assimilated in most ocean DA systems which cover the tropical Pacific in order to constrain the ocean heat content, stratification, and circulation. It is shown that the impacts of observation data depend considerably on the system and application. The presence of model error often makes the results difficult to interpret. Nevertheless there is consensus that the data from TAO/TRITON generally have positive impacts complementary to Argo floats. In the equatorial Pacific, the impacts are generally around the same level or larger than those of Argo. We therefore conclude that, with the current configuration of TPOS, the loss of the TAO/TRITON data is having a significant detrimental impact on many applications based on ocean DA systems. This conclusion needs to be kept under review because the equatorial coverage by Argo is expected to improve in the future.

Send, U, Worcester PF, Cornuelle BD, Tiemann CO, Baschek B.  2002.  Integral measurements of mass transport and heat content in the Strait of Gibraltar from acoustic transmissions. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 49:4069-4095.   10.1016/s0967-0645(02)00143-1   AbstractWebsite

Three 2 kHz acoustic transceivers were deployed on either side of the eastern entrance of the Strait of Gibraltar during April-May 1996 to determine the feasibility of using acoustic methods to make routine, rapidly repeated, horizontally integrated measurements of flow and temperature in straits. Reciprocal transmissions between the transceivers were used to test the feasibility of using traditional ray differential travel times to monitor the component of flow along the acoustic paths. Transmissions directly across the Strait were used to test the feasibility of using horizontal arrival angle fluctuations and acoustic intensity scintillations to monitor the flow perpendicular to the acoustic path. The geometry was selected to provide ray paths that only sample the lower-layer Mediterranean water, so that the feasibility of monitoring the Mediterranean outflow using the various methods could be evaluated. The acoustic scintillation method did not yield useful current estimates, but the experimental parameters were not optimized for this approach. Since the low-frequency variability in log-amplitude was found to be highly correlated at receivers 228 m apart, it is possible that acoustic scintillation measurements using different receiver spacings and more rapid sampling might yield better results. The horizontal deflection method gave encouraging results at the time of neap tides, but less so during spring tides. For this approach, both theoretical estimates and measured phase differences between the horizontally separated receivers suggest that internal-wave-induced horizontal arrival angle fluctuations may fundamentally limit the precision with which arrival angles can be measured. Further work is needed to determine if a smaller horizontal spacing and higher signal-to-noise ratios would yield better results. Reciprocal travel time measurements diagonally across the Strait performed the best of the three methods, giving absolute flow estimates consistent with those derived from current-meter data. The fractional uncertainty variance for the lower layer tidal transport from a single tomographic path was estimated to be 0.017 (i.e. 98% of the a priori tidal transport variance was resolved). The spatial scales of the sub-tidal flow are thought to be significantly shorter than those of the tidal flow, however, which means that a more elaborate monitoring network is required to achieve the same performance for sub-tidal variability. Finally, sum travel times from the reciprocal transmissions were found to provide good measurements of the temperature and heat content in the lower layer. (C) 2002 Published by Elsevier Science Ltd.

Boas, ABV, Ardhuin F, Ayet A, Bourassa MA, Brandt P, Chapron B, Cornuelle BD, Farrar JT, Fewings MR, Fox-Kemper B, Gille ST, Gommenginger C, Heimbach P, Hell MC, Li Q, Mazloff MR, Merrifield ST, Mouche A, Rio MH, Rodriguez E, Shutler JD, Subramanian AC, Terrill EJ, Tsamados M, Ubelmann C, van Sebille E.  2019.  Integrated observations of global surface winds, currents, and waves: Requirements and challenges for the next decade. Frontiers in Marine Science. 6   10.3389/fmars.2019.00425   AbstractWebsite

Ocean surface winds, currents, and waves play a crucial role in exchanges of momentum, energy, heat, freshwater, gases, and other tracers between the ocean, atmosphere, and ice. Despite surface waves being strongly coupled to the upper ocean circulation and the overlying atmosphere, efforts to improve ocean, atmospheric, and wave observations and models have evolved somewhat independently. From an observational point of view, community efforts to bridge this gap have led to proposals for satellite Doppler oceanography mission concepts, which could provide unprecedented measurements of absolute surface velocity and directional wave spectrum at global scales. This paper reviews the present state of observations of surface winds, currents, and waves, and it outlines observational gaps that limit our current understanding of coupled processes that happen at the air-sea-ice interface. A significant challenge for the coming decade of wind, current, and wave observations will come in combining and interpreting measurements from (a) wave-buoys and high-frequency radars in coastal regions, (b) surface drifters and wave-enabled drifters in the open-ocean, marginal ice zones, and wave-current interaction "hot-spots," and (c) simultaneous measurements of absolute surface currents, ocean surface wind vector, and directional wave spectrum from Doppler satellite sensors.

Muccino, JC, Arango HG, Bennett AF, Chua BS, Cornuelle BD, Di Lorenzo E, Egbert GD, Haidvogel D, Levin JC, Luo H, Miller AJ, Moore AA, Zaron ED.  2008.  The Inverse Ocean Modeling system. Part II: Applications. Journal of Atmospheric and Oceanic Technology. 25:1623-1637.   10.1175/2008jtecho522.1   AbstractWebsite

The Inverse Ocean Modeling (IOM) System is a modular system for constructing and running weak-constraint four-dimensional variational data assimilation (W4DVAR) for any linear or nonlinear functionally, smooth dynamical model and observing array. The IOM has been applied to four ocean models with widely varying characteristics. The Primitive Equations Z-coordinate-Harmonic Analysis of Tides (PEZ-HAT) and the Regional Ocean Modeling System (ROMS) are three-dimensional, primitive equations models while the Advanced Circulation model in 2D (ADCIRC-2D) and Spectral Element Ocean Model in 2D (SEOM-2D) are shallow-water models belonging to the general finite-element family. These models. in conjunction with the IOM, have been used to investigate a wide variety of scientific phenomena including tidal. mesoscale, and wind-driven circulation. In all cases, the assimilation of data using the IOM provides a better estimate of the ocean state than the model alone.

The AMODE-MST Group, Birdsall TG, Boyd J, Cornuelle BD, Howe BM, Knox R, Mercer JA, Metzger Jr. K, Spindel RC, Worcester PF.  1994.  Moving ship tomography in the North Atlantic. EOS Trans. AGU. 75:17,21,23. Abstract
Dushaw, BD, Howe BM, Mercer JA, Spindel RC, Baggeroer AB, Menemenlis D, Wunsch C, Birdsall TG, Clark KMC, Colosi JA, Cornuelle BD, Dzieciuch M, Munk W.  1999.  Multimegameter-Range Acoustic Data Obtained by Bottom-Mounted Hydrophone Arrays for Measurement of Ocean Temperature. Ieee Journal of Oceanic Engineering. 24:202-214.   10.1109/48.757271   AbstractWebsite

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 U.S. 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 degree C.

Dushaw, B, Bold G, Chiu CS, Colosi J, Cornuelle B, Desaubies Y, Dzieciuch M, Forbes A, Gaillard F, Gavrilov A, Gould J, Howe BM, Lawrence M, Lynch J, Menemenlis D, Mercer J, Mikhalevsky PN, Munk W, Nakano I, Schott F, Send U, Spindel R, Terre T, Worcester P, Wunsch C.  2001.  Observing the ocean in the 2000's: A strategy for the role of acoustic tomography in ocean climate observation. Observing the Oceans in the 21st Century. ( Koblinsky C, Smith NR, Eds.).:391-418., Melbourne: Bureau of Meteorology Abstract
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.

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.

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.

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.

Baggeroer, AB, Scheer EK, Colosi JA, Cornuelle BD, Dushaw BD, Dzieciuch MA, Howe BM, Mercer JA, Munk W, Spindel RC, Worcester PF.  2005.  Statistics and vertical directionality of low-frequency ambient noise at the North Pacific Acoustic Laboratory site. Journal of the Acoustical Society of America. 117:1643-1665.   10.1121/1.1855035   AbstractWebsite

We examine statistical and directional properties of the ambient noise in the 10-100 Hz frequency band from the NPAL array. Marginal probability densities are estimated as well as mean square levels, skewness and kurtoses in third octave bands. The kurotoses are markedly different from Gaussian except when only distant shipping is present. Extremal levels reached similar to 150 dB re 1 mu Pa, suggesting levels 60dB greater than the mean ambient were common in the NPAL data sets. Generally, these were passing ships. We select four examples: i) quiescent noise, ii) nearby shipping, iii) whale vocalizations and iv) a micro earthquake for the vertical directional properties of the NPAL noise since they are representative of the phenomena encountered. We find there is modest broadband coherence for most of these cases in their occupancy band across the NPAL aperture. Narrowband coherence analysis from VLA to VLA was not successful due to ambiguities. Examples of localizing sources based upon this coherence are included. kw diagrams allow us to use data above the vertical aliasing frequency. Ducted propagation for both the quiescent, and micro earthquake (T phase) are identified and the arrival angles of nearby shipping and whale vocalizations. MFP localizations were modestly successful for nearby sources, but long range ones could not be identified, most likely because of signal mismatch in the MFP replica. 2005 Acoustical Society of America.

Moore, AM, Martini MJ, Akella S, Arango HG, Balmaseda M, Bertino L, Ciavatta S, Cornuelle B, Cummings J, Frolov S, Lermusiaux P, Oddo P, Oke PR, Storto A, Teruzzi A, Vidard A, Weaver AT, Assimilation GOVD.  2019.  Synthesis of ocean observations using data assimilation for operational, real-time and reanalysis systems: A more complete picture of the state of the ocean. Frontiers in Marine Science. 6   10.3389/fmars.2019.00090   AbstractWebsite

Ocean data assimilation is increasingly recognized as crucial for the accuracy of real-time ocean prediction systems and historical re-analyses. The current status of ocean data assimilation in support of the operational demands of analysis, forecasting and reanalysis is reviewed, focusing on methods currently adopted in operational and real-time prediction systems. Significant challenges associated with the most commonly employed approaches are identified and discussed. Overarching issues faced by ocean data assimilation are also addressed, and important future directions in response to scientific advances, evolving and forthcoming ocean observing systems and the needs of stakeholders and downstream applications are discussed.

Worcester, PF, Cornuelle BD, Dzieciuch MA, Munk WH, Howe BM, Mercer JA, Spindel RC, Colosi JA, Metzger K, Birdsall TG, Baggeroer AB.  1999.  A test of basin-scale acoustic thermometry using a large-aperture vertical array at 3250-km range in the eastern North Pacific Ocean. Journal of the Acoustical Society of America. 105:3185-3201.   10.1121/1.424649   AbstractWebsite

Broadband acoustic signals were transmitted during November 1994 from a 75-Hz source suspended near the depth of the sound-channel axis to a 700-m long vertical receiving array approximately 3250 km distant in the eastern North Pacific Ocean. The early part of the arrival pattern consists of raylike wave fronts that are resolvable, identifiable, and stable. The later part of the arrival pattern does not contain identifiable raylike arrivals, due to scattering from internal-wave-induced sound-speed fluctuations. The observed ray travel times differ from ray predictions based on the sound-speed field constructed using nearly concurrent temperature and salinity measurements by more than a priori variability estimates, suggesting that the equation used to compute sound speed requires refinement. The range-averaged ocean sound speed can be determined with an uncertainty of about 0.05 m/s from the observed ray travel times together with the time at which the near-axial acoustic reception ends, used as a surrogate for the group delay of adiabatic mode 1. The change in temperature over six days can be estimated with an uncertainty of about 0.006 degrees C. The sensitivity of the travel times to ocean variability is concentrated near the ocean surface and at the corresponding conjugate depths, because all of the resolved ray arrivals have upper turning depths within a few hundred meters of the surface. (C) 1999 Acoustical Society of America. [S0001-4966(99)04506-3].

Cornuelle, B, Wunsch C, Behringer D, Birdsall T, Brown M, Heinmiller R, Knox R, Metzger K, Spiesberger J, Spindel R, Webb D, Worcester P.  1985.  Tomographic maps of the ocean mesoscale. Part 1: Pure acoustics. Journal of Physical Oceanography. 15:133-152.   10.1175/1520-0485(1985)015<0133:TMOTOM>2.0.CO;2   Abstract

A field test of ocean acoustic tomography was conducted in 1981 for a two month period in a 300 km square at 26°N, 70°W in the North Atlantic (just south of the MODE region). Nine acoustic deep-sea moorings with sea floor transponders for automated position keeping and with provisions for precise time keeping were set and recovered. From the measured travel times between moorings, various displays of the three-dimensional field of sound speed (closely related to temperature) have been obtained by inversion procedures. These procedures use historical ocean data as a reference, but all information from the in situ surveys has been withheld; the “pure” tomographic results were then compared to direct in situ observations. The tomographically derived spatial mean profile compares favorably to an equivalent profile from the in situ observations; both differ significantly from the historical average. Maps constructed at three day intervals for a two month period show a pattern of eddy structure in agreement with the direct observations within computed mapping errors, but these mapping errors are too large for many oceanographic purposes. The mapping errors are the result of an unexpectedly large noise variance in travel time. (A 1983 experiment, using sources with larger bandwidth, reduced this variance to acceptable limits.) The 1981 tomographic results strongly suggest that the ocean sometimes undergoes transitions too rapid to be mapped over such large areas by shipboard observations.

Di Lorenzo, E, Moore AM, Arango HG, Cornuelle BD, Miller AJ, Powell B, Chua BS, Bennett AF.  2007.  Weak and strong constraint data assimilation in the inverse Regional Ocean Modeling System (ROMS): Development and application for a baroclinic coastal upwelling system. Ocean Modelling. 16:160-187.   10.1016/j.ocemod.2006.08.002   AbstractWebsite

We describe the development and preliminary application of the inverse Regional Ocean Modeling System (ROMS), a four dimensional variational (4DVAR) data assimilation system for high-resolution basin-wide and coastal oceanic flows. Inverse ROMS makes use of the recently developed perturbation tangent linear (TL), representer tangent linear (RP) and adjoint (AD) models to implement an indirect representer-based generalized inverse modeling system. This modeling framework is modular. The TL, RP and AD models are used as stand-alone sub-models within the Inverse Ocean Modeling (IOM) system described in [Chua, B.S., Bennett, A.F., 2001. An inverse ocean modeling system. Ocean Modell. 35 137-165.]. The system allows the assimilation of a wide range of observation types and uses an iterative algorithm to solve nonlinear assimilation problems. The assimilation is performed either under the perfect model assumption (strong constraint) or by also allowing for errors in the model dynamics (weak constraints). For the weak constraint case the TL and RP models are modified to include additional forcing terms on the right hand side of the model equations. These terms are needed to account for errors in the model dynamics. Inverse ROMS is tested in a realistic 3D baroclinic upwelling system with complex bottom topography, characterized by strong mesoscale eddy variability. We assimilate synthetic data for upper ocean (0-450 m) temperatures and currents over a period of 10 days using both a high resolution and a spatially and temporally aliased sampling array. During the assimilation period the flow field undergoes substantial changes from the initial state. This allows the inverse solution to extract the dynamically active information from the synthetic observations and improve the trajectory of the model state beyond the assimilation window. Both the strong and weak constraint assimilation experiments show forecast skill greater than persistence and climatology during the 10-20 days after the last observation is assimilated. Further investigation in the functional form of the model error covariance and in the use of the representer tangent linear model may lead to improvement in the forecast skill. (c) 2006 Elsevier Ltd. All rights reserved.