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Duda, TF, Flatte SM, Colosi JA, Cornuelle BD, Hildebrand JA, Hodgkiss WS, Worcester PF, Howe BM, Mercer JA, Spindel RC.  1992.  Measured Wave-Front Fluctuations in 1000-Km Pulse-Propagation in the Pacific-Ocean. Journal of the Acoustical Society of America. 92:939-955.   10.1121/1.403964   AbstractWebsite

A 1000-km acoustical transmission experiment has been carried out in the North Pacific, with Pulses broadcast between a moored broadband source (250-Hz center frequency) and a moored sparse vertical line of receivers. Two data records are reported: a period of 9 days at a pulse rate of one per hour, and a 21 -h period on the seventh day at six per hour. Many wave-front segments were observed at each hydrophone depth, and arrival times were tracked and studied as functions of time and depth. Arrivals within the final section of the pulse are not trackable in time or space at the chosen sampling rates, however. Broadband fluctuations, which are uncorrelated over 10-min sampling and 60-m vertical spacing, are observed with about 40 (ms)2 variance. The variance of all other fluctuations (denoted as low-frequency) is comparable or smaller than the broadband value; this low-frequency variance can be separated into two parts: a wave-front segment displacement (with vertical correlation length greater than 1 km) that varies substantially between rays with different ray identifiers, and a distortion (with vertical correlation length between 60 m and 1 km) of about 2 (ms)2 variance. The low-frequency variance may be explained as the effect of internal waves, including internal tides. The variance of the broadband fluctuations is reduced somewhat but not eliminated if only high-intensity peaks are selected; this selection does not affect the statistics of the low-frequency fluctuations.

Duda, TF, Pawlowicz RA, Lynch JF, Cornuelle BD.  1995.  Simulated Tomographic Reconstruction of Ocean Features Using Drifting Acoustic Receivers and a Navigated Source. Journal of the Acoustical Society of America. 98:2270-2279.   10.1121/1.413341   AbstractWebsite

Numerically simulated acoustic transmission from a single source of known position (for example, suspended from a ship) to receivers of partially known position (for example, sonobuoys dropped from the air) are used for tomographic mapping of ocean sound speed. The maps are evaluated for accuracy and utility. Grids of 16 receivers are employed, with sizes of 150, 300, and 700 km square. Ordinary statistical measures are used to evaluate the pattern similarity and thus the mapping capability of the, system. For an array of 300 km square, quantitative error in the maps grows with receiver position uncertainty. The large and small arrays show lesser mapping capability than the mid-size array. Mapping errors increase with receiver position uncertainty for uncertainties less than 1000-m rms, but uncertainties exceeding that have less systematic effect on the maps. Maps of rms error of the field do not provide a complete view of the utility of the acoustic network. Features of maps are surprisingly reproducible for different navigation error levels, and give comparable information about mesoscale structures despite great variations in those levels. (C) 1995 Acoustical Society of America.

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.

Dushaw, BD, Worcester PF, Cornuelle BD, Howe BM.  1993.  Variability of Heat-Content in the Central North Pacific in Summer 1987 Determined from Long-Range Acoustic Transmissions. Journal of Physical Oceanography. 23:2650-2666.   10.1175/1520-0485(1993)023<2650:vohcit>2.0.co;2   AbstractWebsite

The evolution of the heat content in the central North Pacific Ocean during summer 1987 has been measured using acoustic transmissions between transceivers deployed in a triangle approximately 1000 km on a side. The acoustically determined heat contents of the source-receiver sections agree with heat contents computed from CTD and XBT data obtained during May and September 1987. The accuracy of acoustical measurements of range-averaged heat content is comparable to estimates from CTD and XBT data. Transmissions at four-day intervals allow the continuous observation of heat content and show that it varies on time scales of weeks or less. The magnitude of these variations is of the same order as that observed from XBT sections, which are only occasionally available. Ocean-atmosphere heat exchange from bulk formulas accounts for only about half of the observed heat content increase from May through September 1987, indicating that advective effects are important in the region. The excess heat change is calculated to be of order 50-150 W m(-2). The advective component of the near-surface heat budget is roughly in phase with the surface flux component.

Dushaw, BD, Egbert GD, Worcester PF, Cornuelle BD, Howe BM, Metzger K.  1997.  A TOPEX/POSEIDON global tidal model (TPXO.2) and barotropic tidal currents determined from long-range acoustic transmissions. Progress in Oceanography. 40:337-367.   10.1016/s0079-6611(98)00008-1   AbstractWebsite

Tidal currents derived from the TPXO.2 global tidal model of Egbert, Bennett, and Foreman are compared with those determined from long-range reciprocal acoustic transmissions. Amplitudes and phases of tidal constituents in the western North Atlantic are derived from acoustic data obtained in 1991-1992 using a pentagonal array of transceivers. Small, spatially coherent differences between the measured and modeled tidal harmonic constants mostly result from smoothing assumptions made in the model and errors caused in the model currents by complicated topography to the southwest of the acoustical array. Acoustically measured harmonic constants (amplitude, phase) of M-2 tidal vorticity (3-8 x 10(-9) s(-1), 210-310 degrees) agree with those derived from the TPXO.2 model (2-5 x 10(-9) s(-1), 250-300 degrees), whereas harmonic constants of about (1-2 x 10(-9) s(-1), 350-360 degrees) are theoretically expected from the equations of motion. Harmonic constants in the North Pacific Ocean are determined using acoustic data from a triangular transceiver array deployed in 1987. These constants are consistent with those given by the TPXO.2 tidal model within the uncertainties. Tidal current harmonic constants determined from current meters do not generally provide a critical test of tidal models. The tidal currents have been estimated to high accuracy using long-range reciprocal acoustic transmissions; these estimates will be useful constraints on future global tidal models. (C) 1998 Elsevier Science Ltd. All rights reserved.

Dushaw, BD, Worcester PF, Cornuelle BD, Howe BM.  1994.  Barotropic Currents and Vorticity in the Central North Pacific-Ocean During Summer 1987 Determined from Long-Range Reciprocal Acoustic Transmissions. Journal of Geophysical Research-Oceans. 99:3263-3272.   10.1029/93jc03335   AbstractWebsite

Large-scale depth-integrated currents and relative vorticity were measured in the central North Pacific Ocean during summer 1987 using long-range reciprocal acoustic transmissions between transceivers in a triangle approximately 1000 km on a side. Inverse techniques were used to estimate the depth-averaged (barotropic) current bihourly at 4-day intervals from differential travel times. Tidal constituent amplitudes and phases found from the acoustically determined currents agree with those found from current meters and with the tidal models of Schwiderski (1980) and Cartwright et al. (1992), providing confirmation that the tomographically derived barotropic currents are correct within the expected uncertainties. The estimated low-frequency, large-scale currents are compared with depth-averaged currents determined by point measurements using current meters and bottom-mounted electrometers. Meridional and zonal currents are calculated using the topographic Sverdrup balance with the Fleet Numerical Oceanography Center wind field. The measured time derivative of the areally averaged relative vorticity is shown to be insignificant to the Sverdrup balance. Currents and vorticity calculated using the Sverdrup balance are an order of magnitude smaller than the observations. The magnitude and variability of the large-scale currents and vorticity determined from the Semtner and Chervin (1988) eddy-resolving model of ocean circulation are similar to the direct measurements.

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
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Dushaw, BD, Cornuelle BD, Worcester PF, Howe BM, Luther DS.  1995.  Barotropic and Baroclinic Tides in the Central North Pacific-Ocean Determined from Long-Range Reciprocal Acoustic Transmissions. Journal of Physical Oceanography. 25:631-647.   10.1175/1520-0485(1995)025<0631:babtit>2.0.co;2   AbstractWebsite

Travel times of reciprocal 1000-km range acoustic transmissions, determined from the 1987 Reciprocal Tomography Experiment, are used to study barotropic tidal currents and a large-scale, coherent baroclinic tide in the central North Pacific Ocean. The difference in reciprocal travel times determines the tidal currents, while the sum of reciprocal travel times determines the baroclinic tide displacement of isotachs (or equivalently, isotherms). The barotropic tidal current accounts for 90% of the observed differential travel time variance. The measured harmonic constants of the eight major tidal constituents of the barotropic tide and the constants determined from current meter measurements agree well with the empirical-numerical tidal models of Schwiderski and Cartwright et al. The amplitudes and phases of the first-mode baroclinic tide determined from sum travel times agree with those determined from moored thermistors and current meters. The baroclinic tidal signals are consistent with a large-scale, phase-locked internal tide, which apparently has propagated northward over 2000 km from the Hawaiian Ridge. The amplitude, phase, and polarization of the first-mode M(2) baroclinic tidal displacement and current are consistent with a northward propagating internal tide. The ratio of baroclinic energy to barotropic energy determined using the range-averaging acoustic transmissions is about 8%, while a ratio of 26% was determined from the point measurements. The large-scale, internal tide energy flux, presumed northward, is estimated to be about 180 W m(-1).

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.

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.

Dzieciuch, MA, Cornuelle BD, Skarsoulis EK.  2013.  Structure and stability of wave-theoretic kernels in the ocean. Journal of the Acoustical Society of America. 134:3318-3331.   10.1121/1.4818846   AbstractWebsite

Wave-theoretic modeling can be applied to obtain travel-time sensitivity kernels (TSKs) representing the amount ray travel times are affected by sound-speed variations anywhere in the medium. This work explores the spatial frequency content of the TSK compared to expected ocean variability. It also examines the stability of the TSK in environments that produce strong sensitivity of ray paths to initial conditions. The conclusion is that the linear TSK model is an effective predictor of travel-time changes and that the rays perform nearly as well as the full-wave kernel. The TSK is examined in physical space and in wavenumber space, and it is found that this is the key to understanding how the travel time reacts to ocean perturbations. There are minimum vertical and horizontal length scales of ocean perturbations that are required for the travel time to be affected. The result is that the correspondence between true travel times and those calculated from the kernel is high for large-scale perturbations and somewhat less for the small scales. This demonstrates the validity of ray-based inversion of travel time observations for the cases under study. (C) 2013 Acoustical Society of America.

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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.

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Fontan, A, Cornuelle B.  2015.  Anisotropic response of surface circulation to wind forcing, as inferred from high-frequency radar currents in the southeastern Bay of Biscay. Journal of Geophysical Research-Oceans. 120:2945-2957.   10.1002/2014jc010671   AbstractWebsite

The short-term (less than 20 days) response of surface circulation to wind has been determined in waters of the southeastern Bay of Biscay, using wind impulse response (time domain) and transfer (frequency domain) functions relating high-frequency radar currents and reanalysis winds. The response of surface currents is amplified at the near-inertial frequency and the low-frequency and it varies spatially. The analysis indicates that the response of the ocean to the wind is slightly anisotropic, likely due to pressure gradients and friction induced by the bottom and coastline boundaries in this region. Thus, the transfer function at the near-inertial frequency decreases onshore due to the coastline inhibition of circularly polarized near-inertial motion. In contrast, the low-frequency transfer function is enhanced toward the coast as a result of the geostrophic balance between the cross-shore pressure gradient and the Coriolis forces. The transfer functions also vary with season. In summer, the current response to wind is expected to be stronger but shallower due to stratification; in winter, the larger mixed layer depth results in a weaker but deeper response. The results obtained are consistent with the theoretical description of wind-driven circulation and can be used to develop a statistical model with a broad range of applications including accurate oceanic forecasting and understanding of the coupled atmosphere-ocean influence on marine ecosystems.

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.

Furue, R, Jia YL, McCreary JP, Schneider N, Richards KJ, Muller P, Cornuelle BD, Avellaneda NM, Stammer D, Liu CY, Kohl A.  2015.  Impacts of regional mixing on the temperature structure of the equatorial Pacific Ocean. Part 1: Vertically uniform vertical diffusion. Ocean Modelling. 91:91-111.   10.1016/j.ocemod.2014.10.002   AbstractWebsite

We investigate the sensitivity of numerical model solutions to regional changes in vertical diffusion. Specifically, we vary the background diffusion coefficient, kappa(b), within spatially distinct subregions of the tropical Pacific, assess the impacts of those changes, and diagnose the processes that account for them. Solutions respond to a diffusion anomaly, delta kappa(b), in three ways. Initially, there is a fast response (several months), due to the interaction of rapidly propagating, barotropic and gravity waves with eddies and other mesoscale features. It is followed by a local response (roughly one year), the initial growth and spatial pattern of which can be explained by one-dimensional (vertical) diffusion. At this stage, temperature and salinity anomalies are generated that are either associated with a change in density ("dynamical" anomalies) or without one ("spiciness" anomalies). In a final adjustment stage, the dynamical and spiciness anomalies spread to remote regions by radiation of Rossby and Kelvin waves and by advection, respectively. In near equilibrium solutions, dynamical anomalies are generally much larger in the latitude band of the forcing, but the impact of off equatorial forcing by delta kappa(b) on the equatorial temperature structure is still significant. Spiciness anomalies spread equator ward within the pycnocline, where they are carried to the equator as part of the subsurface branch of the Pacific Subtropical Cells, and spiciness also extends to the equator via western-boundary currents. Forcing near and at the equator generates strong dynamical anomalies, and sometimes additional spiciness anomalies, at pycnocline depths. The total response of the equatorial temperature structure to delta kappa(b) in various regions depends on the strength and spatial pattern of the generation of each signal within the forcing region as well as On the processes of its spreading to the equator.

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Gaillard, F, Cornuelle B.  1987.  Improvement of Tomographic Maps by Using Surface-Reflected Rays. Journal of Physical Oceanography. 17:1458-1467.   10.1175/1520-0485(1987)017<1458:iotmbu>2.0.co;2   AbstractWebsite

The results of the experiment conducted in the northwest Atlantic in 1981 have demonstrated the possibilities of acoustic tomography. The first maps, based only on purely refracted rays, showed the evolution of a cold eddy, confirmed by direct measurement of temperature and salinity. A more complete use of the 1981 dataset, with incorporation of surface-reflected rays, is proposed here. The addition of new data reduces the statistical error on the estimation of the sound speed field. Resolution at levels already well estimated in the earlier computations is improved, and individual maps exhibit a better continuity. Information is now available about the average properties of the upper layers of the ocean, which could not be monitored with purely refracted rays.

Gasparin, F, Roemmich D, Gilson J, Cornuelle B.  2015.  Assessment of the upper-ocean observing system in the equatorial Pacific: The role of Argo in resolving intraseasonal to interannual variability*. Journal of Atmospheric and Oceanic Technology. 32:1668-1688.   10.1175/jtech-d-14-00218.1   AbstractWebsite

Using more than 10 years of Argo temperature and salinity profiles (2004-14), a new optimal interpolation (OI) of the upper ocean in the equatorial Pacific is presented. Following Roemmich and Gilson's procedures, which were formulated for describing monthly large-scale anomalies, here every 5 days anomaly fields are constructed with improvements in the OI spatial covariance function and by including the time domain. The comparison of Argo maps with independent observations, from the TAO/TRITON array, and with satellite sea surface height (SSH), demonstrates that Argo is able to represent around 70%-80% of the variance at intraseasonal time scales (periods of 20-100 days) and more than 90% of the variance for the seasonal-to-longer-term variability. The RMS difference between Argo and TAO/TRITON temperatures is lower than 1 degrees C and is around 1.5 cm when the Argo steric height is compared to SSH. This study also assesses the efficacy of different observing system components and combinations, such as SSH, TAO/TRITON, and Argo, for estimating subsurface temperature. Salinity investigations demonstrate its critical importance for density near the surface in the western Pacific. Objective error estimates from the OI are used to evaluate different sampling strategies, such as the recent deployment of 41 Argo floats along the Pacific equator. Argo's high spatial resolution compared with that of the moored array makes it better suited for studying spatial patterns of variability and propagation on intraseasonal and longer periods, but it is less well suited for studying variability on periods shorter than 20 days at point locations. This work is a step toward better utilization of existing datasets, including Argo, and toward redesigning the Tropical Pacific Observing System.

Gawarkiewicz, G, Jan S, Lermusiaux PFJ, McClean JL, Centurioni L, Taylor K, Cornuelle B, Duda TF, Wang J, Yang YJ, Sanford T, Lien RC, Lee C, Lee MA, Leslie W, Haley PJ, Niiler PP, Gopalakrishnan G, Velez-Belchi P, Lee DK, Kim YY.  2011.  Circulation and Intrusions Northeast of Taiwan: Chasing and Predicting Uncertainty in the Cold Dome. Oceanography. 24:110-121. AbstractWebsite

An important element of present oceanographic research is the assessment and quantification of uncertainty. These studies are challenging in the coastal ocean due to the wide variety of physical processes occurring on a broad range of spatial and temporal scales. In order to assess new methods for quantifying and predicting uncertainty, a joint Taiwan-US field program was undertaken in August/September 2009 to compare model forecasts of uncertainties in ocean circulation and acoustic propagation, with high-resolution in situ observations. The geographical setting was the continental shelf and slope northeast of Taiwan, where a feature called the "cold dome" frequently forms. Even though it is hypothesized that Kuroshio subsurface intrusions are the water sources for the cold dome, the dome's dynamics are highly uncertain, involving multiple scales and many interacting ocean features. During the experiment, a combination of near-surface and profiling drifters, broad-scale and high-resolution hydrography, mooring arrays, remote sensing, and regional ocean model forecasts of fields and uncertainties were used to assess mean fields and uncertainties in the region. River runoff from Typhoon Morakot, which hit Taiwan August 7-8, 2009, strongly affected shelf stratification. In addition to the river runoff, a cold cyclonic eddy advected into the region north of the Kuroshio, resulting in a cold dome formation event. Uncertainty forecasts were successfully employed to guide the hydrographic sampling plans. Measurements and forecasts also shed light on the evolution of cold dome waters, including the frequency of eddy shedding to the north-northeast, and interactions with the Kuroshio and tides. For the first time in such a complex region, comparisons between uncertainty forecasts and the model skill at measurement locations validated uncertainty forecasts. To complement the real-time model simulations, historical simulations with another model show that large Kuroshio intrusions were associated with low sea surface height anomalies east of Taiwan, suggesting that there may be some degree of predictability for Kuroshio intrusions.

Gemba, KL, Sarkar J, Cornuelle B, Hodgkiss WS, Kuperman WA.  2018.  Estimating relative channel impulse responses from ships of opportunity in a shallow water environment. The Journal of the Acoustical Society of America. 144:1231-1244.   10.1121/1.5052259   Abstract

The uncertainty of estimating relative channel impulse responses (CIRs) obtained using the radiated signature from a ship of opportunity is investigated. The ship observations were taken during a 1.4 km (11 min) transect in a shallow water environment during the Noise Correlation 2009 (NC09) experiment. Beamforming on the angle associated with the direct ray-path yields an estimate of the ship signature, subsequently used in a matched filter. Relative CIRs are estimated every 2.5 s independently at three vertical line arrays (VLAs). The relative arrival-time uncertainty is inversely proportional to source bandwidth and CIR signal-to-noise ratio, and reached a minimum standard deviation of 5 μs (equivalent to approximately 1 cm spatial displacement). Time-series of direct-path relative arrival-times are constructed for each VLA element across the 11 min observation interval. The overall structure of these time-series compares favorably with that predicted from an array element localization model. The short-term standard deviations calculated on the direct-path (7 μs) and bottom-reflected-path (17 μs) time-series are in agreement with the predicted arrival-time accuracies. The implications of these observed arrival-time accuracies in the context of estimating sound speed perturbations and bottom-depth are discussed.

Giglio, D, Roemmich D, Cornuelle B.  2013.  Understanding the annual cycle in global steric height. Geophysical Research Letters. 40:4349-4354.   10.1002/grl.50774   AbstractWebsite

Steric variability in the ocean includes diabatic changes in the surface layer due to air-sea buoyancy fluxes and adiabatic changes due to advection, which are dominant in the subsurface ocean. Here the annual signal in subsurface steric height (eta' below 200 db) is computed on a global scale using temperature and salinity profiles from Argo floats. The zonal average of over a season (e.g., eta'(March) - eta'(December)) is compared to the wind-forced vertical advection contribution (Delta eta'(w)) both in the global ocean and in different basins. The results show agreement that extends beyond the tropics. The estimate of Delta eta'(w) is based on the Ekman pumping and assumes that the seasonal vertical velocity is constant over the depth range of interest. This assumption is consistent with annual isopycnal displacements inferred from Argo profiles. The contribution of horizontal advection to Delta eta' is significant in some regions and consistent with differences between Delta eta' and Delta eta'(w).

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.

Gopalakrishnan, G, Hoteit I, Cornuelle BD, Rudnick DL.  2019.  Comparison of 4DVAR and EnKF state estimates and forecasts in the Gulf of Mexico. Quarterly Journal of the Royal Meteorological Society. 145:1354-1376.   10.1002/qj.3493   AbstractWebsite

An experiment is conducted to compare four-dimensional variational (4DVAR) and ensemble Kalman filter (EnKF) assimilation systems and their predictability in the Gulf of Mexico (GoM) using the Massachusetts Institute of Technology general circulation model (MITgcm). The quality of the ocean-state estimates, forecasts, and the contribution of ensemble prediction are evaluated. The MITgcm-Estimating the Circulation and Climate of the Ocean (ECCO) 4DVAR (MITgcm-ECCO) and the MITgcm-Data Assimilation Research Testbed (DART) EnKF (MITgcm-DART) systems were used to compute two-month hindcasts (March-April, 2010) by assimilating satellite-derived along-track sea-surface height (SSH) and gridded sea-surface temperature (SST) observations. The estimates from both methods at the end of the hindcast period were then used to initialize forecasts for two months (May-June, 2010). This period was selected because a loop current (LC) eddy (Eddy Franklin: Eddy-F) detachment event occurred at the end of May 2010, immediately after the Deepwater Horizon (DwH) oil spill. Despite some differences between the setups, both systems produce analyses and forecasts of comparable quality and both solutions significantly outperformed model persistence. A reference forecast initialized from the 1/12 degrees Hybrid Coordinate Ocean Model (HYCOM)/NCODA global analysis also performed well. The EnKF experiments for sensitivity to filter parameters showed enhanced predictability when using more ensemble members and stronger covariance localization, but not for larger inflation. The EnKF experiments varying the number of assimilation cycles showed enhanced short-term (long-term) predictability with fewer (more) assimilation cycles. Additional hindcast and forecast experiments at other times of significant LC evolution showed mixed performance of both systems, which depends strongly on the background state of the GoM circulation. The present work demonstrates a practical application of both assimilation methods for the GoM and compares them in a limited number of realizations. The overall conclusion showing improved short-term (long-term) predictability for EnKF (4DVAR) carries an important caveat that the results from this study are specific to a few 4DVAR and EnKF LC eddy separation experiments in the GoM and cannot be generalized to conclude the relative performance of both methods, especially in other applications. However, some of the concepts and methods should carry over to other applications.

Gopalakrishnan, G, Cornuelle BD, Hoteit I.  2013.  Adjoint sensitivity studies of loop current and eddy shedding in the Gulf of Mexico. Journal of Geophysical Research: Oceans. 118:3315-3335.   10.1002/jgrc.20240   AbstractWebsite

Adjoint model sensitivity analyses were applied for the loop current (LC) and its eddy shedding in the Gulf of Mexico (GoM) using the MIT general circulation model (MITgcm). The circulation in the GoM is mainly driven by the energetic LC and subsequent LC eddy separation. In order to understand which ocean regions and features control the evolution of the LC, including anticyclonic warm-core eddy shedding in the GoM, forward and adjoint sensitivities with respect to previous model state and atmospheric forcing were computed using the MITgcm and its adjoint. Since the validity of the adjoint model sensitivities depends on the capability of the forward model to simulate the real LC system and the eddy shedding processes, a 5 year (2004–2008) forward model simulation was performed for the GoM using realistic atmospheric forcing, initial, and boundary conditions. This forward model simulation was compared to satellite measurements of sea-surface height (SSH) and sea-surface temperature (SST), and observed transport variability. Despite realistic mean state, standard deviations, and LC eddy shedding period, the simulated LC extension shows less variability and more regularity than the observations. However, the model is suitable for studying the LC system and can be utilized for examining the ocean influences leading to a simple, and hopefully generic LC eddy separation in the GoM. The adjoint sensitivities of the LC show influences from the Yucatan Channel (YC) flow and Loop Current Frontal Eddy (LCFE) on both LC extension and eddy separation, as suggested by earlier work. Some of the processes that control LC extension after eddy separation differ from those controlling eddy shedding, but include YC through-flow. The sensitivity remains stable for more than 30 days and moves generally upstream, entering the Caribbean Sea. The sensitivities of the LC for SST generally remain closer to the surface and move at speeds consistent with advection by the high-speed core of the current, while sensitivities to SSH generally extend to deeper layers and propagate more slowly. The adjoint sensitivity to relative vorticity deduced from the sensitivities to velocity fields suggests that advection of cyclonic (positive) relative vorticity anomalies from the YC or the LCFEs accelerate the LC eddy separation. Forward model perturbation experiments were performed to complement and check the adjoint sensitivity analysis as well as sampling the predictability and nonlinearity of the LC evolution. The model and its adjoint can be used in four-dimensional variational assimilation (4D-VAR) to produce dynamically consistent ocean state estimates for analysis and forecasts of the circulation of the GoM.

Gopalakrishnan, G, Cornuelle BD, Hoteit I, Rudnick DL, Owens BW.  2013.  State estimates and forecasts of the loop current in the Gulf of Mexico using the MITgcm and its adjoint. Journal of Geophysical Research: Oceans. 118:3292-3314.   10.1002/jgrc.20239   AbstractWebsite

An ocean state estimate has been developed for the Gulf of Mexico (GoM) using the MIT general circulation model and its adjoint. The estimate has been tested by forecasting loop current (LC) evolution and eddy shedding in the GoM. The adjoint (or four-dimensional variational) method was used to match the model evolution to observations by adjusting model temperature and salinity initial conditions, open boundary conditions, and atmospheric forcing fields. The model was fit to satellite-derived along-track sea surface height, separated into temporal mean and anomalies, and gridded sea surface temperature for 2 month periods. The optimized state at the end of the assimilation period was used to initialize the forecast for 2 months. Forecasts explore practical LC predictability and provide a cross-validation test of the state estimate by comparing it to independent future observations. The model forecast was tested for several LC eddy separation events, including Eddy Franklin in May 2010 during the deepwater horizon oil spill disaster in the GoM. The forecast used monthly climatological open boundary conditions, atmospheric forcing, and run-off fluxes. The model performance was evaluated by computing model-observation root-mean-square difference (rmsd) during both the hindcast and forecast periods. The rmsd metrics for the forecast generally outperformed persistence (keeping the initial state fixed) and reference (forecast initialized using assimilated Hybrid Coordinate Ocean Model 1/12° global analysis) model simulations during LC eddy separation events for a period of 1̃2 months.

Gopalakrishnan, G, Cornuelle BD, Gawarkiewicz G, McClean JL.  2013.  Structure and evolution of the cold dome off northeastern Taiwan: A numerical study. Oceanography. 26:66-79. Abstract

Numerous observational and modeling studies of ocean circulation surrounding Taiwan have reported occurrences of cold water and doming of isotherms (called the cold dome) that result in the formation of coastal upwelling on the northeastern Taiwan shelf. We use a high-resolution (1/24°) ocean model based on the Massachusetts Institute of Technology general circulation model to study the evolution of this distinct shelf-slope circulation phenomenon. We performed a number of model simulations spanning a five-year period (2004–2008) using realistic atmospheric forcing and initial and open boundary conditions. The model solutions were compared with satellite measurements of sea surface height (SSH), sea surface temperature (SST), and historical temperature and salinity observations. The model showed a realistically shaped cold dome with a diameter of ~ 100 km and temperature of ~ 3°C below the ambient shelf waters at 50 m depth. The occurrences of simulated cold dome events appeared to be connected with the seasonal variability of the Kuroshio Current. The model simulations showed more upwelling events during spring and summer when the core of the Kuroshio tends to migrate away from the east coast of Taiwan, compared to fall and winter when the core of the Kuroshio is generally found closer to the east coast of Taiwan. The model also reproduced weak cyclonic circulation associated with the upwelling off northeastern Taiwan. We analyzed the spatio-temporal variability of the cold dome using the model solution as a proxy and designed a "cold dome index" based on the temperature at 50 m depth averaged over a 0.5° × 0.5° box centered at 25.5°N, 122°E. The cold dome index correlates with temperature at 50 m depth in a larger region, suggesting the spatial extent of the cold dome phenomenon. The index had correlation maxima of 0.78 and 0.40 for simulated SSH and SST, respectively, in and around the cold dome box region, and we hypothesize that it is a useful indicator of upwelling off northeastern Taiwan. In addition, both correlation and composite analysis between the temperature at 50 m depth and the East Taiwan Channel transport showed no cold dome events during low-transport events (often in winter) and more frequent cold dome events during high-transport events (often in summer). The simulated cold dome events had time scales of about two weeks, and their centers aligned roughly along a northeastward line starting from the northeastern tip of Taiwan.