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Agnon, Y, Malanotterizzoli P, Cornuelle BD, Spiesberger JL, Spindel RL.  1989.  The 1984 bottom-mounted Gulf Stream tomographic experiment. Journal of the Acoustical Society of America. 85:1958-1966.   10.1121/1.397849   AbstractWebsite

In this paper, data from a Gulf Stream tomographic experiment carried out in October 1984 are analyzed. The experiment used acoustic sources and receivers bottom mounted beneath the stream to measure Gulf Stream dynamics. However, due to an unfortunate electronic malfunction of the source, only 2 days of acoustically measured travel time data are available. Nevertheless, some new and positive results are obtained. Bottom reflected acoustic rays having up to two bottom bounces are unambiguously identified by solving the direct problem of tracing rays both in a reference climatological profile and in actual range‐dependent sound‐speed sections from a hydrographic survey carried out during the experiment. It is also shown that these rays do not appear to be affected by important nonlinearities so that they can be used to provide consistent results in inverse solutions.

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Tiemann, CO, Worcester PF, Cornuelle BD.  2001.  Acoustic remote sensing of internal solitary waves and internal tides in the Strait of Gibraltar. Journal of the Acoustical Society of America. 110:798-811.   10.1121/1.1382617   AbstractWebsite

High-frequency underwater acoustic transmissions across the Strait of Gibraltar are used to examine the feasibility of acoustically measuring several physical processes in the Strait, a difficult area to sample with conventional instruments. Internal undular bores propagate along the interface between an upper layer of Atlantic water and a lower layer of Mediterranean water. As they cross the acoustic path they are recognized by their scattering effects in the acoustic record. The time between internal bore crossings is influenced more by the tidal phase of the bore release at the Camarinal Sill than by variability in the bore's propagation time to the acoustic path. When internal bores were present, the acoustic arrival patterns could be classified as one of three types with different internal bore and internal tide amplitudes. The arrival types alternate during spring to neap tide transitions, suggesting that internal bore amplitude is not linearly related to tidal height. The sensitivity of acoustic observables to several physical parameters is investigated using a forward model, and a demonstration of inverse techniques provides estimates of several physical parameters from spring tidal cycles. (C) 2001 Acoustical Society of America.

Tiemann, CO, Worcester PF, Cornuelle BD.  2001.  Acoustic scattering by internal solitary waves in the Strait of Gibraltar. Journal of the Acoustical Society of America. 109:143-154.   10.1121/1.1329624   AbstractWebsite

High-freqnency underwater acoustic transmissions across the Strait of Gibraltar were used to examine acoustic scattering caused by the unique internal wave field in the Strait. Internal solitary waves of 100 m in amplitude propagate along the interface between an upper layer of Atlantic water and a lower layer of Mediterranean water. The interface is also strongly modulated by internal tides of comparable amplitude. As internal solitary waves cross the acoustic path, they cause sharp soundspeed gradients which intermittently refract acoustic rays away from normal sound channels. Internal tides vertically shift soundspeed profiles for additional travel time variability. Although the acoustic scattering is quite complicated, it is also surprisingly robust, making it a good candidate for modeling. Key features of the acoustic arrival pattern can be accounted for in some detail by a model description of the complex hydraulics in the Strait. (C) 2001 Acoustical Society of America. [DOI: 10.1121/1.1329624].

Cornuelle, BD.  1982.  Acoustic Tomography. IEEE Transactions on Geoscience and Remote Sensing. 20:326-332.   10.1109/tgrs.1982.350450   AbstractWebsite

High-resolution measurements of the density field in the ocean are prohibitively expensive if traditional ship-borne instruments are used. Tomography uses acoustic remote sensing to infer ocean structure, and avoids many of the limitations of direct measurements. Sound pulses follow distinct trajectories through the water from source to receiver, and the travel time for a given pulse is a known functional of the sound speed field. This functional can be inverted to recover an estimate of the sound speed field through which it passed. The inversion is accomplished with either detenninistic linear matrix inversion or stochastic optimal estimation, and the sound speed field estimate returned can be converted to an estimate of density. A numerical simulation of the pilot tomography experiment is presented to demonstrate that tomography can be effective in reproducing significant ocean features.

Song, H, Hoteit I, Cornuelle BD, Subramanian AC.  2010.  An Adaptive Approach to Mitigate Background Covariance Limitations in the Ensemble Kalman Filter. Monthly Weather Review. 138:2825-2845.   10.1175/2010mwr2871.1   AbstractWebsite

A new approach is proposed to address the background covariance limitations arising from undersampled ensembles and unaccounted model errors in the ensemble Kalman filter (EnKF). The method enhances the representativeness of the EnKF ensemble by augmenting it with new members chosen adaptively to add missing information that prevents the EnKF fromfully fitting the data to the ensemble. The vectors to be added are obtained by back projecting the residuals of the observation misfits from the EnKF analysis step onto the state space. The back projection is done using an optimal interpolation (OI) scheme based on an estimated covariance of the subspace missing from the ensemble. In the experiments reported here, the OI uses a stationary background covariance matrix, as in the hybrid EnKF-three-dimensional variational data assimilation (3DVAR) approach, but the resulting correction is included as a new ensemble member instead of being added to all existing ensemble members. The adaptive approach is tested with the Lorenz-96 model. The hybrid EnKF-3DVAR is used as a benchmark to evaluate the performance of the adaptive approach. Assimilation experiments suggest that the new adaptive scheme significantly improves the EnKF behavior when it suffers from small size ensembles and neglected model errors. It was further found to be competitive with the hybrid EnKF-3DVAR approach, depending on ensemble size and data coverage.

Hursky, P, Porter MB, Cornuelle BD, Hodgkiss WS, Kuperman WA.  2004.  Adjoint modeling for acoustic inversion. Journal of the Acoustical Society of America. 115:607-619.   10.1121/1.1636760   AbstractWebsite

The use of adjoint modeling for acoustic inversion is investigated. An adjoint model is derived from a linearized forward propagation model to propagate data-model misfit at the observation points back through the medium to the medium perturbations not being accounted for in the model. This adjoint model can be used to aid in inverting for these unaccounted medium perturbations. Adjoint methods are being applied to a variety of inversion problems, but have not drawn much attention from the underwater acoustic community. This paper presents an application of adjoint methods to acoustic inversion. Inversions are demonstrated in simulation for both range-independent and range-dependent sound speed profiles using the adjoint of a parabolic equation model. Sensitivity and error analyses are discussed showing how the adjoint model enables calculations to be performed in the space of observations, rather than the often much larger space of model parameters. Using an adjoint model enables directions of steepest descent in the model parameters (what we invert for) to be calculated using far fewer modeling runs than if a forward model only were used. (C) 2004 Acoustical Society of America.

Moore, AM, Arango HG, Di Lorenzo E, Miller AJ, Cornuelle BD.  2009.  An Adjoint Sensitivity Analysis of the Southern California Current Circulation and Ecosystem. Journal of Physical Oceanography. 39:702-720.   10.1175/2008jpo3740.1   AbstractWebsite

Adjoint methods of sensitivity analysis were applied to the California Current using the Regional Ocean Modeling Systems (ROMS) with medium resolution, aimed at diagnosing the circulation sensitivity to variations in surface forcing. The sensitivities of coastal variations in SST, eddy kinetic energy, and baroclinic instability of complex time-evolving flows were quantified. Each aspect of the circulation exhibits significant interannual and seasonal variations in sensitivity controlled by mesoscale circulation features. Central California SST is equally sensitive to wind stress and surface heat flux, but less so to wind stress curl, displaying the greatest sensitivity when upwelling-favorable winds are relaxing and the least sensitivity during the peak of upwelling. SST sensitivity is typically 2-4 times larger during summer than during spring, although larger variations occur during some years. The sensitivity of central coast eddy kinetic energy to surface forcing is constant on average throughout the year. Perturbations in the wind that align with mesoscale eddies to enhance the strength of the circulation by local Ekman pumping yield the greatest sensitivities. The sensitivity of the potential for baroclinic instability is greatest when nearshore horizontal temperature gradients are largest, and it is associated with variations in wind stress concentrated along the core of the California Current. The sensitivity varies by a factor of similar to 1.5 throughout the year. A new and important aspect of this work is identification of the complex flow dependence and seasonal dependence of the sensitivity of the ROMS California Current System (CCS) circulation to variations in surface forcing that was hitherto not previously appreciated.

Zhang, XB, Cornuelle B, Roemmich D.  2011.  Adjoint Sensitivity of the Nino-3 Surface Temperature to Wind Forcing. Journal of Climate. 24:4480-4493.   10.1175/2011jcli3917.1   AbstractWebsite

The evolution of sea surface temperature (SST) over the eastern equatorial Pacific plays a significant role in the intense tropical air-sea interaction there and is of central importance to the El Nino-Southern Oscillation (ENSO) phenomenon. Effects of atmospheric fields (especially wind stress) and ocean state on the eastern equatorial Pacific SST variations are investigated using the Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint model, which can calculate the sensitivities of a cost function (in this case the averaged 0-30-m temperature in the Nino-3 region during an ENSO event peak) to previous atmospheric forcing fields and ocean state going backward in time. The sensitivity of the Nino-3 surface temperature to monthly zonal wind stress in preceding months can be understood by invoking mixed layer heat balance, ocean dynamics, and especially linear equatorial wave dynamics. The maximum positive sensitivity of the Nino-3 surface temperature to local wind forcing usually happens similar to 1-2 months before the peak of the ENSO event and is hypothesized to be associated with the Ekman pumping mechanism. In model experiments, its magnitude is closely related to the subsurface vertical temperature gradient, exhibiting strong event-to-event differences with strong (weak) positive sensitivity during La Nina (strong El Nino) events. The adjoint sensitivity to remote wind forcing in the central and western equatorial Pacific is consistent with the standard hypothesis that the remote wind forcing affects the Nino-3 surface temperature indirectly by exciting equatorial Kelvin and Rossby waves and modulating thermocline depth in the Nino-3 region. The current adjoint sensitivity study is consistent with a previous regression-based sensitivity study derived from perturbation experiments. Finally, implication for ENSO monitoring and prediction is also discussed.

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.

Song, HJ, Hoteit I, Cornuelle BD, Luo XD, Subramanian AC.  2013.  An adjoint-based adaptive ensemble Kalman filter. Monthly Weather Review. 141:3343-3359. AbstractWebsite

A new hybrid ensemble Kalman filter/four-dimensional variational data assimilation (EnKF/4D-VAR) approach is introduced to mitigate background covariance limitations in the EnKF. The work is based on the adaptive EnKF (AEnKF) method, which bears a strong resemblance to the hybrid EnKF/three-dimensional variational data assimilation (3D-VAR) method. In the AEnKF, the representativeness of the EnKF ensemble is regularly enhanced with new members generated after back projection of the EnKF analysis residuals to state space using a 3D-VAR [or optimal interpolation (OI)] scheme with a preselected background covariance matrix. The idea here is to reformulate the transformation of the residuals as a 4D-VAR problem, constraining the new member with model dynamics and the previous observations. This should provide more information for the estimation of the new member and reduce dependence of the AEnKF on the assumed stationary background covariance matrix. This is done by integrating the analysis residuals backward in time with the adjoint model. Numerical experiments are performed with the Lorenz-96 model under different scenarios to test the new approach and to evaluate its performance with respect to the EnKF and the hybrid EnKF/3D-VAR. The new method leads to the least root-mean-square estimation errors as long as the linear assumption guaranteeing the stability of the adjoint model holds. It is also found to be less sensitive to choices of the assimilation system inputs and parameters.

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.

Roux, P, Kuperman WA, Cornuelle BD, Aulanier F, Hodgkiss WS, Song HC.  2013.  Analyzing sound speed fluctuations in shallow water from group-velocity versus phase-velocity data representation. Journal of the Acoustical Society of America. 133:1945-1952.   10.1121/1.4792354   AbstractWebsite

Data collected over more than eight consecutive hours between two source-receiver arrays in a shallow water environment are analyzed through the physics of the waveguide invariant. In particular, the use of vertical arrays on both the source and receiver sides provides source and receiver angles in addition to travel-times associated with a set of eigenray paths in the waveguide. From the travel-times and the source-receiver angles, the eigenrays are projected into a group-velocity versus phase-velocity (Vg-Vp) plot for each acquisition. The time evolution of the Vg-Vp representation over the 8.5-h long experiment is discussed. Group speed fluctuations observed for a set of eigenrays with turning points at different depths in the water column are compared to the Brunt-Vaisala frequency. (C) 2013 Acoustical Society of America.

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.

Kim, SY, Cornuelle BD, Terrill EJ.  2009.  Anisotropic Response of Surface Currents to the Wind in a Coastal Region. Journal of Physical Oceanography. 39:1512-1533.   10.1175/2009JPO4013.1   Abstract

Analysis of coastal surface currents measured off the coast of San Diego for two years suggests an anisotropic and asymmetric response to the wind, probably as a result of bottom/coastline boundary effects, including pressure gradients. In a linear regression, the statistically estimated anisotropic response explains approximately 20% more surface current variance than an isotropic wind-ocean response model. After steady wind forcing for three days, the isotropic surface current response veers 42 degrees +/- 2 degrees to the right of the wind regardless of wind direction, whereas the anisotropic analysis suggests that the upcoast (onshore) wind stress generates surface currents with 10 degrees +/- 4 degrees (71 degrees +/- 3 degrees) to the right of the wind direction. The anisotropic response thus reflects the dominance of alongshore currents in this coastal region. Both analyses yield wind-driven currents with 3%-5% of the wind speed, as expected. In addition, nonlinear isotropic and anisotropic response functions are considered, and the asymmetric current responses to the wind are examined. These results provide a comprehensive statistical model of the wind-driven currents in the coastal region, which has not been well identified in previous field studies, but is qualitatively consistent with descriptions of the current response in coastal ocean models.

Zaba, K, Rudnick DL, Cornuelle B, Gopalakrishnan G, Mazloff M.  2018.  Annual and interannual variability in the California Current System: Comparison of an ocean state estimate with a network of underwater gliders. Journal of Physical Oceanography.   10.1175/jpo-d-18-0037.1   Abstract

A data-constrained state estimate of the southern California Current System (CCS) is presented and compared against withheld CalCOFI data and assimilated glider data over the years 2007-2017. The objective of this comparison is to assess the ability of the California State Estimate (CASE) to reproduce the key physical features of the CCS mean state, annual cycles, and interannual variability along the three sections of the California Underwater Glider Network (CUGN). The assessment focuses on several oceanic metrics deemed most important for characterizing physical variability in the CCS: 50 m potential temperature, 80 m salinity, and 26 kg/m3 isopycnal depth and salinity. In the time-mean, the CASE reproduces large-scale thermohaline and circulation structures, including observed temperature gradients, shoaling isopycnals, and the locations and magnitudes of the equatorward California Current and poleward California Undercurrent. With respect to the annual cycle, the CASE captures the phase and, to a lesser extent, the magnitude of upper ocean warming and stratification in late summer to early fall and of isopycnal heave due to springtime upwelling. The CASE also realistically captures near-surface diapycnal mixing during upwelling season and the semiannual cycle of the California Undercurrent. In terms of interannual variability, the most pronounced signals are the persistent warming and downwelling anomalies of 2014-2016 and a positive isopycnal salinity anomaly that peaked with the 2015-2016 El Niño.

Hoteit, I, Cornuelle B, Kim SY, Forget G, Kohl A, Terrill E.  2009.  Assessing 4D-VAR for dynamical mapping of coastal high-frequency radar in San Diego. Dynamics of Atmospheres and Oceans. 48:175-197.   10.1016/j.dynatmoce.2008.11.005   AbstractWebsite

The problem of dynamically mapping high-frequency (HF) radar radial velocity observations is investigated using a three-dimensional hydrodynamic model of the San Diego coastal region and an adjoint-based assimilation method. The HF radar provides near-real-time radial velocities from three sites covering the region offshore of San Diego Bay. The hydrodynamical model is the Massachusetts Institute of Technology general circulation model (MITgcm) with 1 km horizontal resolution and 40 vertical layers. The domain is centered on Point Loma, extending 117 km offshore and 120 km alongshore. The reference run (before adjustment) is initialized from a single profile of T and S and is forced with wind data from a single shore station and with zero heat and fresh water fluxes. The adjoint of the model is used to adjust initial temperature, salinity, and velocity, hourly temperature, salinity and horizontal velocities at the open boundaries, and hourly surface fluxes of momentum, heat and freshwater so that the model reproduces hourly HF radar radial velocity observations. Results from a small number of experiments suggest that the adjoint method can be successfully used over 10-day windows at coastal model resolution. It produces a dynamically consistent model run that fits HF radar data with errors near the specified uncertainties. In a test of the forecasting capability of the San Diego model after adjustment, the forecast skill was shown to exceed persistence for up to 20 h. (C) 2008 Elsevier B.V. All rights reserved.

Kim, SY, Terrill EJ, Cornuelle BD.  2009.  Assessing Coastal Plumes in a Region of Multiple Discharges: The US-Mexico Border. Environmental Science & Technology. 43:7450-7457.   10.1021/es900775p   AbstractWebsite

The San Diego/Tijuana border region has several environmental challenges with regard to assessing water quality impacts resulting from local coastal ocean discharges for which transport is not hindered by political boundaries. While an understanding of the fate and transport of these discharged plumes has a broad audience, the spatial and temporal scales of the physical processes present numerous challenges in conducting assessment with any fidelity. To address these needs, a data-driven model of the transport of both shoreline and offshore discharges is developed and operated in a hindcast mode for a four-year period to analyze regional connectivity between the discharges and the receiving of waters and the coastline. The plume exposure hindcast model is driven by surface current data generated by a network of high-frequency radars. Observations provided by both boat-based CTD measurements and fixed oceanographic moorings are used with the Roberts-Snyder-Baumgartner model to predict the plume rise height. The surface transport model outputs are compared with shoreline samples of fecal indicator bacteria (FIB), and the skill of the model to assess low water quality is evaluated using receiver operating characteristic (ROC) analysis.

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.

Crosby, SC, Cornuelle BD, O'Reilly WC, Guza RT.  2017.  Assimilating Global Wave Model Predictions and Deep-Water Wave Observations in Nearshore Swell Predictions. Journal of Atmospheric and Oceanic Technology. 34:1823-1836.   10.1175/jtech-d-17-0003.1   AbstractWebsite

Nearshore wave predictions with high resolution in space and time are needed for boating safety, to assess flood risk, and to support nearshore processes research. This study presents methods for improving regional nearshore predictions of swell-band wave energy (0.04-0.09 Hz) by assimilating local buoy observations into a linear wave propagation model with a priori guidance from global WAVEWATCH III (WW3) model predictions. Linear wave propagation, including depth-induced refraction and shoaling, and travel time lags, is modeled with self-adjoint backward ray tracing techniques. The Bayesian assimilation yields smooth, high-resolution offshore wave directional spectra that are consistent with WW3, and with offshore and local buoy observations. Case studies in the Southern California Bight (SCB) confirm that the nearshore predictions at independent (nonassimilated) buoy sites are improved by assimilation compared with predictions driven with WW3 or with a single offshore buoy. These assimilation techniques, valid in regions and frequency bands where wave energy propagation is mostly linear, use significantly less computational resources than nonlinear models and variational methods, and could be a useful component of a larger regional assimilation program. Where buoy locations have historically been selected to meet local needs, these methods can aid in the design of regional buoy arrays by quantifying the regional skill improvement for a given buoy observation and identifying both high-value and redundant observations. Assimilation techniques also identify likely forward model error in the Santa Barbara Channel, where permanent observations or model corrections are needed.

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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.  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.

Miller, AJ, Neilson DJ, Luther DS, Hendershott MC, Cornuelle BD, Worcester PF, Dzieciuch MA, Dushaw BD, Howe BM, Levin JC, Arango HG, Haidvogel DB.  2007.  Barotropic Rossby wave radiation from a model Gulf Stream. Geophysical Research Letters. 34   10.1029/2007gl031937   AbstractWebsite

The barotropic Rossby wave field in the North Atlantic Ocean is studied in an eddy-resolving ocean model simulation. The meandering model Gulf Stream radiates barotropic Rossby waves southward through preferred corridors defined by topographic features. The smoother region between the Bermuda Rise and the mid-Atlantic Ridge is a particularly striking corridor of barotropic wave radiation in the 20-50 day period band. Barotropic Rossby waves are also preferentially excited at higher frequencies over the Bermuda Rise, suggesting resonant excitation of topographic Rossby normal modes. The prevalence of these radiated waves suggests that they may be an important energy sink for the equilibrium state of the Gulf Stream.

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Morawitz, WML, Cornuelle BD, Worcester PF.  1996.  A case study in three-dimensional inverse methods: Combining hydrographic, acoustic, and moored thermistor data in the Greenland sea. Journal of Atmospheric and Oceanic Technology. 13:659-679.   10.1175/1520-0426(1996)013<0659:acsitd>2.0.co;2   AbstractWebsite

A variety of measurements, including acoustic travel times, moored thermistor time series, and hydrographic stations, were made in the Greenland Sea during 1988-89 to study the evolution of the temperature held throughout the year. This region is of intense oceanographic interest because it is one of the few areas in the world where open-ocean convection to great depths has been observed. This paper describes how the various data types were optimally combined using linear, weighted least squares inverse methods to provide significantly more information about the ocean than can be obtained from any single data type. The application of these methods requires construction of a reference state, a statistical model of ocean temperature variability relative to the reference state, and an analysis of the differing signal-to-noise ratios of each data type. A time-dependent reference state was constructed from all available hydrographic data, reflecting !he basic seasonal variability and keeping the perturbations sufficiently small so that linear inverse methods are applicable. Smoothed estimates of the vertical and horizontal covariances of the sound speed (temperature) variability were derived separately for summer and winter from all available hydrographic and moored thermistor data. The vertical covariances were normalized before bring decomposed into eigenvectors, so that eigenvectors were optimized to fit a fixed percentage of the variance at every depth. The 12 largest redimensionalized eigenvectors compose the vertical basis of the model. A spectral decomposition of a 40-km correlation scale Gaussian covariance is used as the horizontal basis. The uncertainty estimates provided by the inverse method illustrate the characteristics of each dataset in measuring large-scale features during a diversely sampled time period in the winter of 1989. The acoustic data alone resolve about 70% of the variance in the three-dimensional, 3-day average temperature field. The hydrographic data alone resolve approximately 65% of the variance during the selected period but are much less dense or absent over most of the year. The thermistor array alone resolves from 10% to 65% of the temperature variance, doing better near the surface where the most measurements were taken. The combination of the complete 1988-89 acoustic, hydrographic, and thermistor datasets give three-dimensional temperature and heat content estimates that resolve on average about 90% of the expected variance during this particularly densely sampled time period.

Song, H, Miller AJ, Cornuelle BD, Di Lorenzo E.  2011.  Changes in upwelling and its water sources in the California Current System driven by different wind forcing. Dynamics of Atmospheres and Oceans. 52:170-191.   10.1016/j.dynatmoce.2011.03.001   AbstractWebsite

In the California Current System (CCS), upwelling is one of the most important features that enrich the coastal ecosystem. It is highly dependent on both wind stress and wind stress curl, because they contribute to the upwelling system through Ekman transport away from the coast and Ekman pumping as a result of the surface divergence, respectively. Various wind stress products are known to contain sharply different patterns of wind stress, and well-resolved wind forcing products have been shown to drive stronger upwelling due to their better-resolved wind stress curl in previous studies. However, sensitivities of upwelling to changes in wind stress patterns, and each of their control to the source waters and paths of the upwelling cells, are not yet well known for the CCS. Here we study these effects using the Regional Ocean Modeling System (ROMS) and its adjoint model under idealized wind stress forcing patterns representing three widely-used products in addition to a constant wind stress field (no curl): the NCEP/NCAR Reanalysis, the QuikSCAT satellite observations, and the Regional Spectral Model (RSM) downscaling. Changes in currents and isopycnal patterns during the upwelling season are first studied in ROMS under the four different wind stress fields. The model simulations show that the locations of the core of the equatorward flow and the gradient of the cross-shore isopycnals are controlled by the wind stress curl field. The core of the equatorward flow is found under negative wind stress curl, and a deeper upwelling cell is found as the gradient from positive and negative wind stress curl increases. Source waters for the upwelling in each of the four wind stress patterns are investigated using the ROMS adjoint model. The simulations follow a passive tracer backward in time and track the source waters for upwelling in two key areas of interest: inshore and offshore of the Point Sur region of California. The upwelling source waters depend strongly on the depth of the upwelling cell and the alongshore current location. We further relate these results to recent studies of the observed trends in upwelling favorable winds and consequent wind stress curl changes in the CCS. (C) 2011 Elsevier B.V. All rights reserved.

Villas Bôas, AB, Gille ST, Mazloff MR, Cornuelle BD.  2017.  Characterization of the deep-water surface wave variability in the California current region. Journal of Geophysical Research: Oceans.   10.1002/2017JC013280   Abstract

Surface waves are crucial for the dynamics of the upper ocean not only because they mediate exchanges of momentum, heat, energy, and gases between the ocean and the atmosphere, but also because they determine the sea state. The surface wave field in a given region is set by the combination of local and remote forcing. The present work characterizes the seasonal variability of the deep–water surface wave field in the California Current region, as retrieved from over two decades of satellite altimetry data combined with wave buoys and wave model hindcast (WaveWatch III). In particular, the extent to which the local wind modulates the variability of the significant wave height, peak period, and peak direction is assessed. During spring/summer, regional–scale wind events of up to 10 m/s are the dominant forcing for waves off the California coast, leading to relatively short period waves (8-10 s) that come predominantly from the north–northwest. The wave climatology throughout the California Current region shows average significant wave heights exceeding 2 m during most of the year, which may have implications for the planning and retrieval methods of the Surface Water and Ocean Topography (SWOT) satellite mission.