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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.

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.

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.

Vera, MD, Heaney KD, Grp N.  2005.  The effect of bottom interaction on transmissions from the North Pacific Acoustic Laboratory Kauai source. Journal of the Acoustical Society of America. 117:1624-1634.   10.1121/1.1854491   AbstractWebsite

Acoustic signals transmitted from a 75-Hz broadband source near Kauai as part of the North Pacific Acoustic Laboratory (NPAL), experiment were recorded on an array of receivers near California at a range of 3890 km, and on a vertical line array at a range of 3336 km in the Gulf of Alaska. Because the source is approximately 2 m above the seafloor, and the bottom depth at the receivers near California is approximately 1800 m, acoustic interaction with the bathymetry complicates the identification of the recorded arrivals with those present in numerical simulations of the experiment. Ray methods were used to categorize acoustic energy according to interactions with the sea bottom and surface and to examine the significance of seafloor geometry. A modal decomposition was also used to examine the role of range-dependent bathymetry and to associate the effects on the acoustic field with seafloor features at specific ranges. Parabolic-equation simulations were performed in order to investigate the sensitivity of the received signal to geoacoustic parameters; shear excitations within the seafloor were modeled using a complex-density, equivalent-fluid technique. Incorporation of bottom interaction into models of the propagation enables an identification between experimental and simulated arrivals. (c) 2005 Acoustical Society of America.

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.

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.

Roemmich, D, Cornuelle B.  1992.  The Subtropical Mode Waters of the South-Pacific Ocean. Journal of Physical Oceanography. 22:1178-1187.   10.1175/1520-0485(1992)022<1178:tsmwot>;2   AbstractWebsite

The subtropical mode waters (STMW) of the southwestern Pacific Ocean are described, including their physical characteristics, spatial distribution, and temporal variability. STMW is a thermostad, or minimum in stratification, having temperatures of about 15-degrees-19-degrees-C and vertical temperature gradient less than about 2-degrees-C per 100 m. Typical salinity is 35.5 psu at 16.5-degrees-C. The STMW layer is formed by deep mixing and cooling in the eastward-flowing waters of the separated East Australia Current. Surface mixed layers are observed as deep as 300 m north of New Zealand in winter, in the center of a recurring anticyclonic eddy. The STMW thermostad in the South Pacific is considerably weaker than its counterparts in the North Atlantic and North Pacific, a contrast that may help to discriminate between physical processes contributing to its formation. A quarterly time series of expendable bathythermograph transects between New Zealand and Fiji is used to study the temporal variability of STMW. Large fluctuations are observed at both annual and subannual periods. Based on the quarterly census of STMW volume, the lifetime of the thermostad is estimated to be of order 1 year. During the years 1986-91 wintertime sea surface and air temperature minima warmed by about 1.5-degrees-C. The volume of STMW decreased dramatically during that period, with the 1989-91 census showing only a small fraction of the 1986-87 STMW volume. The observed fluctuations may be due either to long-period change in air-sea heat exchange or to fluctuations in heat transport by ocean currents.