Relative-time inversions in the Labrador Sea acoustic tomography experiment.

One-year long travel-time data from the second deployment period of the Labrador Sea acoustic tomography experiment are analyzed, using a relative-time matched-peak approach, in order to invert for the sound-speed field and simultaneously solve for an unknown travel-time offset. From the relative-time inversions oceanographic information in terms of vertically averaged temperatures are deduced, yielding satisfactory matching with respect to available independent observations. The estimated offsets can be attributed to differential clock drifts, showing a clear parabolic behaviour over the course of the experiment, reaching maximum deviations from linear clock drift corrections (end-point calibrations) of the order of 50 ms. By applying the estimated second-order corrections to the travel-time data, absolute-time matched-peak inversions can then be performed. The used matched-peak approach accounts for the non-linear behaviour of travel times, which is due to the seasonally variable acoustic propagation conditions in the probed region, and turns out to be an appropriate tool in dealing with unknown travel-time offsets

Open-ocean deep convection explored in the Mediterranean

Open-ocean deep convection is a littleunderstood process occurring in winter in remote areas under hostile observation conditions, for example, in the Labrador and Greenland Seas and near the Antarctic continent. Deep convection is a crucial link in the “Great Ocean Conveyor Belt” [Broecker, 1991], transforming poleward flowing warm surface waters through atmosphere-oceaninteraction into cold equatorward flowing water masses. Understanding its physics, interannual variations, and role in the global thermohaline circulation is an important objective of climate change research. In convection regions, drastic changes in water mass properties and distribution occur on scales of 10–100 km. These changes occur quickly and are difficult to observe with conventional oceanographic techniques. Apart from observing the development of the deep-mixed patch of homogeneous water itself, processes of interest are convective plumes on scales <1 km and vertical velocities of several cm s−1 [Schott et al., 1994] that quickly mix water masses vertically, and instability processes at the rim of the convection region that expedite horizontal exchanges of convected and background water masses [e.g., Gascard, 1978]

Multipurpose acoustic networks in the integrated arctic ocean observing system

The dramatic reduction of sea ice in the Arctic Ocean will increase human activities in the coming years. This activity will be driven by increased demand for energy and the marine resources of an Arctic Ocean accessible to ships. Oil and gas exploration, fisheries, mineral extraction, marine transportation, research and development, tourism, and search and rescue will increase the pressure on the vulnerable Arctic environment. Technologies that allow synoptic in situ observations year-round are needed to monitor and forecast changes in the Arctic atmosphere-ice-ocean system at daily, seasonal, annual, and decadal scales. These data can inform and enable both sustainable development and enforcement of international Arctic agreements and treaties, while protecting this critical environment. In this paper, we discuss multipurpose acoustic networks, including subsea cable components, in the Arctic. These networks provide communication, power, underwater and under-ice navigation, passive monitoring of ambient sound (ice, seismic, biologic, and anthropogenic), and acoustic remote sensing (tomography and thermometry), supporting and complementing data collection from platforms, moorings, and vehicles. We support the development and implementation of regional to basin-wide acoustic networks as an integral component of a multidisciplinary in situ Arctic Ocean observatory

Ocean acoustic tomography based on peak arrivals

The recently introduced notion of peak arrivals [Athanassoulis and Skarsoulis, J. Acoust. Soc. Am. 97, 3575–3588 (1995)], defined as the significant local maxima of the arrival pattern, is studied here as a modeling basis for performing ocean tomography. Peak arrivals constitute direct theoretical counterparts of experimentally observed peaks, and offer a complete modeling of experimental observables, even in cases where ray or modal arrivals cannot be resolved. The coefficients of the resulting peak‐inversion system, relating travel‐time with sound‐speed perturbations, are explicitly calculated in the case of range‐independent environments using normal‐mode theory. To apply the peak‐inversion scheme to tomography the peak identification and tracking problem is examined from a statistical viewpoint; maximum‐likelihood and least‐square solutions are derived and discussed. The particular approach adopted treats the identification and tracking problem in close relation to the inversion procedure; all possibilities of associating observed peaks with background arrivals are examined via trial inversions, and the best peak identification is selected with respect to a least‐square criterion. The feasibility of peak tomography is subsequently demonstrated using first synthetic data and then measured data from the THETIS‐I experiment. In the synthetic case the performance of the overall scheme is found to be satisfactory both with noise‐free and noisy data. Furthermore, the identification, tracking, and inversion results using experimental acoustic data from THETIS‐I are in good agreement with independent field observations

Acoustic thermometry of the western Mediterranean basin

Ocean acoustic tomography is used to obtain heat-content estimates for the western Mediterranean basin. Travel-time data from 13 tomography sections of the Thetis-2 experiment (January–October 1994) are analyzed with a matched-peak inversion approach. The underlying analysis involves the use of peak arrivals and nonlinear model relations between travel-time and sound-speed variations. Slice inversion results are combined with temperature covariance functions for the western Mediterranean to obtain heat-content estimates for the basin. These estimates compare favorably with ECMWF data over the nine-month period of the Thetis-2 experiment. Furthermore, estimates for the basin-average temperature of the western Mediterranean deep water are obtained. © 2004 Acoustical Society of America

Acoustic observations of heat content across the Mediterranean Sea

The ability to monitor the heat content of oceans over long distances is becoming increasingly important for understanding the role of oceans in climate change, for determining the variability of the state of the oceans, for operational ocean observing systems, and for studying large-scale ocean processes such as water-mass formation. Although the properties of the upper layers of the ocean can be routinely measured on large scales by satellite remote sensing (providing altimetric and infrared data) and with expendable probes dropped from commercial vessels, the deep interior of the ocean is more difficult to monitor. Ocean acoustic tomography1 is a promising technique for such applications, as it has the potential to provide systematic, instantaneous and repeated measurements of the ocean interior over large parts of an ocean basin. Here we demonstrate the capability of this technique for measuring the heat content across an entire (albeit small) ocean basin—the western Mediterranean Sea