Acoustic Thermometry of Ocean Climate (ATOC)

Kauai Timeseries

The Kauai acoustic source began transmitting in late 1997, continuing through Fall 1999. After a hiatus of 2 years while marine mammal permitting issues were sorted out, the Kauai acoustic source resumed transmissions in January 2002. Transmissions are expected to continue for another 5 years at regular 4-day intervals. Timeseries obtained from the Kauai source trasmissions compared to TOPEX/POSEIDON data and the ECCO ocean model state estimates may be seen HERE. The costs for aquiring these data are about $150K per year, with perhaps another $150K per year for data archiving, reduction, and analysis (and posting the data here).

See the references below for more information.

Time series last updated November 2005.

A brief description of the time series and data assimilation plans as a CLIVAR Exchanges Note (1.4 MB). Published March 2003.

Pioneer Timeseries

The Pioneer Seamount acoustic source began transmissions in early 1996. It continued to transmit for about 24 months at irregular intervals in accord with marine mammal research protocols. This acoustic source has now been removed in accordance with marine mammal permitting agreements. Timeseries obtained from the Pioneer source trasmissions may be seen HERE.

See the references below for more information.

Last updated January 2003.

ATOC

ATOC is directed at using the travel time data obtained from a few acoustic sources and receivers located throughout the North Pacific basin to study the climatic variability of the thermal field at the largest scale (The ATOC Consortium, 1998). The ATOC project has now completed several important phases. An acoustic source off the coast of California (Pioneer Seamount) began transmissions in early 1996; this source transmitted for about 24 months at irregular intervals in accord with marine mammal research protocols. A second acoustic source north of the Hawaiian island of Kauai transmitted signals from late 1997 through Fall 1999. Transmissions from the Kauai source began again in January 2002, and they are expected to continue at regular 4-day intervals for another 5 years. The acoustic receivers are those of opportunity such as the U.S. Navy SOSUS receivers, as well as two dedicated vertical line arrays that were located near Hawaii and Kiritimati. In addition, signals transmitted from the California source were detected by a temporary receiver (a single hydrophone) located to the east of the North Island of New Zealand at 10 Mm range (Tindle and Bold, 1999).

The data obtained during ATOC have shown that acoustic ray arrivals may be resolved and identified to at least 5-Mm range so that these data can be used for acoustic thermometry. The estimated uncertainties in the range- and depth-averaged temperature measurements were about 10 m°C. The time series obtained using the California acoustic source showed a clear annual cycle whose amplitude was similar to that derived from climatology (World Ocean Atlas 94: Levitus et al., 1994; Levitus and Boyer, 1994) and from XBTs of opportunity, yet smaller than the amplitude of the annual cycle derived from TOPEX/POSEIDON altimetry (The ATOC Consortium, 1998). The time series obtained using the Kauai acoustic source is of similar quality in its ability to measure the thermal variability, but it shows greater variability at 100-day timescales. Focus of the ATOC research has recently shifted from establishing the integrity of the acoustical measurements (Dushaw et al., 1999; Dushaw, 1999; Worcester et al., 1999) to employing the data oceanographically. One original and continuing goal of the ATOC program is to use the line-integral data to detect patterns of climate variability such as the Pacific Decadal Oscillation (PDO) (Latif and Barnett, 1994).

Tomographic measurements can be made without risk of calibration drift; these measurements have the accuracy and precision required for large-scale ocean climate observation. Tomographic measurements directly probe the existence and nature of signals at the lowest wavenumbers. These data are also sensitive to variability almost to the ocean bottom, and thus can detect changes below the depths at which XBT and float data are obtained. The impact of the integral measurements on the quality of ocean estimation using numerical models remains an open question, however. This impact is best assessed when all data types - tomography, ARGO, XBTs, TOPEX, etc., are used. The transbasin acoustic measurements may offer a signal-to-noise capability for ocean climate variability that is not readily attainable by an ensemble of point measurements.

At this time, greater coverage of the world's oceans by acoustic tomography can be implemented at an annual cost that is no greater than other observational approaches. The major costs of tomography are the initial capital costs of the instrumentation and its installation. Efforts are currently under way to reduce the costs of sources and receivers. Once the instruments have been installed, however, the operational costs to make continuing measurements are low. The amortized cost of the technique is therefore attractive, even using present-day source and receiver technology.

A prototypical array for acoustic thermometry that spans the globe. Instruments for this array consist of 20 nominal locations for permanent DEOS buoys and the existing ATOC instruments in the North Pacific. An acoustic array of this type would be greatly augmented by simple, cheap receivers deployed either nearshore or on moorings of opportunity.

Marine Mammals and Acoustic Tomography. The ATOC project included a Marine Mammal Research Program (MMRP) to study the potential effects, if any, of the ATOC sound sources on marine mammals and other marine life. The MMRP did not find any overt or obvious short-term changes in the distribution, abundance, behavior, or vocalizations of marine mammals in response to the playback of ATOC-like sounds or in response to the transmissions of the ATOC sound sources themselves. No species vacated the areas around the sound sources during transmissions. Statistical analyses of the data showed some subtle, but statistically significant, shifts in the distribution of humpback (and possibly sperm) whales during transmission periods, as well as some subtle changes in the behavior of humpback whales. The MMRP investigators concluded that these subtle effects would not adversely impact the survival of an individual whale or the status of the North Pacific marine mammal populations.

Nonetheless, monitoring of the distribution and abundance of marine mammals around the Kauai source is planned as part of the proposed continuing operation of that source over the next five years, to look for possible longer-term changes in distribution and abundance, if any. The ATOC source provides one of the few controlled sound sources available for such longer-term studies.

References:

The ATOC Consortium, 1998. Ocean Climate Change: Comparison of Acoustic Tomography, Satellite Altimetry, and Modeling, Science, 281, 1327-1332.

Dushaw, B. D., G. Bold, C.-S. Chui, J. Colosi, B. Cornuelle, Y. Desaubies, M. Dzieciuch, A. Forbes, F. Gaillard, J. Gould, B. Howe, M. Lawrence, J. Lynch, D. Menemenlis, J. Mercer, P. Mikhaelvsky, W. Munk, I. Nakano, F. Schott, U. Send, R. Spindel, T. Terre, P. Worcester, and C. Wunsch, 2001. "Observing the ocean in the 2000's: A strategy for the role of acoustic tomography in ocean climate observation" in Observing the Oceans in the 21st Century, edited by C. J. Koblinsky and N. R. Smith (GODAE Project Office and Bureau of Meteorology, Melbourne), pp. 391-418. Download pdf file (7.0 MB)

Dushaw, B., and P. Worcester, Acoustic remote sensing of the North Pacific on gyre and regional scales, Pacific CLIVAR International Implementation Workshop, International Pacific Research Center at the University of Hawaii, Honolulu, Hawaii, February 5-8, 2001, 9 pp. Download pdf file (2.1 MB)

Dushaw, B. D., and the ATOC Group, 1999. A comparison of acoustic thermometry, XBT, TOPEX, and HOT observations of ocean temperature in the Northeast Pacific Ocean, Proc. of the Intl. Conf. on the Ocean Observing System for Climate (OCEANOBS99), St. Raphael, France, 18-22 October 1999, 2 pp. Download pdf file (91 KB)

Dushaw, B. D. and the ATOC group, The Acoustic Thermometry of Ocean Climate (ATOC) Project: Towards depth-averaged temperature maps of the North Pacific Ocean, Proc. of the Intl. Symp. on Acoustic Tomography and Thermometry, Tokyo, Japan, 8-9 February 1999, 10 pp. Download pdf file (1.7 MB)

Dushaw, B. D., B. M. Howe, J. A. Mercer, R. C. Spindel, and the ATOC Group, 1999. Multimegameter range acoustic data obtained by bottom-mounted hydrophone arrays for measurement of ocean temperature. IEEE J. Oceanic Engineering, 24, 202-214.

Dushaw, B. D., 1999. Inversion of multimegameter range acoustic data for ocean temperature. IEEE J. Oceanic Engineering, 24, 215-223.

Dushaw, B. D., P. F. Worcester, B. D. Cornuelle, and B. M. Howe, 1993. Variability of heat content in the central North Pacific in summer 1987 determined from long-range acoustic transmissions. J. Phys. Oceanogr., 23, 2650-2666.

Dushaw, B. D., P. F. Worcester, B. D. Cornuelle, and B. M. Howe, 1993. On equations for the speed of sound in seawater. J. Acoust. Soc. Am., 93, 255-275.

Worcester, P. F., B. D. Dushaw and the ATOC Group, A comparison of acoustic thermometry, satellite altimetry, and other observations of ocean temperature in the North Pacific Ocean, Proc. 5th Pacific Ocean Remote Sensing Conf. (PORSEC), Goa, India, 5-8 December 2000, vol. I, 1-4. Download pdf file (250 KB)

ATOC poster presented at the 1999 OceanObs and TOPEX/POSEIDON Science Team meetings

Entitled: "A Comparison of Acoustic Thermometry, TOPEX, XBT and HOT Observations of Ocean Temperature in the Northeast Pacific Ocean"
Download poster (1 MB) (a large sized poster).

ATOC poster presented at the 2002 WOCE meeting, San Antonio, TX

Entitled: "Acoustic thermometry time series in the North Pacific Ocean"
Download poster (3.7 MB) (a large sized poster).