Low-frequency acoustic propagation loss in the Arctic Ocean: results of the Arctic climate observations using underwater sound experiment

Acoustic data from the Arctic climate observations using underwater sound (ACOUS) experiment are analyzed to determine the correlation between acoustic propagation loss and the seasonal variability of sea ice thickness. The objective of this research is to provide long-term synoptic monitoring of sea ice thickness, an important global climate variable, using acoustic remote sensing. As part of the ACOUS program an autonomous acoustic source deployed northwest of Franz Josef Land transmitted tomographic signals at 20.5 Hz once every four days from October 1998 until December 1999. These signals were received on a vertical array in the Lincoln Sea 1250 km away. Two of the signals transmitted in April 1999 were received on a vertical array at ice camp APLIS in the Chukchi Sea north of Point Barrow, Alaska, at a distance of approximately 2720 km from the source. Temporal variations of the modal propagation loss are examined. The influence of ice parameters, variations of the sound speed profile, and mode-coupling effects on the propagation losses of individual modes is studied. The experimental results are compared to the results of the earlier experiments and the theoretical prediction using numerical modeling

The Transarctic Acoustic Propagation Experiment and Climate Monitoring in the Arctic

In April 1994, coherent acoustic transmissions were propagated across the entire Arctic basin for the first time. This experiment, known as the Transarctic Acoustic Propagation Experiment (TAP), was designed to determine the feasibility of using these signals to monitor changes in Arctic Ocean temper- ature and changes in sea ice thickness and concentration. CW and maximal length sequences (MLS) were transmitted from the source camp located north of the Svalbard Archipelago 1000 km to a vertical line array in the Lincoln Sea and 2600 km to a two-dimensional horizontal array and a vertical array in the Beaufort Sea. TAP demonstrated that the 19.6-Hz 195-dB (251-W) signals propagated with both sufficiently low loss and high phase stability to support the coherent pulse compression processing of the MLS and the phase detection of the CW signals. These yield time-delay measurements an order of magnitude better than what is required to detect the estimated 80-ms/year changes in travel time caused by interannual and longer term changes in Arctic Ocean temperature. The TAP data provided propagation loss measurements to compare with the models to be used for correlating modal scattering losses with sea ice properties for ice monitoring. The travel times measured in TAP indicated a warming of the Atlantic layer in the Arctic of close to 0.4 degrees, which has been confirmed by direct measurement from icebreakers and submarines, demonstrating the utility of acoustic thermometry in the Arctic. The unique advantages of acoustic thermometry in the Arctic and the importance of climate monitoring in the Arctic are discussed. A four-year program, Arctic Climate Observations using Underwater Sound is underway to carry out the first installations of sources and receivers in the Arctic Ocean. ACOUS is a joint project being executed under a bilateral memorandum of understanding with Russia and is part of the Gore-Chernomyrdin (now Gore-Primakov) Commission, Science and Technology Committee