The Arctic Ocean is undergoing dramatic changes in both the ice cover and ocean structure. Changes in sea ice and the water column affect both acoustic propagation and ambient noise. The implication is that what was learned about Arctic acoustics during the Cold War is now obsolete. Signals at frequencies well above the very low frequencies (≤ 20 Hz) previously required for long-range propagation in the Arctic may now be practical, for example.
In response to these changes, ONR has funded the Canada Basin Acoustic Propagation Experiment (CANAPE), consisting of a yearlong experiment in the Canada Basin of the Arctic Ocean during 2016–2017, preceded by a short Pilot Study in July-August 2015, in order to understand the effects of changing Arctic conditions on low-frequency, deep-water propagation and on the low-frequency ambient noise field. We plan in 2016–2017 to combine measurements of acoustic propagation and ambient noise with the use of acoustic remote sensing methods to help characterize the large-scale sound-speed field in this difficult-to-measure region that is still ice covered during much of the year. The overall goal of the CANAPE experiments is to determine the fundamental limits to signal processing in the Arctic imposed by ice and ocean processes.
THin-ice Arctic Acoustic Window (THAAW)
The Canada Basin is the largest of the subbasins in the Arctic Ocean. In the past, the clockwise and convergent motion of the Beaufort Gyre meant that sea ice tended to be trapped within the Canada Basin, becoming thicker and more ridged than ice elsewhere that was transported by the Transpolar Drift toward Fram Strait. As the conditions in the Arctic have changed, however, the greatest loss of both first-year and multiyear ice has occurred in the Canada Basin. These changing conditions make the Canada Basin an ideal location to test the hypothesis put forward by P. Mikhalevsky (personal communication) that changing Arctic conditions will contribute to a THin-ice Arctic Acoustic Window (THAAW):
- The Arctic is now dominated by 1–2 year ice with reduced pressure ridging, resulting in lower transmission loss and allowing operation at higher frequencies.
- Reduced pressure ridging also results in more frequent periods of low ambient noise.
- Ice cover is still present throughout much of the year, insulating the ocean from wind and solar forcing and preserving the stable Arctic acoustic channel.
The goals of the CANAPE experiments include (1) understanding the impacts of changing sea ice and oceanographic conditions on acoustic propagation and fluctuations; (2) characterizing the depth dependence and temporal variability of the ambient noise field; and (3) measuring the spatial and temporal variability in the upper ocean throughout the annual cycle by combining acoustic and other data with ocean models. Acoustic transmissions are being used to both study acoustic propagation and scattering and help characterize the large-scale oceanographic structure in the Beaufort Gyre.