Page 58 - Spring2020

Arctic Acoustic Oceanography
Arctic basin margins and ridges, guided by the bathymetry and becoming deeper as it travels. It is present throughout the Arctic Ocean. Data from 1950 to 2010 show that the Atlantic
water core (maximum) temperature steadily warmed begin- ning in the 1970s (Polyakov et al., 2012).
In addition to the changes associated with the warm- ing Atlantic layer, the western Arctic is also influenced by waters entering from the Pacific Ocean through the shallow Bering Strait between Russia and Alaska. These Pacific- origin waters take two forms in the Canada Basin north of Alaska: the fresher Pacific Summer Water (PSW) between approximately 40 and 100 meters in depth characterized by a local temperature (and sound speed) maximum and the more saline Pacific Winter Water (PWW) characterized by a local temperature (and sound speed) minimum (McLaughlin et al., 2011). The PSW warmed and thickened starting in about 2000 (Timmermans et al., 2014) while the depth of the PWW increased from 150 to 200 meters.
Central Arctic Acoustics
Acoustic Propagation
In general, sound speed increases monotonically with depth in the eastern central Arctic (Figure 4). The sound speed pro- file is, therefore, everywhere, upward refracting, and sound interacts repeatedly with the ice as it propagates. Some of the acoustic energy is reflected with each interaction, but some is scattered by the rough ice and some is converted to compres- sional and shear waves within the ice. The resulting losses increase with increasing frequency; the Arctic waveguide is effectively a low-pass filter. During the Cold War, propagation to ranges in excess of a few hundred kilometers was limited to frequencies below about 30 Hz or wavelengths greater than 50 meters (Mikhalevsky, 2001). The strong ice interactions mean that the changes in the ice cover that are occurring have important implications for acoustic propagation. As the frequency becomes even lower, however, the vertical extent of the acoustic normal modes increases and they begin to interact with the seafloor and lose energy to it. The resulting losses increase with decreasing frequency, leading to a lower frequency bound of 5-10 Hz for long-range propagation.
Acoustic propagation in the western Arctic differs from that in the eastern Arctic because of the presence of the Pacific- origin waters. Sound speed increases with depth, except for the sound speed minimum at the depth of the PWW, which forms an acoustic duct. As the PSW has warmed and thickened in recent years, the sound speed duct, sometimes referred to
as the Beaufort duct, has become stronger (i.e., the difference in sound speed between the maximum in the PSW and the minimum in the PWW has increased). Signals transmitted from sources within the duct can be trapped and propagate to long ranges without interacting with the ice cover or the seafloor. An acoustic navigation and communications system deployed in the Beaufort Sea achieved ranges in excess of 400 kilometers using 900-Hz sources deployed at 100-meter depth in the Beaufort duct, for example (Freitag et al., 2015). The duct is sufficiently weak, however, that signals at frequencies below a few hundred hertz are not fully trapped.
Ambient Sound
The sources and propagation of ambient sound in the Arctic differ substantially from those at midlatitudes. The prevailing sound in ice-covered regions of the Arctic is largely gener- ated when the ice cover deforms and fractures in response to forcing from wind, swell, currents, and thermal stresses (e.g., Mikhalevsky, 2001; Johannessen et al., 2003). In contrast, low-frequency (~20- to 500-Hz) sound at lower latitudes is predominately caused by distant shipping, whereas higher frequency (~500- to 100,000-Hz) sound is mostly due to spray and bubbles associated with breaking waves. More episodic sources of sound in the Arctic include marine life (especially in the marginal ice zone that separates pack ice from open water), earthquakes, and anthropogenic sound due to seismic surveys and ice breakers (Figure 5). There is a broad peak in the spectrum at 15-20 Hz in the central Arctic due to the band-pass nature of propagation from distant
Figure 5. Spectrogram from a hydrophone in Fram Strait showing bowhead whale calls, a ship transiting, and airgun pulses from a seismic survey. Reproduced from Moore et al., 2012, with permission of Oxford University Press.
58 | Acoustics Today | Spring 2020