Fig. 4. Ice compactness for December 2010 and acoustic tracks.
Task 3: Acoustic modeling of Arctic basin.
The acoustic monitoring system for averaged ocean temperature will be designed to ful fill the following requirements 1) the signal to noise ratio is as good as possible without having to use a too energetic acoustic source, 2) the acoustic signal travels though the regions and water layers where significant changes in ocean climate are predicted by climate modeling, 3) the effect of averaged temperature changes are separated out from the effects caused by changes in the ice conditions. In order to meet these requirements ambient noise frequency spectra from different regions are analyzed and the acoustic propagation are studied by numerical simulations.
Makris and Dyer (1991) reports that the ambient noise frequency spectrum has a maximum between 15-25 Hz in the interior Arctic. By analyzing ambient noise data we have found that the ambient noise have a maximum at frequencies between 40-100 Hz in the Greenland Sea and between 100-400 Hz in the Barents Sea (Sagen, 1998). These changes are related to the temperature stratification, ice conditions, water depths and geological conditions. One should also be aware of the strong effect of inertial oscillations and tidal current on low frequency ambient noise levels (less than 125 Hz) in ice covered shallow water regions (Johannessen et al., 1992).
Numerical simulations are generated by the acoustic propagation model OASES (Schmidt, 1997). In order to reduce the complexity of the propagation problem the first tracks has been selected to avoid regions of shallow waters and MIZ. Furthermore, it is assumed that the environment is range independent and the sea ice is modeled as a thin and smooth elastic plate. Fig. 5 shows the transmission loss as a function of range and frequency for different source and receiver positions in a deep water region (3800m) characterized by a 100m deep surface duct with cold water covered with 2.0 m thick ice. It is clear from Fig. 5 that the decrease of acoustic intensity is much less in a
