Abstract
The application of ocean acoustic tomography in Fram Strait requires a careful assessment of the accuracy to which estimates of sound speed from tomography can be converted to estimates of temperature. The Fram Strait environment is turbulent, with warm, salty, northward-flowing North Atlantic water interacting with cold, fresh, southward-flowing Arctic water. The nature of this environment suggests that salinity could play an important role with respect to sound speed. The properties of sound speed with respect to temperature and salinity in this environment were examined using climatological and in situ glider data. In cold water, a factor of about 4.5 m s(-1) °C(-1) can be used to scale between sound speed and temperature. In situ data obtained by gliders were used to determine the ambiguities between temperature, salinity, and sound speed. Tomography provides a depth-averaging measurement. While errors in the sound speed-temperature conversion at particular depths may be 0.2 °C or larger, particularly within 50 m of the surface, such errors are suppressed when the depth is averaged. Using a simple scale factor to compute temperature from sound speed introduced an error of about 20 m °C for depth-averaged temperature, a value less than formal uncertainties estimated from acoustic tomography.
Highlights
The application of tomography for observing Fram Strait (Fig. 1), and high-latitude regions in general, has been of considerable theoretical interest in the past (Chiu et al, 1987; De Marinis et al, 2003; Naugolnykh et al, 1998; Mikhalevsky et al, 2015), it is only in recent years that these observations have been implemented in Fram Strait (Sagen et al, 2016; Skarsoulis et al, 2010; Mikhalevsky et al, 2015)
A simple scale factor of about 4.5 m sÀ1 CÀ1 would appear to be suitable for conversion of sound speed variations to temperature in most circumstances, giving an accuracy of a few percent
An even more careful assessment of the sound speed/ temperature conversion could be made by looking at the time dependence of Atlantic water temperature in the West Spitsbergen Current and its seasonal cycle
Summary
The application of tomography for observing Fram Strait (Fig. 1), and high-latitude regions in general, has been of considerable theoretical interest in the past (Chiu et al, 1987; De Marinis et al, 2003; Naugolnykh et al, 1998; Mikhalevsky et al, 2015), it is only in recent years that these observations have been implemented in Fram Strait (Sagen et al, 2016; Skarsoulis et al, 2010; Mikhalevsky et al, 2015). In situ perturbations of temperature and salinity with respect to this reference ocean were computed from hydrographic data obtained from gliders deployed in the region of the tomography experiment at intervals from 2008 to 2012. The in situ data allowed the relative effects of temperature and salinity variations on sound speed, their potential influences on tomographic measurements of the Fram Strait environment, to be determined with precision.
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