Abstract
Acoustic tomography systems have been deployed in Fram Strait over the past decade to complement existing observing systems there. The observed acoustic arrival patterns are unusual, however, consisting of a single, broad arrival pulse, with no discernible repeating patterns or individual ray arrivals. The nature of these arrivals is caused by vigorous acoustic scattering from the small-scale processes that dominate ocean variability in Fram Strait. Simple models for internal wave and mesoscale variability were constructed and tailored to match the variability observed by moored thermisters in Fram Strait. The internal wave contribution to variability is weak. Acoustic propagation through a simulated ocean consisting of a climatological sound speed plus mesoscale and internal wave scintillations obtains arrival patterns that match the characteristics of those observed, i.e., pulse width and travel time variation. The scintillations cause a proliferation of acoustic ray paths, however, reminiscent of "ray chaos." This understanding of the acoustic forward problem is prerequisite to designing an inverse scheme for estimating temperature from the observed travel times.
Highlights
The application of tomography for observing Fram Strait (Fig. 1) has been examined by theory or simulation in the past (Chiu et al, 1987; Naugolnykh et al, 1998), and the conclusions of those studies suggested a practical utility to such observations
The character of the observed acoustic arrival patterns is significantly different than that described by the previous theoretical studies; the actual ocean is of a different character than that employed by the theoretical studies
Travel-time data for acoustic tomography obtained in the Fram Strait region during the DAMOCLES and ACOBAR deployments has the property that clear, individual ray arrivals are not often observed
Summary
The application of tomography for observing Fram Strait (Fig. 1) has been examined by theory or simulation in the past (Chiu et al, 1987; Naugolnykh et al, 1998), and the conclusions of those studies suggested a practical utility to such observations. Travel-time data for acoustic tomography obtained in the Fram Strait region during the DAMOCLES and ACOBAR deployments has the property that clear, individual ray arrivals are not often observed. The conjecture of the analysis described here is that if scattering of acoustic energy by small-scale variability is an important aspect of the observed arrival patterns, properly addressing this scattering may lead to more accurate formulations of the inverse problem, more accurate estimates of ocean temperature The aim of this analysis was to examine the issue of the scattering of acoustic rays by the small-scale variability that characterizes the oceanographic conditions of the Fram Strait region to see if the observed arrivals could be more accurately modeled. The stratification in Fram Strait is such that internal wave contributions to sound speed scintillations are quite weak Variability at both short and long time scales likely resulted from smaller and larger mesoscale variability, respectively.
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