Mode-Coupling Effects in Acoustic Thermometry of the Arctic Ocean
The effects of mode coupling on modal travel times and amplitudes are considered with respect to the robustness and accuracy of acoustic thermometry in a range-dependent ocean waveguide. An approximate analytic solution for the complex amplitudes of coupled modes in a slowly varying waveguide is used to analyze the character of mode coupling effects. Change in the source-receiver transfer function for individual modes, due to the mode coupling, is discussed. It is shown that the variations of the modal travel times measured by the modal phases are much less sensitive to the mode coupling than those measured by locating the modal arrival time in pulse-like signals. The numerical solution of the coupled-mode problem has been used to model broadband acoustic propagation at 20 Hz over 1200 km from a source northwest of Franz Josef Land to a receiver in the Lincoln Sea, crossing the Eurasian continental slope, the deep-water Arctic Basin, and the Canadian continental slope. Year-round acoustic measurements are currently being made on this path as part of the Arctic Climate Observations using Underwater Sound (ACOUS) experiment that started in October 1998. The results of modeling show that the mode coupling limits the accuracy of acoustic thermometry on this path. However this limit should not exceed 5 millidegree C for integrated temperature changes on this path if the acoustic travel time variations are measured using the modal phase. Variations of the modal amplitudes due to mode coupling are examined using the results of the Transarctic Acoustic Propagation (TAP) experiment in 1994.
KeywordsArctic Ocean Continental Slope Mode Coupling Modal Transfer Function Arrival Pattern
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