Acoustic waveguide characteristics of the Indian Ocean - north of equator
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The acoustic characteristics of the waters of the Arabian Sea and Bay of Bengal, constituting the northern Indian Ocean, are analysed based on a climatological mean sound speed field. In the upper 30-40 m in the Bay of Bengal a weak surface duct is noticed. The depth of the SOFAR axis varies from 1400-1950 m in the Arabian Sea and from 1100-1750 m in the Bay of Bengal. While the depth of the SOFAR axis increases towards the northern latitudes in the Arabian Sea, it decreases in the Bay of Bengal. The axial sound speed varies from 1492-1496 m/s in the Arabian Sea and from 1492-1494 m/s in the Bay of Bengal. The eastern Bay and the Andaman Sea are characterisEd. by low axial depth and high axial sound speed. A feature important and common to both Arabian Sea and Bay of Bengal is the depth limited nature of the sound speed profile. As a consequence the effective waveguide lies much below the sea surface (100-300 m). The acoustic ray arrival pattern, obtained from ray computations, reveals that rays with emergence angles between 6.5 and 7.5 degrees in the Arabian Sea reach the receiver first followEd. by the flat angle rays and steep RBR rays. In the Bay of Bengal, flat angle rays reach faster than the purely refracted steep angle rays. This arrival pattern is, due to the presence of a weak acoustic waveguide in the northern Indian Ocean, contrary to the arrival sequences reported for the Atlantic and the Pacific Oceans. The early arrivals of 6.5-7.5 degree rays in the Arabian Sea are attributed to the presence of warm, high saline waters in the depth limits of 500-1200 m, as compared to the Bay of Bengal, which is scanned more often by these rays than any other rays. The arrival span of the refracted rays vary from 495 ms (Arabian Sea) to 344 ms (Bay of Bengal) for a range of 300 km.
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