A short duration test experiment was conducted in the eastern Arabian Sea by deploying two transceivers, near the sound channel axis, on deep sea moorings at water depths of about 4250m and 4175m respectively.
Acoustic transmissions were conducted 12.5ﾟ N latitude for a duration of ten days (2-12 May 1993), with two transceiver systems separated by a range of 270.92km. Hourly reciprocal transmissions were carried with a time lag of 30minutes between eachdirection. From the multiple arrival patterns, significant peaks corresponding to the predicted ray arrivals of the most stable rays were identified (Fig. 7). The travel time perturbations enabled reconstruction of temperature anomaly from the sound speed perturbation. A linear relation was used to transform sound speed perturbations in the vertical plane to temperature perturbations. The 2-D anomaly derived from the six hourly mean travel-time data showed a gradual warming of the top layers, signatures of diurnal variability and intrusion of Red Sea waters.
Numerical simulation and acoustic receptions of Heard Island signals
Simulations showed a gradual deepening of the ray paths from the southern ocean towards tropics. Also the axis of the sound channel deepens from 150m to 1600m (Fig. 9, upper panel). Sudden jumps were noticed in the acoustic ray paths in the vicinity of the Subtropical Convergence Zone (Fig. 9, lower panel). Measured acoustic signals received at the Indian listening station have a signal to noise ratio (SNR) of 20dB on an average and confirmed their detectability over distances as far as 7000km. The phase stability of these signals allowed coherent averaging time of 20minutes for a corresponding increase in SNR. Measured intensity losses were about 135dB in contrast to the computed value of 150dB .
ATOC Program
Following the success of the Heard Island Feasibility Test (HIFT) in early 1990s, a new concept has been advanced by the school in the form of Acoustic Thermometry of Ocean Climate (ATOC) to measure global ocean warming using acoustic techniques. ATOC field program involving deployment of acoustic sources and receivers to acquire environmental data in the form of travel times of acoustic signals. ATOC program is now well established in the Pacific Ocean. Similar network is envisaged for the Indian Ocean. In view of this, the OAT group took up the acoustic environmental study of the Indian Ocean-Southern Ocean combine. The results of the analysis are presented below.
Watermass structure and circulation:
Circulation of the eastern Indian Ocean has been examined from the vertical distributions of temperature (Fig. 10), salinity (Fig. 11), sigma-theta, theta-S curves (Fig. 12). Large variability in the watermass structure was noticed as one moves from the northern to southern latitudes. The salinity of the surface waters at northern latitudes (5ﾟN) is low in January at the easterntransect and high during March at the western transect. Below the sea surface, the upper 100m water column is influenced by the high salinity waters of the Arabian Sea origin. At the equator, a thick (50-75m) low salinity layer is noticed. This layer decreases in its thickness away from the equator. The low saline surface waters, between 3ﾟS and 8ﾟS, encountered have been traced to the Indonesian through flow directed towards west into the Indian Ocean.
