Large-scale processes in the upper layers of the Indian Ocean inferred from temperature climatology

Determination of amplitudes and phases for the annual and semi-annual cycle of the temperature in the Indian Ocean north of 20S from Levitus temperature climatology (1982) gives maximum amplitudes of the seasonal cycle at 100 m with the dominance of semi-annual cycle in the equatorial region and annual cycle elsewhere in the domain. The Bay of Bengal shows characteristics of the westward-propagating Rossby waves of annual period, while the Arabian Sea shows the dominance of Ekman pumping in the central region and westward-propagating features in the eastern region. Qualitative evidences obtained from the distribution of depth of 20°C isotherm and computed Ekman pumping velocities are consistent with the above inferences. From the time-longitude plot of the depth of the 20°C isotherm, the phase speed of westward propagating features from the west coast of India along 10.5N and 15.5N are found to be 7.8 cm s−1 and 5.2 cm s−1 respectively. This is consistent with the corresponding values computed and verified with theory for the Bay of Bengal (Prasanna Kumar and Unnikrishnan, 1995)

Understanding our seas: National Institute of Oceanography, Goa

The present article summarizes the research done at the CSIR–National Institute of Oceanography in 2014 in ocean science, resources and technology. Significant research has been conducted on air–sea interactions and coastal circulation, biogeochemistry, biology, marine geophysics, palaeoceanography, marine fishery, gas hydrates and wave energy. Technological advances covered topics like oceanographic tools. Major strides have been made in marine resources research and evaluation

Understanding our seas: National Institute of Oceanography, Goa

The present article summarizes the research done at the CSIR–National Institute of Oceanography in 2014 in ocean science, resources and technology. Significant research has been conducted on air–sea interactions and coastal circulation, biogeochemistry, biology, marine geophysics, palaeoceanography, marine fishery, gas hydrates and wave energy. Technological advances covered topics like oceanographic tools. Major strides have been made in marine resources research and evaluation

Sound field computations in the Bay of Bengal using parabolic equation method

179-183Effect of the cold core eddy in the Bay of Bengal on acoustic propagation was analysed by parabolic equation (PE) method. Source depth, frequency and propagation range considered respectively for the two numerical experiments are 150 m, 400 Hz, 650 km (case i) and 1000 m, 70 Hz, 5000 km (case ii). Acoustic intensity levels varied from -20 to -35 dB in the upper 500 m (case i). In the presence of the eddy, isolines exhibited deviations between 250 and 500 km range extending from 100 m to the abyssal region. Maximum insonified region is present in the vicinity of the channel axis depth which showed variations from 800 to 1500 m in case ii

Acoustic propagation within a surface duct in the western Bay of Bengal

236-238Sound speed structure forms a surface duct in the upper 50 m layer in the western Bay of Bengal during late July A range-dependent acoustic ray computation shows that some rays emanating from a source within the upper 30 m, get trapped within this duct while the untrapped rays propagating as refracted bottom-reflected (RBR) rays. The extent of acoustic energy trapped depends on the critical angle of approximately 1 degree. When the source depth coincides with the axis of the surface duct, all the rays within the above critical angle get trapped fully. With further decrease in the source depth, the limit of energy trapping reduces to 0.5 of a degree. The intensity level for a trapped ray is 10-15 dB higher than that of an untrapped RBR ray. The leakage of + 0.6 degree ray out of the surface duct can clearly be seen as change in the intensity level of about 15-20 dB which is attributed to the weakening of the duct at that range

Sound speed structure in the Arabian Sea and the Bay of Bengal

17-20Sound speed computed from annual mean temperature and salinity data of Levitus reveals that spatial variation in the Arabian Sea is greater than that in the Bay of Bengal. Maximum spatial variation of sound speed in the Arabian Sea noticed between the depth levels 200 to 1500 m is due to the combined effect of variation in the salinity (to a larger extent) and temperature. In the Bay of Bengal though the spatial variation of sound speed is marginal, variations observed between 1500 and 2800 m are brought about by warm Andaman Sea waters. The depth limited nature of annual mean sound speed profile for 2 marginal seas suggests that the effective acoustic channel lies much below (approximately 300 m) the surface. This results in acoustic propagation in the form of surface-refracted bottom-reflected (RBR) rays within the SOFAR channel. Acoustic ray arrival pattern, obtained from the ray-tracing software, reveals that in the Arabian Sea middle order rays with launch angles from +/-6.5-degrees to +/-7.5-degrees reach the receiver early. These are followed by rays with flat angle or near-axial rays and the RBR rays. The purely refracted rays with steep as well as flat angles can adequately be resolved in time-a characteristic that make the ray identifications easier at the experimental stages. On the contrary, in the Bay of Bengal, near axial, flat angle rays with launch angles from -4-degrees to +4-degrees reach the receiver faster than the purely refracted, steep angle rays. Further, flat angle rays arrive almost simultaneously making the ray identification complex and difficult. The rays with launch angles between +/-4-degrees and +/-7-degrees undergo total refraction and have a better spread as far as travel time is concerned. The early arrival of the middle order rays in the Arabian Sea has been attributed to the frequent scanning of warm and high saline waters in the depth range of 500-1200 m by these rays. The early arrival of the axial rays in the Bay of Bengal results from weak gradients within the SOFAR channel

Numerical simulation and measurements of acoustic transmissions from Heard Island to the equatorial Indian Ocean

173-178Simulated acoustic propagation showed a gradual deepening of the ray paths from the southern ocean towards tropics. Also the axis of the sound channel deepens from 150 m to 1600 m. Sudden jumps were noticed in the acoustic ray paths in the vicinity of the Sub Tropical Convergence. Measured acoustic signals, received at the Indian listening station has a signal to noise ratio (SNR) of 20 dB on an average and confirmed their detectability over distances as far as 7000 km. The phase stability of these signals allowed coherent averaging time of 20 minutes for corresponding increase in the SNR. Measured intensity losses were about 135 dB in contrast to those computed values of 150 dB