These studies were confined to understanding of the acoustic field of the two major basins-Bay of Bengal and Arabian Sea including the region north of the equator.
While the oceanographic features of the north Indian Ocean underline the importance of the dynamics of these two marginal seas, one needs detailed investigations in a time series mode to address (a) the nature of advective field within the ocean interior, (b) degree of orderliness of the observed equatorward and poleward Western Boundary Currents, (c) the nature of local and remote forcings on the flow field and the associated upwelling and (d) the influences of deep water circulation.
Sound speed structure in the north Indian Ocean
The spatial distribution of sound speed reveals a northward decrease at the surface (Fig. 1a) while an increase in the northward direction is seen in the deeper layers. The sound speed at the axis shows variation up to 4m/s and 2m/s respectively (Fig. 1b). The eastern Bay of Bengal and the Andaman Sea are characterized by low axial depths and high axial sound speeds. The depth of sound channel axis varies between 1450 and 1850m in the Arabian Sea as compared to 1100-1750m in the Bay. In general, the depth of channel axis increases towards the northern latitudes in the Arabian Sea, while it decreases in the Bay of Bengal (Fig. 1c). The mean depths of the channel axes are 1750 and 1500m in the Arabain Sea and the Bay of Bengal respectively.
The annual mean sound speed profiles in the north Indian Ocean were obtained from quasi-synthetic environmental data (Fig. 2). In the upper 40-m of the Arabian Sea, these profiles show uniform values while they increase with depth in the Bay of Bengal. An interesting feature common to both the marginal seas is the depth limited nature of sound speed profile, where surface sound speed exceeds the near bottom value. In the Arabian Sea sound speed shows maximum spatial variations in the depth layer between 200 to 1500m due to a combined effect of salinity (to a large extent) and temperature. Spatial variations in sound speed in the Bay of Bengal are marginal as compared to the Arabian Sea. The sound speed profile in the Arabian Sea shows the presence of large gradients above the axis of the SOFAR channel in contrast to that in the Bay of Bengal and also a relatively higher sound speed values at any given depth indicating a strong wave guide nature.
Numerical simulations
Acoustic model simulation experiments related to the forward and inverse aspects of ocean tomography have been developed. For the forward model, ray theoretical approach and to a less extent the parabolic equation method were followed for the prediction of arrival pattern (Fig. 3) and signal strength (Figs. 4 & 5) while for the inverse model deterministic and stochastic inverse methods were used. The deterministic method is based on the generalized inverse approach for the estimation of the model parameter (sound speed perturbation) in a near stable or stratified ocean. Resolution matrices obtained through singular value decomposition helped to examine the correctness of the solution. In the stochastic method of estimation, the vertical structure of the ocean is modeled through the quasi-geostrophic theory while the horizontal structure is assumed to have a gaussian covariance with a length scale of 100km. A good agreement has been observed between the assumed and the reconstructed sound speed perturbation field.
