Influence of Bathymetry on the Performance of Regional Scale Model

 A three dimensional ocean circulation model (Princeton Ocean Model) is utilised to study the thermohaline variability of the eastern Arabian Sea associated with changes in the three input bathymetry data sets, viz. ETOPO5 (E5), Modified ETOPO5 (ME5) and ME5 further modified based on actual fine resolution data collected using Multibeam echo-sounder (MEN5). The temperature and salinity measurements made onboard INS Sagardhwani for the period July 2000 is utilised to validate the model. Simulations of temperature using Princeton Ocean Model show good improvement in the coastal region with MEN5 bathymetry data (RMS error of 0.71 °C and correlation coefficient of 0.98). The study highlights the choice of fine resolution bathymetry data in the simulation of nearshore processes, where bathymetry is very complex

Transmission Loss Variability Associated with Upwelling and Downwelling Off the Southwest Coast of India

Fine resolution spatial survey carried out off the west coast of India during June and December 2004 was utilised to study the transmission loss (TL) variability associated with the upwelling and downwelling processes in this region. During June, the upwelling was confined to the upper 80 m. Downsloping of isotherms below this depth towards the coast and the occurrence of low saline waters indicated the presence of undercurrent. Between the periods of upwelling and downwelling, temperature and salinity in the surface layers increased by 1-2 oC and 2 PSU, respectively, while at the sub-surface levels, the corresponding increase was ~8 oC and ~0.5 PSU. A range-dependent acoustic propagation model based on parabolic equation method was utilized to compute TL for these two periods. The model was run with a source frequency of 3 kHz kept at 5m depth for different environmental setup, viz. propagation along the constant-depth contour, range-independent and range-dependent environment, and upslope/downslope propagation. The computations revealed significant variability in the TL characteristics between the upwelling and downwelling scenario, though bathymetry and geo-acoustic properties were the same. The analysis also stressed the need of range-dependent acoustic propagation model for realistic prediction of transmission loss variability.Defence Science Journal, 2010, 60(5), pp.476-482, DOI:http://dx.doi.org/10.14429/dsj.60.57

On the Possible Mechanisms for Saltening of the Bay of Bengal

The Bay of Bengal (BoB) is a low saline basin owing to large influx of freshwater from precipitation and river runoff. To maintain the salt balance of the BoB, the incessant lowering of salinity is to be balanced by the inflow of saltier water into the basin. In the present work, various processes that contribute to the saltening of the BoB, viz. coastal upwelling, eddies and their interaction, lateral advection from Arabian Sea and tropical cyclones are discussed. In the near-shore regions, the coastal upwelling due to wind induced Ekman transport plays a dominant role in increasing the surface salinity. On the other hand, in the open ocean, the divergence induced by eddies and their mutual interaction contributes significantly to the salt water pumping. In the southern BoB, the advection from the Arabian Sea increases the salinity. The formation of cyclones in the BoB also leads to an increase in the surface salinity. However, the magnitude of saltening of the Bay due to these processes varies from north to south. The uplift of saltier water from subsurface levels increases the salinity in the surface layers thereby creating a salinity gradient and a salinity front

Shallow Water Internal Waves and Associated Acoustic Intensity Fluctuations

Physical oceanographic and acoustic data were simultaneously collected from the coastalwaters of the Arabian Sea. Acoustic transmissions were carried out from an anchored vesselusing 620 Hz transducer and received by an array of hydrophones moored at ~5 km away fromthe anchorage. Thermal structure in this region was characterised by a tri-layer structure, ie, astrong thermocline (> 0.4 oC/m) sandwiched between an upper (< 10 m) and bottom (> 25 m)homogeneous layer. High-resolution (sampled at 10 s interval) temperature data from mooredsensors revealed intense internal wave activity. The maximum value of Brunt-Vaisala frequency,which is the maximum frequency limit of internal waves in the thermocline, suggests that theupper frequency limit of the internal wave, which can be generated during this period, is 23 cph(2.6 min). High and low frequency waves caused variations of ~3 oC and ~5 oC respectively inthe temperature field. But the low frequency internal waves were found to contain maximumenergy compared to the high frequency waves. Fluctuations of 8-12 dB were noticed in themeasured acoustic intensity values in the presence of low frequency internal waves. Simulationstudies carried out using parabolic equation model using 620 Hz source indicated well-definedducted propagation with minimum transmission loss, when the source was kept within thehomogeneous layer. The presence of tri-layer thermal structure, ie, a strong gradient layersandwiched between an upper and bottom homogeneous layer, caused surface and bottom channelpropagation in this region

Arabian Sea Mini Warm Pool and its Influenceon Acoustic Propagation

A systematic experiment was conducted in the eastern Arabian Sea for the first time exclusivelyto study the characteristics of the Arabian Sea mini warm pool1. The analysis revealed complexnature of the thermohaline and sonic layer distributions across the Arabian Sea mini warm pool.This mini warm pool was identified between 67.5 oE and 75 oE, where the sea surface temperaturewas in excess of 30.25 oC. At the core of this mini warm pool, warmer (>31.2 oC) and low saline(<34.6 PSU) waters were noticed. Further, very thin sonic layer (< 5 m) was noticed at the miniwarm pool core, which increased eastward and westward. In this study, the acoustic propagationcharacteristics across and outside of the core, i.e., (i) within the mini warm pool core, (ii) easternside of the mini warm pool core, and (iii) western side of the mini warm pool core, were assessedbased on the output of a range-dependant acoustic model. In general, the occurrence of this miniwarm pool was found to alter the propagation characteristics. Better propagation was obtainedwhen the simulation was carried out on the eastern side of this mini warm pool, with source nearthe coast (i.e., downslope condition

Match beam inversion of geoacoustic parameters from towed hydrophone streamer array data

183-194<span style="font-size:9.0pt;font-family: " times="" new="" roman","serif";mso-fareast-font-family:"times="" roman";mso-ansi-language:="" en-us;mso-fareast-language:en-us;mso-bidi-language:ar-sa"="" lang="EN-US">The study estimates geoacoustic parameters of the sediment by inversion of acoustic data acquired from seismic hydrophone streamer array.  Inversion is performed by beamforming technique as the data is pertaining to shallow water and there is no prior information on the sediment type and the profiles of subbottom sediment layers. Initially, the estimates of sediment speed are obtained from critical angle derived from beamformer output by conventional beamforming. The sediment speed is used to set the parameter search bounds for further inversion by beamforming technique. The search bounds for density and compressional attenuation coefficient is set accordingly. The feasibility of inversion is understood by undertaking synthetic runs followed by inversion of field data. The beam cutoff angles are determined from the beampattern and the arrivals pertaining to the sea bottom.  The geoacoustic model comprises of a water column, two sediment layers and a sediment half space.  The inversion results of field data sets are compared with results obtained by modal inversion. The results show negligible variation in estimated sound speed indicating similar sediment type. The sediment density and compressional attenuation coefficient exhibits lesser sensitivity toward inversion process<span style="font-size: 11.0pt;font-family:" times="" new="" roman","serif";mso-fareast-font-family:"times="" roman";="" mso-ansi-language:en-us;mso-fareast-language:en-us;mso-bidi-language:ar-sa"="" lang="EN-US">.</span