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A D Rao

A D Rao

Indian Institute of Technology Delhi
India

Title: Identification of internal waves in the Western Bay of Bengal: Observations and modelling

Biography

A.D. Rao is a Professor at the Centre for Atmospheric Sciences, IIT Delhi. He received his Ph.D from IIT Delhi in 1982 in physical oceanography. His research interests include development of numerical models for storm surges, internal waves, ocean state forecast, air-sea interaction processes. He has also developed finite-element based models to compute coastal inundation and associated water levels due to storm surges. His contribution to the surge prediction system is very significant as the inland intrusion of surge waters is the main cause for extensive damage along the coast. He has published more than 180 papers in various national and international journals.

Abstract

The existence of internal waves (IWs) in the northwest Bay of Bengal is observed through in situ data and Synthetic Aperture Radar (SAR) images. As suggested by the observations, the dominant type of internal wave is the internal tide, formed by barotropic tidal flow over topography. The near-shore bathymetry of east coast of India tends to be shallow north of 18oN. The alongshore topographic gradient that inhabits in the northern coastal region also supports to the generation of IWs. Further, the region shows a perennial gradient in salinity, both in the horizontal and in the vertical, which augments IWs generation. The increase in the tidal range towards north of this region also contributes for the more prevalence of IWs. To compliment and comprehend the observations, MITgcm is configured for the western continental shelf of the Bay of Bengal to get more insight on the generation and propagation of IWs. The model is forced with astronomical tides, daily winds, fluxes and monthly climatological density fields as initial conditions. The in situ time series observations of temperature and currents at various depths and SAR imageries collected during different seasons in 2006, 2007, 2012 and 2013 are used for validation of the model simulations. The numerical investigation definitely enhances the understanding of generation and propagation of IWs of the region. The simulations suggest that the core of the energy is essentially in the low-frequency range which is in good agreement with the observations and the energy associated with semi-diurnal frequency reaches maximum in the thermocline. The effect of coastal geometry on the behaviour of IWs is also studied by analysing the simulations at different vertical cross-sections adjoining the coast. The experiments imply that the energy variation depends on the combined effects of tidal forcing, local coastal geometry and bottom topography. It is important to note that IW energy is spread over a larger area in the northern cross-sections due to wider shelf and relatively high-tides in the region. However in the southern cross-section near Machilipatnam, energy is trapped over a narrow area due to crescent shape of local geometry. Steep continental slope in the region reflects a considerable amount of IW energy backwards and reduces its intrusion into the continental shelf. If the shelf-break lies in the thermocline region, the IW solitons could travel up to the coast along the gentle continental shelf and could affect the coastal circulation. However, the peak energy is always found near the shelf-break region irrespective of any bottom topography. The propagation of IWs up to the shelf of the coast enhances the local mixing causing reduction in the intensity of temperature inversions (TI) of the study region. From the analysis of energy estimate associated with IWs, variability of dominant semi-diurnal IWs is addressed on both spatial and temporal scales. The computations of the estimate infer the peak estimate during spring-neap tidal period is confined to the depths of the thermocline in the spring tide.

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