Internal Wave Characteristics from high frequency measurements in the shelf area of the western Bay of Bengal

Abstract

Characteristics of internal waves (IWs) observed before the onset of Southwest monsoon (May 2015) at a shallow water site off the coast of Visakhapatnam in the Bay of Bengal are presented. The combination of considerable heating and presence of low saline waters in the upper layers of the coastal waters during the pre-monsoon period leads to high stratification in the water column. In this paper, the temperature data obtained from a 36-hour thermistor chain mooring with a sampling interval of 2 seconds in a water depth of ~70 m is analyzed to understand the spectral characteristics of IWs. The vertical mooring consists of 24-channel thermistor array, with sensors placed in the water column from 7.5 m depth to 62.5 m depth at an interval of 2.5 m. The data measured during the pre-monsoon period show rhythmic oscillation of isotherms in the thermocline (between 25-45 m), indicating dominant IW activity. The time series data spanning the water column exhibit high frequency (HF) IW trains (> 3 c/h) riding on semi-diurnal IWs (~0.08 c/h). Vertical displacements estimated from HF temperature data show the propagation of very large amplitude oscillations (> 15 m) with tidal periodicity. Results also indicate that the amplitude of HF fluctuations is comparatively large near the crest of the IWs. The vertical displacement spectra obtained at depths with minimum and maximum IW amplitude are compared. Vertical displacement spectral slope β at 40m depth is -2.57 for internal wave frequency between 0.08 - 4 c/h and -3.83 for frequency > 6 c/h.