Tidally-modulated high frequency internal waves in Gautami-Godavari estuary, east coast of India

Abstract

Time series temperature and salinity (at 1 hour interval) and currents data (10 min interval) at surface (3 m) and bottom (14 m), collected in the Gautami-Godavari estuary during 25-27 September 2008 were utilized to document the characteristics of Internal waves (IWs) and to infer their generation mechanism. Based on the stability criteria, two High Frequency (HF) significant modes in the Internal Wave (IW) field at frequencies 96.67 cph (10 m depth) and 71.15 cph (14 m depth) have been identified. At these modes, IW parameters viz. wave length (L), wave number (κ), Potential Energy (PE), Baroclinic Potential Energy (BPE) and phase speed (c_i) and displacement function (η(z,t)) have been computed (with salinity and currents data) objectively by adopting well known harmonic technique through Principal Component Analysis (PCA). Results revealed that (1) first mode: IW was found to move with a phase speed of 0.13 m s^-1 and wave length of 0.0056 km (wave number = 178.64 cycles km^-1) having energies of 0.0006 J m^-2 (potential) and 0.0027 J m^-2 (baroclinic) whereas (2) second mode: 0.09 m s^-1, 0.0076 km, 131.48 cycles km^-1, 0.0055 J m^-2, 0.020 J m^-2 and 0.094 m s^-1 for L, κ, PE, BPE and c_i) respectively. η(z,t) was found to be positive (elevation-type IWs) and negative values (oscillation-type) during IW propagation of two modes respectively wherein mode 2 is more energetic (8 times) than mode 1. Tidally-modulated force may be the possible mechanism to generate HF IWs under ebb period.