Observations of internal wave oscillations in the upper thermocline based on CTD-yoyo measurements during ARMEX-I

Abstract

A CTD yoyo experiment was conducted onboard ORV Sagar Kanya (SK-179) during the second cruise of ARMEX Phase-I to observe the extent of internal wave oscillations which induce sound speed fluctuations along their propagation path. The Mini CTD of Sensor data was attached with the Sea-Bird CTD available onboard SK. Both the systems were continuously lowered and hauled up (yoyo) within the upper thermocline, between 45 and 205 m of the water column at a deep water location (15° 23.12¢ N and 72° 10.55¢ E) while the ship was allowed to drift. There were total 36 profiles (down cast and up cast) collected within a period of 2 hours and 10 minutes. The observations made by Mini CTD and Sea-Bird CTD were interpolated in depth (50 to 200 m at 1m interval) and time (3.6 minutes – duration of each cast) for comparison as they have different accuracies. The profiles collected during the start of the yoyo and on completion clearly revealed a well defined mixed layer (nearly 43 m) and its temperature remained practically same. The isotherms during the time-series clearly revealed internal wave oscillations of height and period ranging from 2 to 4 m and 18 to 40 minutes respectively excluding the higher frequencies. The upward and downward movements of 2 to 20 m along the depth/vertical were attributed to well known tidal internal waves.