A snapshot of internal waves and hydrodynamic instabilities in the southern Bay of Bengal

Abstract

Measurements conducted in the southern Bay of Bengal (BoB) as a part of the ASIRI-EBoB Program portray the characteristics of high-frequency internal waves in the upper pycnocline as well as the velocity structure with episodic events of shear instability. A 20 h time series of CTD, ADCP, and acoustic backscatter profiles down to 150 m as well as temporal CTD measurements in the pycnocline at z = 54 m were taken to the east of Sri Lanka. Internal waves of periods ∼10–40 min were recorded at all depths below a shallow (∼20–30 m) surface mixed layer in the background of an 8 m amplitude internal tide. The absolute values of vertical displacements associated with high-frequency waves followed the Nakagami distribution with a median value of 2.1 m and a 95% quintile 6.5 m. The internal wave amplitudes are normally distributed. The tails of the distribution deviate from normality due to episodic high-amplitude displacements. The sporadic appearance of internal waves with amplitudes exceeding ∼5 m usually coincided with patches of low Richardson numbers, pointing to local shear instability as a possible mechanism of internal-wave-induced turbulence. The probability of shear instability in the summer BoB pycnocline based on an exponential distribution of the inverse Richardson number, however, appears to be relatively low, not exceeding 4% for Ri < 0.25 and about 10% for Ri < 0.36 (K-H billows). The probability of the generation of asymmetric breaking internal waves and Holmboe instabilities is above ∼25%.