Internal Waves and Shear Instability in the Southern Bay of Bengal

Iossif Lozovatsky1, Hemantha W Wijesekera2, Ewa Jarosz2, William J Teague2, Madis-Jaak Lilover3, Annunziata Pirro4, Luca Raffaele Centurioni5, Zachariah Silver6 and Harindra Joseph Fernando6, (1)University of Notre Dame, Notre Dame, IN, United States, (2)Naval Research Laboratory, Stennis Space Center, MS, United States, (3)Marine Systems Institute at TUT, Tallinn, Estonia, (4)University of Notre Dame, Department of Civil Engineering, Environmental and Earth Sciences, Notre Dame, IN, United States, (5)Scripps Institution of Oceanography, La Jolla, CA, United States, (6)University of Notre Dame, Civil & Environmental Engineering & Earth Sciences, Notre Dame, IN, United States
Abstract:
Recent measurements in the southern Bay of Bengal (BoB) conducted in July 2014 as a part of the ASIRI-EBOB program facilitated by the US Office of Naval Research, shed light on basic characteristics of high-frequency internal waves in the upper pycnocline and corresponding velocity structure affected by episodic events of shear instability. A 20 hour series of CTD, ADCP, and acoustic backscatter profiles (down to 150 m) plus temporal CTD measurements at z = 54 m in the pycnocline were taken at the northeastern periphery of the cold Sri Lanka Dome (evident from satellite images and drifter trajectories). Quasi-harmonic internal waves with periods ranged from ~ 10 to 40 min were registered at all depths below a shallow (~ 20 – 30 m) surface mixed layer in the background of a 10 m amplitude internal tide. Periodic (about every 6 hr) increase/decrease of the wave kinetic energy links it to the tidal motions. Vertical displacements associated with high-frequency waves followed the Weibull distribution with the median value 2.3 m and a 95% quintile 6.5 m. Sporadic appearance of high-amplitude (> 5 m) vertical displacements mainly coincided with patches of low Richardson number, pointing to local shear instability as possible mechanism of internal-wave induced turbulence. However, the probability of shear instability in the summer BoB pycnocline is relatively low, not exceeding 5% for Ri < 0.25 and 35% for Ri < 1.

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