Internal tidal currents and solitons in the southern Bay of Bengal

Abstract

Tidal currents and higher-frequency internal solitons are observed from current and hydrographic fields that were recently measured at six deep-water moorings in the southern Bay of Bengal by the Naval Research Laboratory. Current, temperature and salinity were measured in the upper 500 m of the water column between December 2013 and August 2015. Harmonically-fitted (coherent) tidal currents, in the upper 500 m, showed maximum amplitudes for the M2 constituent, with velocities of the order of 3 cm s−1; M2 tidal ellipses were generally oriented towards the north-northeast. Incoherent internal tides accounted for at least 60% of the total semidiurnal tide energy. Solitons were generated at multiple source regions along the Andaman Islands and contribute significantly to the high frequency energy (greater than 1 cph). Solitons were present throughout the measurement period but their activity was reduced during the 2014–2015 winter monsoon. Soliton and internal-tide amplitudes show a fortnightly modulation, consistent with the spring-neap cycle of the semidiurnal tide. The interaction of tides and the associated high-frequency waves (20–40 min periods) with mesoscale eddies produced regions of enhanced shear and strain in the thermocline. High-strain regions were found within cyclonic eddies and high-shear regions were found within anticyclonic eddies. Shear-strain-based dissipation models indicate that mixing rates can be enhanced by a factor of 3–5 by tidal and higher frequencies during interactions with eddies. This study further suggests that tidal harmonics and internal tides have an important impact on the near-surface currents away from coastal boundaries, in the interior of the Bay of Bengal, while the effects of the depth-averaged tides are more important deeper in the water column.