Rapid Near-inertial Internal Wave Group Propagation Through the Transition Layer from Float and Glider Observations in the Bay of Bengal

Two Spray gliders and 1 SOLO-II float were deployed in 2013 and 2014 as components of ONR's Air-Sea Interactions in the Northern Indian Ocean (ASIRI) experiment. Shallow (10-50 m) salinity-controlled mixed layers in the Bay of Bengal isolate the rest of the deeper isothermal layer and ocean interior from winds. The transition layer is a deeper stratification maximum (20-100 m), which separates the upper ocean from the interior.

Downward near-inertial internal wave (NIW) groups are observed here in potential density fluctuations and can rapidly (a few inertial periods) transfer energy out of the mixed layer into the stratified interior. (Inertial periods are T = 2*pi/f = 2 - 3 days from 9 - 17°N, where f is the Coriolis frequency.) When isopycnals shoal at fronts, the transition layer is brought closer to the mixed layer allowing for faster downward group speed due to the higher stratification. With about 10 inertial wind events in the NCEP reanalysis over the observation period of about 21 weeks, we find 3 NIW groups with clear downward energy (upward phase) propagation into the interior.

The groups reach 200 m within 2-3 T and have vertical wavelengths of about 200 m. This implies horizontal wavelengths of about 200 km if the waves have a frequency of 1.1f. This horizontal wavelength and propagation time scale appear consistent with surface wind forcing correlation scales from 3-day highpassed wind products and decay estimates from surface drifters and theory (Park et al., 2009). Our results extend this previous work by making subsurface observations and measuring further equatorward. The mesoscale appears to mediate: (a) the conversion from mixed layer inertial oscillations into propagating NIW and (b) NIW propagation into the interior.