Abstract: Propagation of semi-diurnal internal tides through mesoscale currents in the Kauai Channel, Hawaii.

 

 

Authors: Cedric Chavanne, Pierre Flament, Doug Luther and Klaus-Werner Gurgel

Observations of semi-diurnal surface currentsin the Kauai Channel, Hawaii, are interpreted in light of theinteraction of internal tides with energetic surface-intensified mesoscale currents.A cyclone of 55 km diameter and 100 m vertical decay scale,and vorticity waves of 90-100 km wavelength and 100-200 mvertical decay scales are investigated. M2 internal tiderays with initial horizontal wavelength of 44 km and verticalwavelength O(1000 m) are propagated using ray tracing through idealized mesoscale currents approximating the observed currents. Despite the lack of scale separation,especially in the vertical, the results are qualitativelyconsistent with observations.The Doppler-shifted intrinsic frequency is assumed to satisfythe hydrostatic internal waves dispersion relation in an oceanat rest, but the buoyancy frequency is allowed to be modified by thebackground currents.We show that internal tides energy is mainly affected by stratification variations induced by the mesoscale thermal or gradient winds, with Doppler-shifting playinga secondary but not negligible role.Energy can be increased by a factor 10 at the surface in thepresence of the cyclone, and surfacing time can be delayed byup to 5 hours (about 150 degrees phase lag), resulting in incoherentsignals at fixed locations on the surface over long periods of timeas the internal tides propagate through variable mesoscalefeatures. Furthermore, the vertical wavelength can be reducedby a factor 4 near the surface in the presence of the cyclone,which, combined with the energy amplification, leads toincreased vertical shear, and could trigger instabilities,turbulence and dissipation.