title: Internal tides and deep diel fades in acoustic intensity.
abstract: A mechanism is presented by which the observed acoustic intensity is made to vary due to changes in the acoustic path that are caused by internal-tide vertical fluid displacements. The position in range and depth of large-scale caustic structure is determined by the background sound-speed profile. Internal tides cause a deformation of the background profile, changing the positions of the caustic structures-which can introduce intensity changes at a distant receiver. Gradual fades in the acoustic intensity occurring over timescales similar to those of the tides were measured during a low-frequency (284-Hz) acoustic scattering experiment in the Philippine Sea in 2009 [White et al., J. Acoust. Soc. Am. 134(4), 3347-3358 (2013)]. Parabolic equation and Hamiltonian ray-tracing calculations of acoustic propagation through a plane-wave internal tide environmental model employing sound-speed profiles taken during the experiment indicate that internal tides could cause significant gradual changes in the received intensity. Furthermore, the calculations demonstrate how large-scale perturbations to the index of refraction can result in variation in the received intensity.
authors.affiliation: Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA.
authors.forename: Andrew W
authors.initials: AW
authors.lastname: White
authors.affiliation: Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA.
authors.forename: Frank S
authors.initials: FS
authors.lastname: Henyey
authors.affiliation: Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA.
authors.forename: Rex K
authors.initials: RK
authors.lastname: Andrew
authors.affiliation: Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA.
authors.forename: James A
authors.initials: JA
authors.lastname: Mercer
authors.affiliation: Scripps Institution of Oceanography, University of San Diego, La Jolla, California 92093, USA.
authors.forename: Peter F
authors.initials: PF
authors.lastname: Worcester
authors.affiliation: Scripps Institution of Oceanography, University of San Diego, La Jolla, California 92093, USA.
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Dzieciuch
authors.affiliation: Department of Oceanography, Naval Postgraduate School, Monterey, California 93943, USA.
authors.forename: John A
authors.initials: JA
authors.lastname: Colosi