FY 1983

A satellite study of ocean internal waves

Sawyer, C.

NOAA Tech. Memo. ERL PMEL-46, NTIS: PB83-248708, 70 pp (1983)

Ocean internal-wave slicks over the eastern U.S. continental shelf imaged by the satellite Landsat are described and analyzed. This study confirms generation by semidiurnal tides, locates the site of the generation at the edge of the continental shelf, and establishes the time of generation near the time of maximum flood current. Characteristics such as wave-packet extent, water depth, and packet lifetime are quantitatively described. The wavelength spectrum derived from spacing of slicks observed near generation is far from a smooth "universal spectrum." The dominant peak at 26 km corresponds to packet spacing, and a second peak near 450 m corresponds to spacing of slicks within packets. The longest within-packet spacing depends on total water depth. The shortest spacing changes with season and so is probably determined by the depth of the upper warm layer. Slick visibility is discussed in terms of modification of surface-wave amplitude by the near-surface current system of the internal waves. Possible mechanisms are (1) damping of surface waves by film concentrated at current convergences, and (2) damping of surface waves in a parallel current, and (3) amplification in an adverse current. Calculations indicate that the latter mechanism provides a better quantitative description of the observed contrast between slicks and rough bands. Though the derived dependence on sun altitude and windspeed corresponds to some observed characteristics, the simplified theory and average parameters used in this trial application only partially explain the observed latitude distribution of visible slicks. From uniformly spaced series of packets, and from similar packet images recorded on successive days, the median lifetime of slick packets is found to be two to three tidal cycles, much longer than the time in which waves are expected to disperse or to break. The balance of non-linear terms in the cnoidal solution of the Korteweg-de Vries equation that permits a stable wave profile relates amplitude and wave number in terms of the elliptic parameter, m. Solutions progress from a train of sinusoidal waves at m = 0, through trains of waves with flat crests and widely separated troughs, to the solitary wave of infinite length at m = 1. The cnoidal solution describes the stability and other observed features of the internal waves, but the variable spacing of slicks within a packet demands a more complex description.