On internal wave groups

The pattern of disturbance left by internal wave groups traveling in a unif ormly stratified ocean is examined. Particular attention is given to the te mporal and spatial reoccurrence of extreme values of some parameter a, such as the Richardson number or the wave slope, which may determine, for examp le, the onset of wave breaking in the group or the wave group's refraction of smaller-scale waves. Extreme values reoccur with a period T, equal to th e period of the internal waves, and are sustained along a direction that de pends on the wave frequency, but that, over much of the frequency range fro m f (the Coriolis frequency) to N (the constant buoyancy frequency) of the internal waves, is nearly horizontal. The size of regions in which extreme values are achieved depends on the aspect ratio of the region of a wave gro up, termed the "group breaking region," V, within which values of Q exceed some threshold Q(c). Conditions in which regions of past exceedence of Q(c) ("scars" left by waves in passing wave groups) overlap, so as to be always observed by vertical or horizontal profile measurements, depends on the ra tio tau/T, where tau is the time for which Q > Q(c) as a wave passes throug h V. Near-inertial and semidiurnal tidal internal waves are more likely to leave overlapping scars and may lead to mon general mixing of the ocean tha n, for example, internal wave groups generated by tidal flow over small hor izontal scale (1-3 km) topography. It is suggested that wave groups may be evident, and consequently their effects in promoting turbulence may be larg est, near the site of internal wave generation, just where recent observati ons suggest is the region of enhanced turbulent dissipation in the abyssal ocean.