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On Internal Wave Groups

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  • 1 Department of Oceanography, Southampton Oceanography Centre, Southampton, United Kingdom
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Abstract

The pattern of disturbance left by internal wave groups traveling in a uniformly stratified ocean is examined. Particular attention is given to the temporal and spatial reoccurrence of extreme values of some parameter Q, such as the Richardson number or the wave slope, which may determine, for example, 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 the period of the internal waves, and are sustained along a direction that depends on the wave frequency, but that, over much of the frequency range from 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 group, termed the “group breaking region,” V, within which values of Q exceed some threshold Qc. Conditions in which regions of past exceedence of Qc (“scars” left by waves in passing wave groups) overlap, so as to be always observed by vertical or horizontal profile measurements, depends on the ratio τ/T, where τ is the time for which Q > Qc as a wave passes through V. Near-inertial and semidiurnal tidal internal waves are more likely to leave overlapping scars and may lead to more general mixing of the ocean than, for example, internal wave groups generated by tidal flow over small horizontal scale (1–3 km) topography. It is suggested that wave groups may be evident, and consequently their effects in promoting turbulence may be largest, near the site of internal wave generation, just where recent observations suggest is the region of enhanced turbulent dissipation in the abyssal ocean.

Corresponding author address: Dr. S. A. Thorpe, Department of Oceanography, University of Southampton, Southampton Oceanography Centre, European Way, Southampton S014 3ZH, United Kingdom.

Email: sxt@mail.soc.soton.ac.uk

Abstract

The pattern of disturbance left by internal wave groups traveling in a uniformly stratified ocean is examined. Particular attention is given to the temporal and spatial reoccurrence of extreme values of some parameter Q, such as the Richardson number or the wave slope, which may determine, for example, 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 the period of the internal waves, and are sustained along a direction that depends on the wave frequency, but that, over much of the frequency range from 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 group, termed the “group breaking region,” V, within which values of Q exceed some threshold Qc. Conditions in which regions of past exceedence of Qc (“scars” left by waves in passing wave groups) overlap, so as to be always observed by vertical or horizontal profile measurements, depends on the ratio τ/T, where τ is the time for which Q > Qc as a wave passes through V. Near-inertial and semidiurnal tidal internal waves are more likely to leave overlapping scars and may lead to more general mixing of the ocean than, for example, internal wave groups generated by tidal flow over small horizontal scale (1–3 km) topography. It is suggested that wave groups may be evident, and consequently their effects in promoting turbulence may be largest, near the site of internal wave generation, just where recent observations suggest is the region of enhanced turbulent dissipation in the abyssal ocean.

Corresponding author address: Dr. S. A. Thorpe, Department of Oceanography, University of Southampton, Southampton Oceanography Centre, European Way, Southampton S014 3ZH, United Kingdom.

Email: sxt@mail.soc.soton.ac.uk

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