Observations on the energy balance of internal waves during JASIN

Abstract

Depth-integrated horizontal kinetic energy (HKE) in the frequency band 0.1-4 cycles per hour is used to estimate the time variation at one site of internal wave energy over a 40 day period during JASIN 1978. The HKE smoothed over three days varies from 400 to 1500J m -2 . The canonical Garrett-Munk total energy of 3800J m -2 would provide about 1400J m -2 of hke in this high frequency band; a scaled-down estimate based on local mean buoyancy frequency and water depth suggests 450J m -2 . The first part of the record (1 to about 16 August) slowly grows then decays in energy to the record minimum at rates between 0.6 and — 1 mW m -2 , followed by about 11 days of sporadic growth and decay at rates between 3 and — 0.8 mW m -2 to the record maximum, and then a week of fast decay at rates between — 2 and — 1 mW m -2 . A speculative balance of possible energy sources, sinks, and advection/ propagation is discussed. The tentative conclusion is that horizontal shears and wind stress fluctuations are the principal energy sources, the latter possibly via interacting surface waves, and that loss of energy from the internal wave continuum to the near-inertial band may be a major sink of hke. A surprising heuristic correspondence is shown between the amplitude of the local surface wavefield, and the internal wave energy 11 days later.