Generation, propagation and dissipation of near-inertial waves in the tropical ocean

Several aspects of the generation, propagation and dissipation of near-inertial waves were examined for tropical regions using a suite of numerical models and observational data. The primary goals were to investigate how the wind input of inertial kinetic energy partitions between loss by turbulent dissipation at the base of the mixed layer versus downward radiation of near-inertial waves, how deep the near-inertial energy penetrates into the interior, and how the background circulation and stratification impact the radiation of near-inertial waves. Results from a 1D model for upper ocean turbulence indicate that, in the eastern tropical Pacific, approximately 50% of the energy input by the wind is radiated downwards into the thermocline as near-inertial waves, despite displaying significant variability between forcing events. Estimates of the vertical energy flux for near-inertial wave packets observed in data collected in the tropical Indian Ocean are in general agreement with the 1D model results, such that 30% to 40% of the input of inertial energy per unit area by a single wind event is estimated to be radiated by a single packet seen in the 60 to 90 m depth range. The observations show that the near-inertial wave energy penetrates down to at least 200 m, where relatively high turbulent kinetic energy dissipation rates are estimated to have dissipated ≈20% of an energetic wave packet. A process study of the generation and propagation of these near-inertial waves using a linear continuously stratified model suggests that the inclusion of relative vorticity is a key mechanism that promotes localized regions of increased near-inertial energy, as well as increased depth penetration of energy in the vicinity of anticyclonic features. Furthermore, a combination of relative vorticity of order 10% of the local inertial frequency and increased thermocline stratification yields the highest near-inertial energy penetration into the thermocline.