Excitation mechanism of near-inertial waves in baroclinic tidal flow caused by parametric subharmonic instability

Parametric subharmonic instability (PSI) transfers energy from low-mode semidiurnal baroclinic tidal flow to high-mode near-inertial waves at latitudes ∼30°, inducing strong ocean mixing and hence affecting the global ocean circulation. Nevertheless, intuitive descriptions of the physical mechanism for energy transfer by PSI are very sparse. In this study, we reformulate this phenomenon to present a visual image of its mechanism based on a combination of simple classical theories such as beats and parametric excitation without adhering to a strict mathematical formula. It is shown that two small-scale near-inertial waves with slightly different wavenumbers propagating in opposite directions superpose to create beats. When the resulting beats have the peak-to-peak length and the phase velocity equal to the wavelength and the phase velocity of large-scale semidiurnal baroclinic tidal flow, respectively, continuous acceleration of near-inertial motions takes place under the effects of convergence and horizontal shear of the background semidiurnal baroclinic tidal flow. The resonant condition for PSI can thus be easily understood by introducing the well-known concept of beats which also provides a natural explanation for the large difference in spatial scales between the semidiurnal baroclinic tidal flow and the resulting near-inertial waves.