[A-CG47] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[ACG47-08] Numerical experiments on excitation of internal gravity waves by surface waves in the ocean

*Takumi Nishikawa¹, Yutaka Yoshikawa¹, Haruka Imamura¹ (1.kyoto university)

Keywords:Surface gravity waves, Internal gravity waves, Langmuir circulations, Triad resonance

Surface gravity waves (SWs) exist at the interface between the ocean and atmosphere. SWs then generate internal gravity waves (IWs) in stratified layer below. Secondary circulations, induced by the interaction between SW and wind-induced current (Langmuir circulations) that pump the mixed layer base (top of the stratified layer) are one possible mechanism generating the IWs (Polton et al. (2008)). Another generation mechanism is nonlinear interaction (triad resonance) between SWs and IWs (e.g. Olbers (2016)). Here, we refer to these mechanisms as the Secondary circulation mechanism and the Triad resonance mechanism, respectively. Both mechanisms generate IWs with frequencies close to the local buoyancy frequency. Such IWs are trapped near the mixed layer base, and are suggested to contribute to turbulent mixing there. Olbers (2016) estimated the dissipation rate of the turbulent kinetic energy and the vertical diffusivity based on the Triad resonance mechanism. Czeschel and Eden (2019) on the other hand demonstrated that eddies in the mixed layer such as Langmuir circulations advected by the mixed layer mean flow generate IWs which vertically radiate kinetic energy from the mixed layer base into deeper stratified layer. These studies suggest that IWs excited by SWs contribute to the exchange of heat and energy between the mixed layer and the stratified ocean interior by triggering turbulent mixing or energy radiation. Understanding of generation mechanism of IWs by SWs is crucial both for air-sea interactions through the mixed layer temperature (sea surface temperature) that is affected by the turbulent mixing at the mixed layer base and for oceanic phenomena beneath the mixed layer via the energy radiation from the mixed layer base into the ocean interior.
However, previous numerical experiments for the Secondary circulation mechanism relied on wave-phase averaged equations in which wave orbital motion (and hence the triad resonance) is not resolved but parameterized by the vortex force. Previous studies for the wave triad mechanism on the other hand do not consider the secondary circulations. Consequently, whether both mechanisms work together or the one mechanism dominates the other remains unknown.
To investigate the excitation processes of IWs by SWs, we conducted numerical experiments using the wave resolving free-surface nonhydrostatic model (Imamura et.al (2025)). We performed experiments similar to the LES experiment of Polton et al. (2008) with freely propagating third order Stokes wave as an initial condition (exp.Free-Surface; FRS). For comparison, we also ran a model with rigid-lid approximation at the surface in which the SW effects are represented by vortex force (exp.Rigid-lid-surface; RLS). Under weak SWs condition, IWs were not generated in exp.RLS, whereas they were excited in exp.FRS. Under stronger SWs condition, exp.RLS showed weaker IWs than exp.FRS. These results suggest the importance of wave orbital motion of SWs in IWs excitation. In the presentation, we will show these results with possible cause of the difference of these experiments.