A numerical study on energy transfer between near-inertial internal waves and super-inertial internal waves in the South China sea under the influence of a typhoon

The energy transfer between near-inertial internal waves (NIWs) and super-inertial internal waves (SIWs) in the South China Sea (SCS) after the passage of Typhoon Son-tinh was investigated on the basis of a three-dimensional numerical model. Our model nicely reproduces the spectral peak (6.6 × 10⁻² m² s⁻² cpd⁻¹) within the near-inertial frequency band, which agrees well with that (6.5 × 10⁻² m² s⁻² cpd⁻¹) in the in-situ observations at the Xisha mooring. Model results demonstrate that the energy transfer rate from NIWs (0.8f∼1.8f) to SIWs (>1.8f) within the mixed layer in the wake of typhoon Son-tinh is an order of magnitude larger than that during the typhoon-free period. Analogously, the super-inertial shear variance increases by nearly an order of magnitude as well. The increase in the energy of SIWs is mainly due to the energy cascade of NIWs through the nonlinear wave-wave interaction. The interaction between NIWs and SIWs was also revealed by the bicoherence spectrum. Compared with that on the left side of the typhoon track, the NIW kinetic energy on the right side is stronger, where the interaction between NIWs and SIWs is more intense. Several sensitivity experiments were designed to further investigate the effects of three typhoon parameters, namely the radius of maximum typhoon wind speed ($R_{\max }$), the maximum typhoon wind speed ($V_{\max }$) and the moving speed of typhoon ($U_m$), on the energy transfer rate (ETR) from NIWs to SIWs. It is shown that, the ETR increases linearly with $R_{\max }$. Among three parameters, $V_{\max }$ has the strongest effects on the ETR which grows in power with increasing $V_{\max }$. The ETR increases and then decreases with the enhancement of $U_m$, which gets maximum when $U_m$ is about 6∼6.5 m/s. Overall, our results highlight the energy transfer between near-inertial internal waves and super-inertial internal waves under the influence of a typhoon.