Observation of Near-Inertial Waves Induced by Typhoon Lan in the Northwestern Pacific: Characteristics, Energy Fluxes and Impact on Diapycnal Mixing

Based on mooring observations from October 10 to 5 November 2017 at four stations in the Northwestern Pacific, the characteristics of four strong near-inertial wave (NIW) packets generated by the typhoon Lan were examined. The wave-packet analysis revealed that for the NIWs with larger horizontal wavelengths, as their interactions with the background currents were weakened, the observed frequency was close to the intrinsic frequency. The near-inertial kinetic energy (NIKE) between two cyclonic eddies penetrated deeper (similar to 620 m) than that in a negative vorticity region. A ray-tracing model suggested that it was the strong positive vorticity to the north that caused the northward propagating NIWs to be reflected, and then the reflected NIWs were accelerated to propagate downward at similar to 70 m depth where the stratification was strongest. In these two cases, furthermore, the efficiency of the downward propagation of NIKE was at a comparable level of 21%-25%. Energy budget analysis indicated that about 5%-25% of the near-inertial wind work was injected into the upper 50-200 m, approximately 6%-25% of which could be further radiated to the deeper ocean. On average, after the passage of the typhoon Lan, the dissipation rate increased by 3-8 times, and for the enhanced diapycnal mixing, similar to 42% of the energy was provided by the typhoon-induced strong NIWs. Near-inertial waves (NIWs) are a type of internal waves in the ocean that are primarily generated by the winds. Based on mooring observations at four stations in the Northwestern Pacific, four strong near-inertial wave packets were observed after the typhoon Lan in 2017. The four wave packets exhibited distinct characteristics associated with different background flow fields. In addition to the magnitude, their penetration depths varied largely from 130 to 620 m. The underlying mechanisms were then explored using the wave packet analysis method and the ray-tracing model. Energy budget analysis was also conducted, estimating that 5%-25% of the near-inertial wind work could be injected into the upper 50-200 m, and about 6%-25% of the vertical energy flux in the upper 50-200 m layer reached the lower 200-400 m layer. The strong NIWs provided similar to 42% of energy for the diapycnal mixing. All these results in the present study provide a benchmark for further studies of NIWs in the Northwestern Pacific. Four strong near-inertial wave packets were observed after the passage of typhoon Lan The deepest penetration depth of the local near-inertial kinetic energy was observed between two cyclonic eddies The near-inertial wave packets provided similar to 42% of energy for the diapycnal mixing