Study On The Generation And Propagation Characteristics Of Near-inertial Internal Waves In The Western Philippine Sea

Near-inertial waves(NIWs)are an important component of high-frequency oscillations in the ocean with a frequency near the inertial frequency f and appear as prominent peaks in both the continuum internal wave spectrum and the shear spectrum.NIWs are widespread globally and have significant variability in both the spatial and temporal distributions,such as seasonality and differences between latitudinal bands.NIWs are not only important carriers for the energy transport from wind to the deep ocean but also one of the main energy sources for deep ocean internal waves.Therefore,NIWs play a crucial role in ocean mixing and energy dissipation and have a nonnegligible impact on climate.Almost any force can generate near-inertial motion,and NIWs are susceptible to a variety of dynamic processes such as background flow,mesoscale eddies and stratification during propagation.Although the generation,propagation and dissipation processes of NIWs have received extensive attention,the understanding of the generation mechanism and propagation characteristics of NIWs is still insufficient.Based on five sets of mooring observations and HYCOM data,this paper statistically analyses the similarities and differences between typhoon and non-typhoon generated NIWs in the Philippine Sea,explore the mechanism of spontaneous generation of NIWs and the interaction between NIWs and anticyclonic eddies,and describe the seasonal variability of NIWs east of Taiwan.The main work and conclusions of this paper are as follows:1)Using 32 months of mooring observations deployed east of the Philippine Islands,this paper investigated the statistical characteristics of the near-inertial kinetic energy(NIKE)at the observation points.During the whole observation period,the ratio of the typhoon-induced NIKE to the total NIKE was only approximately 1/3,while processes other than typhoons contributed approximately 2/3.The research depth was divided into four layers to explore the relative importance of NIKE.It was found that the ratio of NIKE to the total kinetic energy was highest in the 400 m ～600m layer,not the 50 m ～200 m layer.Four cases of strong NIKE were observed in the non-typhoon period.The characteristics of these non-typhoon generated NIWs differ significantly from those caused by typhoons,with relatively deep core depth,more concentrated energy,smaller vertical group velocity,and frequency very close to the local Coriolis frequency f.Correlation analysis suggests that vertical shear of the background flow is a possible mechanism for the spontaneous generation of NIWs.The ratio of NIKE generated by background flow vertical shear to the total NIKE in the non-typhoon period is 14.1%.In the presence of an anticyclonic eddy,the wind-generated NIWs captured by the eddy and NIWs stimulated by background flow vertical shear together constitute the strongest near-inertial case during the non-typhoon period.2)Based on two sets of subsurface mooring observations and HYCOM data,this paper explores the propagation characteristics of NIWs generated by typhoon MITAG east of Taiwan.Since HYCOM data can provide a reasonable reproduction of this near-inertial event,HYCOM data were mainly used to reveal the role played by anticyclonic eddies in the propagation and dissipation process of NIWs.The results show that most typhoon-generated NIWs propagate towards negative vorticity.In the anticyclonic eddy,NIWs are maintained near the eddy centre and propagate down to800 m.The e-folding time scale for these NIWs is two weeks and a significant enhancement of near-inertial energy flux occurs near the critical layer(400 m ～600 m).In contrast,NIWs outside the anticyclonic eddy is unable to cross the thermocline and dissipates rapidly,which is mainly controlled by low modes.In the anticyclonic eddy,it is mainly high mode NIWs that propagate to the deep layers,and the state of continuous energy growth of higher modes can be maintained for more than a week.In addition,high mode NIWs propagating to the deep layers can be carried westwards by the advection of the mean background flow.3)Using five sets of mooring velocity data obtained in the western Philippine Sea,seasonal variation of NIKE at the observation points are given by calculating the vertical profile of NIKE and dynamic mode decomposition.The results show that NIKE east of Taiwan is strongest in autumn,followed by winter and weaker in spring and summer.But the distribution of NIKE with depth is characterized differently at different observation points.The mooring observations outside the Kuroshio have more energy below 400 m,which may be spontaneously generated or propagated from other regions.In contrast,the NIKE in the Kuroshio is mainly concentrated above 300 m.The mode decomposition results indicated that the NIKE is dominated by the first and the third modes at all three moorings.Only under the influence of strong typhoons will the energy of the second mode dominate for a relatively short period of time.The closer the observation point is to the main axis of the Kuroshio,the higher the proportion of high mode energy.The NIKE at the observation points east of the Philippines is dominated by strong typhoons and can be stronger in summer,autumn or winter.The effect of strong typhoons causes the maximum NIKE to occur in the 100 m～ 300 m layer.