| 18 Mar 2026
Energetic near-inertial waves induced by winter storms and mesoscale eddies in the subtropical Northwestern Pacific Ocean
Abstract. Near-inertial waves (NIWs) play a fundamental role in transferring wind energy into the ocean interior and sustaining diapycnal mixing, yet their wintertime characteristics and interactions with mesoscale eddies remain insufficiently understood. Using subsurface mooring observations and reanalysis products in the subtropical Northwestern Pacific Ocean, we investigate the generation, downward propagation, and modal characteristics of NIWs associated with two winter storm events. Although the wind energy input into mixed layer during the first storm is approximately three times larger than that during the second storm, the observed near-inertial kinetic energy (NIKE) in the thermocline is comparable between the two events. Energy transfer analyses show that mesoscale eddies extract about 46 % of the wind-generated near-inertial energy during the first event, whereas they supply approximately 43 % of the wind input to NIWs during the second event, leading to similar observed NIKE intensity. Additionally, the two NIW events exhibit distinct vertical wavelengths, group velocities, and modal structures. The first event is characterized by a larger vertical wavelength, faster downward group velocity, and dominance of low baroclinic modes, with the first four modes accounting for nearly half of the total NIKE. In contrast, the second event displays shorter vertical wavelengths and enhanced high-mode energy, with modes five to eight contributing about 41 % of the total NIKE. These differences are attributed to the combined effects of mesoscale eddy modulation and the modal projection of wind energy. Our results highlight the critical roles of winter storms and eddy-wave interactions in shaping NIW propagation and characteristics in wintertime.
Summary:
This manuscript addresses an important question regarding the propagation of wind-generated near-inertial internal waves. The fate of near-inertial waves' energy is an important topic because the breaking of near-inertial waves serves as a source of energy for vertical mixing in the ocean interior, particularly in the upper ocean. This study investigates the propagation of near-inertial wave energy into the interior and the energy transfer rate between waves and eddies for two winter storm events, based on the data from the mooring system and reanalysis product. Their results highlight the need to account for the interactions between waves and eddies when interpreting the propagating near-inertial energies and their dissipation, leading to turbulent mixing. While the paper is well-motivated and contributes to science, I believe the manuscript requires substantial revision before it can be considered for publication.
Major comments:
A) The introduction section is well-structured, citing observational studies of wind-generated near-inertial internal waves. However, the novel aspects of the present study compared to these prior studies, as well as the motivation for this research, remain somewhat unclear to readers who are not familiar with these fields. I think it would be better if these points were summarized as the first few sentences of the final paragraph of the Introduction.
B) In Section 4.1, the authors explain that the similar NIKE values between the two events—despite significant differences in the input of wind-generated near-inertial energy—can be understood by taking into account the difference in energy transfer rates between mesoscale eddies and NIW. However, is it reasonable not to consider other energy sinks for the wind-generated NIW's energy inputs? Kunze et al. (1995) assumed that there are three possible energy sinks for wind-generated NIW in the anticyclonic eddy, including i) loss to the mean flow, ii) loss to un-trapped waves that can freely propagate, and iii) instability of shear, leading to turbulence production. Although this manuscript considers only i), is it acceptable to ignore the other two candidates in the interpretation of your observational data? I understand that quantification is difficult—particularly regarding point iii), given the absence of turbulence observations—but the possible effects by ii) and iii) should at least be discussed in the discussion section.
My other comments are mostly on details that could be improved or on which I had minor questions.
Minor comments:
Line 86: Please consider showing the deployment depth of each instrument (ADCPs, SBE 37-SM, and SBE 56) as a figure. It could be helpful for readers to see the vertical resolution of the observed density profile.
Line 93: What value of the damping parameter is applied in the slab model?
Line 99-101: Please clarify how the mixed layer depth (MLD) is defined in the present study.
Line 101-103: To verify the validity of the reanalysis product, please add a depth-time plot of the squared buoyancy frequency N2 and the plots of the mixed layer depth (MLD) of both the observations and the reanalysis product in the supp info. The reproducibility of these variables is important considering that they are used to estimate the near-inertial energy inputs and their modal decomposition.
Line 110: Please check if the citations for this sentence are correct. I guess that the other citations (e.g., Gill (1982)?) would be more appropriate.
Line 127: Please clarify the definition of background horizontal velocity (U, V). Do the authors apply the low-pass filtering spatially and/or temporally to estimate ‘background’ velocity from the daily outputs of the reanalysis product?
Line 137: Is the velocity vector V the same as the background flow (U, V) used in Eq. (4)? Please clarify it.
Line 138: I think that |V| in this sentence should be V.
Line 140: Do both kz and m represent the vertical wavenumber? Please use the consistent representation.
Line 142: Please remove ‘horizontal’ in this sentence. Only the vertical group velocity is defined here.
Line 144: I think Equation (9) needs a minus sign. Please check it.
Line 159: It would be helpful to include a comparison between the uni derived from the slab model and the bandpass filtered uni from the observations, even if it's just in the supplementary information.
Line 177-179: Why does the total NIKE (Fig 5c) appear to be smaller than the decomposed downward energy (Fig 5d) at certain time periods and depths, such as 150 m depth in WS1 and WS2?
Line 182-184: Please clarify how to estimate decay time here. Is the decay time defined as the time it takes for the energy propagating along a ray path determined by the group velocity to decrease to one-tenth of its maximum? It might be helpful to understand if the authors indicate the time and depth of 1) the maximum NIKE and 2) the one-tenth of maximum NIKE in Fig. 5c.
Line 201: Please consider indicating the values of 1.01f₀ and 0.98f₀ as vertical lines in Fig. 6. I would like to check whether the red-shifted near-inertial peak shown in Fig. 6c corresponds to 0.98f₀.
Line 208-210: How is the vertical wavenumber (or vertical wavelength) estimated in the present study? Is the wavenumber estimated as m=d(atan(uni/vni))/dz, same as in Chen et al. (2023)? Please clarify it.
Table 1: How are the errors (±) in Table 1 estimated?
Line 252-253: Why does the Fig 9a show the estimated P “within” the mixed layer, rather than “below” the mixed layer, where near-inertial internal waves can freely propagate? Does the author assume that the energy transfer between the near-inertial motions and eddies within the mixed layer is dominant for the net energy budget throughout the water column?
Line 256-257: Please consider showing the definition of the OW parameter used in the present study.
Line 264-265: Please also show the time-integrated energy transfer rate ∫∫ρPdzdt during WS1 and WS2 and compare them with the time-integrated energy flux ∫Fdt.
Line 268: I understand that it is not possible to estimate the vertical variation of relative vorticity from the data obtained by the single mooring site. However, I am concerned about whether there is any reason why and can be used in P in Eq. (4) but are not used to estimate relative vorticity?
Line 295: By presenting an equation, please clarify how the authors estimate the modal projection of wind-induced energy flux following Raja et al. (2022).