**5. Conclusions**

We analyzed the baroclinic responses to barotropic forcing by investigating two aspects, namely, stratification variability and energy transfer, using the MITgcm model [27]. We first validated the model results and thus proved the reliability of our simulation. The stratification variability was investigated by analyzing profiles of temperature and salinity, as well as time series of buoyancy frequency and potential energy anomaly [40]. The energy transfer was investigated based on the depth-integrated barotropic and baroclinic energy equations [24,41,42], specifically by diagnosing the relative kinetic and potential energy terms and the conversion rate in the equations.

We found that the stratification in the Luzon Strait exhibits daily variation caused by daily variations of baroclinic tidal flows and fortnightly variability mainly caused by rectified baroclinic flows. The interaction between asymmetric barotropic forcing and topography generates intense baroclinic flows and thus offers an approach to increasing the stratification without buoyancy inputs like precipitation, fresh water from rivers, and surface heat fluxes. This interaction can also decrease the stratification, thus resulting in the fortnightly variability of stratification. In a scenario without surface buoyancy fluxes, we demonstrated that the stratification in the Luzon Strait can be periodically redistributed by the interaction between periodic asymmetric barotropic forcing and topography. Each barotropic and baroclinic energy component reflects a spring-neap cycle overlaid on the daily variation. The phases of the fortnightly cycle of baroclinic potential energy and conversion rate at L1, which is one location of internal wave generation in the Luzon Strait, do not match the phase of the barotropic energy component, which indicates that the internal wave generation is affected by this fortnightly stratification variability and that the maximum disturbance of these internal waves may not be generated during the maximum barotropic forcing. Extended to the whole Luzon Strait, this lead–lag relation between barotropic tidal forcing and maximum baroclinic response within the fortnightly tidal cycle generally exists in the source of internal waves. In summary, we infer that the fortnightly variability of stratification in the Luzon Strait due to rectified baroclinic flows can significantly affect energy transfer and internal wave generation.

The exact length of the lead–lag relation that determines the accuracy of internal wave amplitude prediction might be affected by the mixing parameterization in our model and by other processes such as mesoscale eddy intrusion, Kuroshio intrusion, and strong upper-layer mixing induced by winds in the real ocean. In order to improve the ability to predict internal waves, each of the effects of the above factors needs further investigation.

**Author Contributions:** Conceptualization, Z.Z., X.C., and T.P.; methodology, Z.Z.; software, Z.Z.; validation, Z.Z., X.C., and T.P.; formal analysis, Z.Z.; investigation, Z.Z.; resources, X.C and T.P.; data curation, Z.Z.; writing—original draft preparation, Z.Z.; writing—review and editing, Z.Z., X.C., and T.P.; visualization, Z.Z.; supervision, X.C. and T.P.; project administration, X.C.; funding acquisition, X.C. and T.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by National Natural Science Foundation of China (no. U1706218), and the German BMBF CLISTORM under grant no. 03F0781A.

**Data Availability Statement:** The model results presented in this study are available on request from the corresponding author.

**Acknowledgments:** We acknowledge the National Supercomputing Center of Jinan for providing the computing resources. The bathymetric data were obtained from the general bathymetric chart of the oceans 2008 http://www.gebco.net (last access: 26 June 2021). The barotropic forcing data were obtained from the OSU TOPEX/Poseidon Global Inverse Solution 7.2 http://g.hyyb.org/ archive/Tide/TPXO/TPXO\_WEB/global.html (last access: 26 June 2021). The initial field data were obtained from World Ocean Atlas 2009 https://www.nodc.noaa.gov/OC5/WOA09/pr\_woa09.html (last access: 26 June 2021). Two anonymous reviewers provided numerous helpful suggestions for improving the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
