Formation of a Quasi-Universal Internal Wave Spectrum by Wind Forcing Alone: Idealized Modeling and Mooring Observations

Abstract Oceanic internal waves (IWs) play a key role in diapycnal mixing that sustains the global overturning circulation. Their energy follows a quasi-universal Garrett–Munk (GM) spectrum, traditionally attributed to nonlinear wave–wave interactions under combined wind and tidal forcing. However, growing evidence points to the importance of eddy–wave interactions, but the specific energy contributions from mesoscale and submesoscale motions to this GM spectrum remain unclear. More importantly, the generation of this GM spectrum by eddy–wave interactions has not been observed in the real ocean. In the present work, an idealized numerical model is used to simulate the formation of a GM-like spectrum by eddy–wave interaction under wind forcing alone. Energy transfers are diagnosed in both Eulerian and Lagrangian coordinates using a multiscale energy and vorticity analysis. It is found that IW energy supplied by mesoscale eddies is comparable to that from submesoscale motions in Lagrangian coordinate, whereas this mesoscale contribution appears much weaker in Eulerian coordinate. Vertically, mesoscale eddies transfer energy to IWs in the upper ocean, while this energy transfer is reversed in the pycnocline. In situ mooring observations from the Southern Ocean further support the role of eddies in facilitating the generation of a GM-like spectrum under wind forcing alone. Together, these findings clarify the energy pathways through which mesoscale and submesoscale dynamics energize IWs and offer new insight into the dynamical processes that help maintain the GM spectrum. Significance Statement A quasi-universal internal wave (IW) spectrum is traditionally thought to be created by wave–wave interactions through simultaneous wind and tidal forcing. Using idealized simulations and Southern Ocean observations, this study provides evidence that wind forcing over an eddying ocean can generate a Garrett–Munk (GM)-like spectrum. Lagrangian energy transfer analyses reveal comparable mesoscale and submesoscale contributions to IW energy, whereas direct extraction of mesoscale energy is less apparent in Eulerian coordinate. Vertically, mesoscale eddies energize IWs in the upper ocean, while IWs return energy to mesoscale eddies in the pycnocline, thus limiting the penetration of wind-induced IWs. These results clarify the dynamical pathways feeding IWs, providing new insights into the formation of a GM-like spectrum under wind forcing alone.