Global Near-Inertial Wave Spectra Shaped by Mesoscale Eddies

Abstract

Wind-forced near-inertial waves (NIWs) propagate through a sea of mesoscale eddies, which can fundamentally alter their evolution. The nature of this NIW--mesoscale interaction depends on how dispersive the waves are. For weakly dispersive waves, ray tracing suggests that the NIW frequency should be shifted by $\frac{1}{2}ζ$, where $ζ$ is the mesoscale vorticity, and that the waves are refracted into anticyclones. Strongly dispersive waves, in contrast, retain the large-scale structure of the wind forcing and exhibit a small negative frequency shift. Previous in situ observational studies have indeed revealed varying degrees of NIW--mesoscale interaction. Here, observations of NIWs from drifters are used to map the geography of NIW--mesoscale interactions globally, and idealized simulations and a simple model are used to identify the underlying physical processes. Almost everywhere in the ocean, with the notable exception of the North Pacific, the NIW frequency is strongly modulated by the mesoscale vorticity, with the slope of the frequency shift vs. vorticity taking values of approximately $0.4$. Concentration of NIW energy into anticyclones is a common feature throughout the ocean. Other aspects of the observations, however, show signatures of strongly dispersive waves: a negative frequency shift and weaker concentration into anticyclones in regions with strong eddies as well as weak modulation of the NIW frequency by mesoscale eddies in the North Pacific. The signatures of both weakly and strongly dispersive NIW behavior can be rationalized by the geography of the wave dispersiveness and the fact that wind forcing excites multiple vertical modes with different wave dispersiveness. These results have implications for NIW-induced mixing in the upper-ocean.