Low-Mode Near-Inertial Wave Generation

Abstract Near-inertial currents can be generated by abrupt shifts in wind. Some of these currents project onto low-mode near-inertial waves (NIWs), which can travel thousands of kilometers. Here, a reduced-physics model [the coupled-mode shallow water (CSW) model] is proposed with the goal of simulating global low-mode NIW generation and propagation. In this study, CSW performance is analyzed in an idealized setting based on the Ocean Storms Experiment (D’Asaro et al.). We show that NIW generation is analogous to internal-tide generation, except waves are excited by inertial pumping (convergences and divergences in inertial currents) rather than by barotropic flow over sloping topography. A theoretical solution for internal-tide generation (Llewellyn Smith and Young) predicts CSW NIW generation on an f plane. Numerical simulations on f and β planes confirm that NIWs are only generated when inertial pumping occurs along the inertia–gravity dispersion curve, as predicted by theory. Therefore, the frequency bandwidth of inertial pumping (due to the β effect or mesoscale vorticity) limits the generation of NIWs at very short wavelengths, even if inertial pumping occurs at small scales. We also show that weak damping associated with linearized bottom drag (or unresolved processes and imperfect numerical methods) can significantly alter the fraction of wind work that is radiated as NIWs.