Simulating the Long-Range Swell of Internal Waves Generated by Ocean Storms

Near-inertial waves (NIWs) are a special class of internal gravity waves with periods set by planetary rotation and latitude (e.g., at 300 latitude, one cycle per 24 hours). They are notable because they contain most of the observed shear in the ocean and around half the kinetic energy. As such, they have been demonstrated to mix the upper ocean and to have the potential to mix the deep ocean enough to be important for climate simulations. NIWs are principally generated as a result of a resonant coupling between upper-ocean currents and mid-latitude atmospheric cyclones. Here, we report on simulated NIWs in an eddy-resolving general circulation model that is forced by a realistic atmosphere, and we make comparisons to NIWs observed from moored and shipboard measurements of currents. The picture that emerges is that as much as 16% of NIW energy (which is season dependent) radiates out of the mixed layer and equatorward in the form of low-mode, long-lived internal gravity waves; they transmit energy thousands of kilometers from their regions of generation. The large amount of energy in near-inertial motions at a given site is a combination of a local response to wind forcing and waves that have traveled far from where they were generated.