Interactions between near-inertial waves and vortical motion in the ocean

Wind forcing of the ocean generates a spectrum of inertia-gravity waves that is sharply peaked near the local inertial (or Coriolis) frequency. The corresponding near-inertial waves (NIWs) are highly energetic and play a significant role in the slow, large-scale dynamics of the ocean. To analyse this role, we develop a new model of the nondissipative interactions between NIWs and mean flow using a variational implementation of the generalised-Lagrangian-mean formalism. The new model couples the Young & Ben Jelloul model of NIWs with a quasi-geostrophic model in which the relation between potential vorticity and streamfunction is modified by a quadratic wave term. The model reveals that NIWs act as an energy sink for the mean flow: NIWs forced at large scales experience a horizontal scale cascade through refraction and advection; as a result, their potential energy increases at the expense of the mean energy. The implications for ocean energetics are discussed.