A unifying view of averaging for wave-mean flow decomposition

Abstract

Geophysical flows are routinely decomposed into mean flow and waves, and the procedure used determines how the motion is divided between the two. Ideally, the wave and mean motions of interest evolve on distinct timescales, in which case they can be separated by taking an infinite-time average over all wave periods. Individual waves average away, while exactly resonant interactions contribute to the averaged dynamics because they are steady on the fast timescale. We generalize this averaging to finite windows, yielding a family of decompositions that additionally retain near-resonant interactions whose phases do not fully cancel over the time window. Such averaging necessarily invokes a choice of reference frame; the same flow averaged in two frames may yield mean flows differing by a drift. The averaging formulation developed here enables us to express Stokes drift through resonant and near-resonant interactions, confirming the intuition that it is a slow quantity generated by fast waves and revealing a coherent near-resonant contribution generally present in Stokes drift calculated over finite time from broadband wave fields. We use the fast-wave evolution underlying the exact average to define both the wave reference frame, in which the fast phase is steady, and a wave-frame average. We cast familiar Eulerian and Lagrangian averages and this wave-frame average in a common form to further illustrate how the chosen frame determines which motions enter the mean. This view of averaging provides a common basis for comparing decompositions and choosing among them according to the physical processes one seeks to isolate.