Critical and near-critical reflections of near-inertial waves off the sea surface at ocean fronts

Abstract

In a balanced oceanic front, the possible directions of the group velocity vector for internal waves depart from the classic Saint Andrew's cross as a consequence of sloping isopycnals and the associated thermal wind shear. However, for waves oscillating at the Coriolis frequency f, one of these directions remains horizontal, while the other direction allows for vertical propagation of energy. This implies the existence of critical reflections from the ocean surface, after which wave energy, having propagated from below, cannot propagate back down. This is similar to the reflection of internal waves, propagating in a quiescent medium, from a bottom that runs parallel to the group velocity vector. We first illustrate this phenomenon with a series of linear Boussinesq numerical experiments on waves with various frequencies, ω, exploring critical (ω=f), forward (ω>f), and backward (ω