Equatorial Ocean Response to Rapidly Translating Wind Bursts

Abstract

The response of the ocean at low latitude to idealized westerly wind bursts can be described as a wave wake composed of equatorial gravity and Rossby-gravity modes. The excited waves are those with phase speeds that match the zonal translation speed of a wind burst, typically 10 m s?1. These modes sum to produce oscillations near the local inertial frequency at each latitude, analogous to near-inertial internal gravity waves generated by moving storms at midlatitude. Linear theory predicts that typical wind burst amplitudes (stresses of 0.1 Pa) will generate substantial current oscillations [O (1 m s?1)] in the upper ocean. Response is initially confined to the region directly beneath a wind burst, after which the wake descends and refracts equatorward as a propagating beam. Waves are of sufficient amplitude to dominate shear and vertical strain in the upper ocean. Phase differences between oscillations at neighboring latitudes induce motion in the meridional-vertical plane at ever-decreasing meridional scales. Mixing associated with predicted low Richardson numbers is expected to check development of nonlinearity from vertical and meridional advection by the waves.