Equatorial Ocean Response to Rapidly Translating Wind Bursts

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  • 1 School of Oceanography, University of Washington, Seattle, Washington
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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.

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.

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