Energy exchanges between a two-dimensional front and internal wave modes

TL;DR

This study develops a quasilinear model to analyze energy exchanges between a two-dimensional oceanic front and internal wave modes, revealing mechanisms like deformation shear production and convergence production during frontogenesis.

Contribution

The paper introduces a novel quasilinear model that quantifies KE exchanges between fronts and internal waves during different stages of frontogenesis, including a newly identified convergence production mechanism.

Findings

High-frequency IWs efficiently extract KE via deformation shear production.

Minimum frequency IWs remain locked to the front and exchange energy with the circulation.

Convergence production is a key mechanism during superexponential front sharpening.

Abstract

Fronts and near-inertial waves are energetic motions in the upper ocean that can interact and provide a route for kinetic energy (KE) dissipation of balanced oceanic flows. A quasilinear model is developed to study the KE exchanges between a two-dimensional geostrophically-balanced front undergoing strain-induced semigeostrophic frontogenesis and internal wave (IW) vertical modes. The quasilinear model is solved numerically for variable imposed strain magnitudes, initial IW vertical modes, and for both minimum frequency (near-inertial, NI) and high-frequency IWs. The front-IW KE exchanges are quantified separately during two frontogenetic stages -- an exponential sharpening stage that is characterized by a low Rossby number and is driven by the imposed geostrophic strain, followed by a superexponential sharpening stage that is characterized by an order-one Rossby number and is driven by the convergence of the ageostrophic secondary circulation. It is demonstrated that high-frequency IWs quickly escape the frontal zone and are very efficient at extracting KE from the imposed geostrophic strain field through the deformation shear production (DSP) mechanism. Part of the extracted KE is then converted to wave potential energy. Minimum frequency IWs remain locked to the frontal zone and therefore exchange energy with the ageostrophic frontal circulation. During the exponential stage, IWs extract KE from the geostrophic strain through DSP and transfer it to the frontal secondary circulation via the ageostrophic shear production (AGSP) mechanism. During the superexponential stage a newly identified mechanism, convergence production (CP), which is directly linked to the convergent secondary circulation, plays an important role in the NIW KE budget.