Modulational instability, wave breaking and formation of large scale dipoles in the atmosphere

TL;DR

This study uses DNS to explore how modulational instability causes wave breaking and large-scale dipole formation in the atmosphere, impacting energy dissipation and air-water interactions.

Contribution

It demonstrates the role of wave steepness in generating atmospheric dipoles and quantifies energy dissipation in air versus water during wave breaking.

Findings

Wave breaking leads to large-scale dipoles in the atmosphere.

Energy dissipation in air exceeds that in water during breaking.

Multiple wave steepening events produce propagating dipoles.

Abstract

In the present Letter we use the Direct Numerical Simulation (DNS) of the Navier-Stokes equation for a two-phase flow (water and air) to study the dynamics of the modulational instability of free surface waves and its contribution to the interaction between ocean and atmosphere. If the steepness of the initial wave is large enough, we observe a wave breaking and the formation of large scale dipole structures in the air. Because of the multiple steepening and breaking of the waves under unstable wave packets, a train of dipoles is released and propagate in the atmosphere at a height comparable with the wave length. The amount of energy dissipated by the breaker in water and air is considered and, contrary to expectations, we observe that the energy dissipation in air is larger than the one in the water. Possible consequences on the wave modelling and on the exchange of aerosols and gases between air and water are discussed.