Laboratory experiments on air-sea interaction dynamics for shoaling waves.*

When the surface wave field is in local equilibrium with wind forcing, the air-sea transfer coefficients can be expressed as a function of a reference wind speed only. However, such conditions are generally not present in the ocean and exchanges of mass, momentum, gases, etc., at the air-water interface are strongly influenced by sea state conditions. Recent studies show that surface roughness is significantly modified in coastal waters, as shoaling waves slow down and steepen. In particular, an enhancement of the drag coefficient (up to 55%) has been observed for shoaling waves compared to deep water conditions. A correct parametrization of the air-sea transfer coefficients requires detailed knowledge of air-water boundary conditions and it is crucial to obtain reliable high-resolution atmosphere-wave-ocean models. To fulfill this gap, we performed detailed laboratory measurements of the airflow above shoaling waves. The 2D+time measurements allowed us to retrieve the velocity field as a function of the wave phase. In turn, we estimate the relative partition between wave, turbulent, and viscous stresses, and show the phase-dependent turbulent kinetic energy, vorticity, and pressure fields during the shoaling process.