High-frequency wave dynamics in mixed seas: The influence of swells on wind waves characteristics

Abstract

The effect of the swell in modifying wind stress and short wave properties is well-recognized in the oceanographic community. Nevertheless, the swell is generally neglected in studies of fetch-limited wind waves despite its everyday presence in most of the world’s seas. During the early stages of wind wave development, ocean surface waves play a crucial role in modulating heat, momentum, and gas transfer between air–sea interfaces. In a measurement campaign conducted in the Gulf of Mexico, continuous measurements with a very high temporal resolution of the directional spectrum of water waves and turbulent Reynolds stresses were obtained using a spar buoy. Events associated with cold fronts were selected. The swell was separated from the local wind-generated waves using the method proposed by Hanson and Phillips (2001). It was observed that the swell dampens young wind-sea energy. In particular, under conditions of swell opposite to the wind. In the equilibrium region, the wind-sea energy is much lower when the swell is present than when it is not. In addition, the swell modifies the energy level of the wind-sea related to the Toba constant, bringing it to a sub-saturated energy level. Swell steepness and swell index are parameters that contribute to modulating short-wave energy. Short waves lose energy by the physical mechanism of dissipation, by the instability of their crest, and therefore break. In a secondary role, the swell reduces the energy extracted from the wind by reducing the surface roughness associated with short waves and less capacity to extract momentum from the atmosphere. It is concluded that the impact of the swell on the wind-sea depends on several factors, such as the swell direction relative to the wind, the swell energy ratio, and the swell steepness.