CoAuthors

José C.B. da Silva (University of Porto, Faculty of Sciences, Department of Geosciences, Environment and Spatial Planning and Institute of Earth Sciences, Polo Porto, Portugal); Jorge M. Magalhaes (CIIMAR - Interdisciplinary Centre of Marine and Environmental Research, Portugal); Thomas Moreau (CLS – Collecte Localisation Satellites, France); Claire Maraldi (CNES - Centre national d'études spatiales, France); Franck Borde (ESA ESTEC – European Space Agency, European Space Technology and Research Centre, Netherlands); Craig Donlon (ESA ESTEC – European Space Agency, European Space Technology and Research Centre, Netherlands)

Wave breaking is a common phenomenon when there is high to medium wind speeds over the ocean. But the wave breaking can have various sources, being the wind only one of them. For instance, wave breaking occurs near inhomogeneous unsteady currents, when waves propagate into an opposing current steepen, shorten and may break; or more simply, as waves shoal over shallow bathymetry. Wave breaking limits the height of surface waves, mixes the ocean surface, generates currents, and enhances air-sea fluxes of heat, mass and momentum through the generation of turbulence, the entrainment of air and the creation of spray and aerosols (Melville, 2018). In this work, we study the wave breaking caused by large amplitude and nonlinear Internal Solitary Waves (ISWs). Internal Waves play an important role in determining the near-surface sea temperature structure and the air–sea exchange processes, being therefore important for understanding the evolution of the climate system. In the presence of strong ISW-Surface Wave interaction, breaking surface waves are known to occur and hence, it is expected that wave energy dissipates and the wave energy spectrum is altered. Furthermore, it has been recently shown that ISWs are successfully detected by using satellite altimetry.
Here, we select two different regions of the ocean, namely the tropical Atlantic Ocean off the Amazon shelf and the Banda Sea in the Indian Ocean, where there are scenes of Sentinel-3 OLCI (Ocean Land Colour Instrument) acquired simultaneously with along-track SAR mode altimeter, which included signatures of large amplitude ISWs. New data of unfocused SAR (UF-SAR) and fully-focused SAR (FF-SAR) modes is analysed. It has been observed a strong decrease in normalized radar cross section (NRCS) over the rough part of the ISWs, and a small increase in the smooth part relatively to the unperturbed ocean background (Santos-Ferreira et al., 2018). Moreover, we demonstrate that the Significant Wave Height (SWH) parameter is significantly attenuated, after the passage of an ISW, considering length scales of about 10 km before and after the ISW crest (i.e. in 20 km length scales). It is suggested that the cause of this SWH attenuation is related to the wave breaking associated with the ISWs, characterized by surface wave energy dissipation, turbulence effects and air emulsion. Furthermore, Sentinel-2 images are analysed and provide insights admittedly into this same phenomenon: white-capping of two different kinds are reported, the first being a traditional radiance increase at all (visible) wavelengths extended in time scales of tens of seconds, and a second kind associated to quick transient “flashes” of enhanced radiance depicted in different coloured pixels in RGB composite images, with typical time scales of one second or less. Fraction of modulation of breaking waves in the presence of internal waves are presented.