Spectral transformations of swell in ocean eddy dipoles from SAR observations and numerical simulations

Mesoscale eddies are energetic oceanic structures that contribute substantially to the ocean’s kinetic energy. They frequently occur in pairs, known as eddy dipoles or mushroom-like eddies, and are potentially ubiquitous in the global ocean. These dipoles generate an intense surface current along their central axis, forming a narrow jet that is considerably stronger than the surrounding flow. We investigate the influence of the central jet of an eddy dipole on the properties and spectral shape of wind waves propagating in opposition to the current. To capture the complex modifications induced by wave–current interactions, a novel inversion method is applied to retrieve directional wave spectra from Synthetic Aperture Radar (SAR) data acquired along an Envisat ground track that crossed an eddy dipole on the Sao Paulo Plateau in the Southwestern Atlantic. The observational analysis is complemented by idealized numerical simulations using state-of-the-art wave and ocean circulation models. Furthermore, the positions and structures of the eddy dipoles and their associated jets are identified using absolute dynamic topography and derived absolute surface geostrophic velocities from satellite altimetry. The results demonstrate that, under favorable geometric conditions, eddy dipoles act as focal lenses for surface waves, refracting them toward the central jet and substantially enhancing wave height—and thus steepness—in the counter-current region. The presence of the dipole broadens the wave spectra in both frequency and directional space, with the magnitude of this broadening modulated by the initial directional spreading of the waves.