Internal solitary waves refraction and diffraction from interaction with eddies off the Amazon Shelf from SWOT

Abstract

Off the Amazon shelf, internal solitary waves (ISWs) generated by internal tides interact with mesoscale eddies, leading to significant modifications in their propagation and structure. For the first time, these interactions are directly observed from repeated high-resolution satellite measurements, notably those provided by the newly launched SWOT (Surface Water and Ocean Topography) mission. The study investigates ISWs detection and the modifications in ISW's characteristics resulting from interactions with eddies across three contrasting scenarios: ISW propagation in the absence of mesoscale eddies, ISW refraction by a cyclonic eddy, and ISW diffraction by an anticyclonic eddy. ISW crests are extracted using a band-pass filtering approach, enabling precise tracking of essential geometric and dynamical features, including propagation direction, crest-to-crest spacing, and wavefront shape. Prior to any interaction with eddies, three mode-1 ISW packets propagate in a steady and coherent manner, exhibiting planar wavefronts and consistent direction, with Absolute Dynamic Topography (ADT) amplitudes ranging from 8 to 14 cm. The results highlight the diversity of ISW responses depending on eddy conditions. In the absence of eddies, interaction with a seamount induces energy transfer from mode-1 to mode-3 ISWswaves (13 crests detected), while propagation direction is maintained and the ADT signature weakens to below 8 cm. In the presence of a cyclonic eddy above the seamount, ISW trajectories are refracted westward by roughly 50 degrees (7 crests detected), with increased wavecrest curvature and intensification of mode-3 generation, while ADT amplitudes decrease below 6 cm. Conversely, near the western boundary of an anticyclonic eddy, ISWs split into two distinct pathways: a western branch is refracted with flatter wavefronts, reduced crest spacing, and ADT amplitudes below 0.2 cm (9 crests detected), while an eastern branch follows the eddy's edge, displaying enhanced curvature, recognizable surface signatures of wave packets, and ADT values exceeding 9 cm (6 crests detected). Together, these findings demonstrate the ability of SWOT-based monitoring to capture the complexity of ISW dynamics and provide new insight into their nonlinear response to interactions with mesoscale and submesoscale oceanic structures.