Guillaume Dodet (IFREMER, France); Clémence Chupin (ENSTA, France); Taina Postec (CNRS, France); Laurent Testut (LIENSs, France); Lisa Maillard (CNES, France); Swen Jullien (Laboratoire d’Océanographie Physique et Spatiale (LOPS), Univ. Brest, CNES, CNRS, IRD, Ifremer, IUEM, Brest, 29200, France, France); Frédéric Bouchette (Université de Montpellier, CNRS, France)

Atoll reef islands are highly vulnerable to oceanic climate changes due to their low elevation, high exposure to oceanic swells, and strong dependence on the coral reef health. The dynamics of atoll water levels is directly influenced by tide- and wind-generated waves. Incident swells generate a super-elevation of the lagoon water levels through wave setup and inward flows across the shallow reef sections as a result of wave breaking and bottom friction. Tidal wave propagating through the narrow shallow passes induce strong nonlinear tidal distortion, which modify the relative duration of ebb and flood tides and the lagoon flushing capacity. Finally, resonant processes may also occur in (semi-)enclosed lagoons as a result of long-wavelength oceanic (e.g., infragravity waves) or atmospheric forcings (e.g., wind setup). Due to sparse measurements and the limitations of local sensors, spatial variations in sea surface height within large atolls have been understudied. The most extensive studies to date rely on high-resolution modelling systems combining atmospheric, circulation, and wave models, although these models are rare and typically validated at only a few sites with available in situ measurements. The Rangiroa atoll, located in the Tuamotu Archipelago, is the world’s second-largest atoll. Recent analyses of SWOT LR 2km gridded maps of sea surface height (SSH) in Rangiroa have revealed significant gradients within the lagoon, between the atoll’s exterior and interior, as well as between the northern and southern regions. These spatial gradients align with observed water level variations between lagoon tide gauge measurements and simulated water levels on the atoll’s southern and northern sides. However, discrepancies exist between SWOT observations and tide gauge data, possibly due to uncertainties in the vertical datum, geoid spatial gradients, or inaccuracies in atmospheric and sea state bias corrections. In this study, we further analyze SWOT L2 and L3 products over Rangiroa atoll in order to reconstruct accurate SSH, mean sea surface (MSS) and SSH anomaly (SSHA) maps at 250-m resolution and investigate the different source of uncertainties in SWOT products. The high-resolution SSHA maps reveal uniquely observed spatial features that illustrate the complex interactions between tides and waves at Rangiroa atoll. These observations are validated against tide gauge data as well as static and cinematic GNSS SSH data acquired during an extensive field campaign conducted in May 2025. The major source of uncertainties are presented based on the combined analysis of this new dataset.