Simon Dechamps (ENSTA Bretagne, France); Swen Jullien (IFREMER, France); Maxime Duphil (UMR ENTROPIE, France)

The hydrodynamic functioning of barrier reef-lagoon (BRL) systems is key to understand the state of health of reef environment marine ecosystems. Moreover, coral reefs act as natural barriers, protecting coastlines from erosion and reducing the impact of storms and waves on the mainland. The dynamics of BRL systems is strongly controlled by incident waves. But in the case of extensive systems, such as the south-west lagoon of New-Caledonia, local winds blowing in the direction of maximum fetch are able to generate wind seas that can dominate the sea state conditions in the lagoon. To date, the spatial variability of sea states within extensive BRL systems has been little investigated because of the lack of data in such systems, and the inadequacy of local in situ sensors to characterize spatial patterns. The most comprehensive studies are certainly based on high-resolution modelling systems, coupling atmospheric, circulation and wave models, but the performance of these models are generally assessed only at a few specific locations, where in situ measurements are available. In this study, we investigate the capacity of satellite-borne radar measurements to monitor significant wave height (Hs) variability in the large south-west reef lagoon system of New-Caledonia over the period 2016-2021. We use high-resolution (20Hz) along-track measurements acquired from the SAR Radar Altimeter (SRAL) on board Sentinel-3A, which intersects the barrier reef with a near normal incidence. The 20Hz records are first filtered using an adaptive denoising method based on Empirical Mode Decomposition in order to enhance the signal-to-noise ratio at scales below 50km. A selection of S3A tracks occurring during energetic synoptic conditions are analyzed and reveal strong Hs gradients from offshore to the reef slope, with maximum reduction in the vicinity of the reef barrier as well as some systematic fluctuations within the lagoon. These patterns are investigated in combinations with in situ observations of wind and waves, as well as numerical model results, and are discussed to the light of existing knowledge on reef-lagoon dynamics and coastal altimetry performance.