Philippe Schaeffer (CLS, France); Maxime Ballarotta (CLS, France); Antoine Delepoulle (CLS, France); Marie-Isabelle Pujol (CLS, France); Gerald Dibarboure (CNES, France)

The mean sea surface (MSS) represents the mean of the sea surface height (SSH) over a given period. It allows to define the so-called sea surface height anomaly (SSHA) in use for plenty of ocean applications ranging from geostrophic currents determination to extreme events detection or climate change monitoring. Errors in the mean sea surface are automatically reflected as sea surface height anomaly errors, which affects the performance of the applications. The data provided by the Ka-band Radar Interferometer (KaRIn) from the Surface Water and Ocean Topography (SWOT) mission is expected to bring a revolution to the altimetry field, and mean sea surface is no exception. SWOT KaRIn strength relies on two key advantages: its outstanding precision and its 2-dimensional swath data. The improvement of precision should make possible the resolution of small-scale ocean features previously unattainable. The 2-dimensional measurements will reveal the spatio-temporal coherence of the oceanic features, not only along-track but also across-track. In this work, we deliver a new MSS model: the CNES CLS 2025. Starting from a MSS first guess for the long wavelengths, the new model takes advantage of SWOT KaRIn data to improve the lower-mesoscale accuracy. It also benefits from the long-term temporal stability of thirty years of nadir altimetric data used through the static part of a Multiscale Inversion of Ocean Surface Topography (MIOST) mapping. The CNES CLS 2025 MSS unveils seamounts or other geodetic structures never seen before and outperforms state-of-the-art MSS model on many metrics of interest. These results using SWOT data underline the potential of swath altimetry for the future of ocean topography.