The first global survey of the Earth’s surface waters with the SWOT satellite mission

Abstract

The NASA/CNES international Surface Water and Ocean Topography (SWOT) mission was launched in December 2022 to provide the first global survey of the Earth’s surface waters. The fundamental advancement of SWOT is the capability to observe the elevation of the ocean and terrestrial surface waters with the resolution of SAR imagery.  The 2D spatial resolution is more than an order of magnitude better than conventional satellite altimetry, enabling the study of water storage and exchange from millions of small-scale lakes, rivers and floodplains, as well as observing the small-scale ocean eddies and fronts that are essential to the ocean’s heat and carbon uptake from the atmosphere. The increased resolution will also advance the study of nearshore processes to assess the coastal impact of sea level rise, flooding and severe weather. SWOT’s global coverage up to 78° in latitude allows enhanced observation of the ocean circulation and sea-ice dynamics in the rapidly-changing polar oceans. During the first 6 months of the mission, SWOT was in a 1-day fast-repeat orbit, firstly for engineering checkout and then for scientific validation. This provided a unique data set to explore the rapid evolution of the small-scale dynamics and water storage and exchange. From mid-July 2023 onwards, SWOT has provided global coverage in its 21-day orbit, and will continue for a minimum of 3 years. Numerous field campaigns were conducted in 2023 to validate SWOT in the open ocean, nearshore and coastal regions, and over the lakes and rivers sampled by SWOT’s repeat orbits. SWOT’s unique, high-resolution 2D observations of water elevation and SAR imagery, combined with field campaign data, and other satellite data and models, provide a new vision of many small-scale dynamical phenomena: from ocean and nearshore zones (mesoscale eddies, dynamical fronts, tides and internal tides, effects of air-sea interactions) to coastal and estuarine contexts (tidal deformation, multi-scale water level changes, flooding) and the global freshwater water storage and exchange between lakes, reservoirs, and rivers. Yet the rapid changes of the ocean and inland waters revealed by the 1-day repeat data pose a challenge to the analysis of the 21-day repeat data from the global mapping phase of the mission.  New mapping techniques are required to enable us to retain the small-scale, rapidly-evolving structures that are observed as snapshots during the global 21-day sampling phase. The SWOT mission team are reaching out to machine learning, data inversion and assimilation specialists to help us reap the rewards from a wealth of new global information about the Earth’s surface waters. Finally, SWOT is the first global mission demonstrating excellent results with SAR-Interferometry, paving the way for future planned operational missions in the 2030s (eg Copernicus Sentinel-3 Next Generation Topography missions).