Three decades of sea level multi-mission satellite data reprocessed to improve mesoscale quality while ensuring climate scale consistency

Abstract. Since the launch of TOPEX/Poseidon in 1992, more than 15 satellite altimetry missions have gathered measurements of ocean surface topography. These observations contributed to significant advancements in our understanding of ocean dynamics in the open ocean, coastal and polar areas and at scales ranging from 10 km and a few days to global averages over decades. Heterogeneity across missions and long update cycles of altimeter instrument processing facilities remains a challenge to assemble a multi-mission, consistently processed, state-of-the-art dataset serving the needs of various user types from data assimilation into ocean circulation models to climate science.

In this context, the Delayed Time DT-2024 satellite altimetry reprocessing is a massive endeavor spanning over more than a 100 years' worth of data, from 3 decades, 15 satellites, and 5 climate reference altimeters. In our effort to enhance the user-oriented "reliability" of sea level measurements, we focus on the refinement of altimetry satellite standards (radar processing algorithms and geophysical models) and on the cross-mission consistency. Reliability is here treated as a multi-dimensional spectrum encompassing coverage, precision, accuracy, and stability. These four pillars are essential, not only to capture short-term ocean variability (large and small eddies) for the open, coastal and polar oceans, but also to detect seasonal and long-term climate signals such as global mean sea level rise.

The DT-2024 standards introduce new radar processing algorithms and geophysical corrections. In coastal areas, the error is reduced by 5,6 cm² (or 17%), enhancing monitoring of applications as storm surges and upwelling. In polar regions, error reduction of variance at crossovers exceed 7,7 cm² (or 27%) in the Arctic and 5,9 cm² (or 18%) in the Antarctic, potentially enhancing observation of freshwater fluxes and circulation around ice-covered zones. In open ocean, the error of sea surface height variance at crossovers is reduced by 1,2 cm² (or 6%). Although the contributions of each standard are relatively balanced in the open ocean, the significant improvements observed in coastal and polar regions are largely attributable to the FES-22B tide model, which alone contributes approximately 70% of the gains in these areas. Additional gains come from the TUGO atmospheric correction (forced with ERA5), the CLS/DTU/SIO Hybrid-23 mean sea surface and updated instrumental corrections. These refinements could potentially aid in the detection of mesoscale features, contribute to the assimilation of data into ocean models, and offer insights into dynamic processes such as fronts and internal tides. These improvements stack up with similar gains from previous reprocessing campaigns (e.g. DT-2021), highlighting a continuous progress made in satellite altimetry with every reprocessing cycle since the nineties.

Ensuring the stability of sea level measurements is also crucial for accurate climate monitoring and analysis (Cazenave et al., 2019, Meyssignac et al. 2023). To support climate applications, the DT-2024 definition was rigorous because new algorithms could affect the sea level trends of the multi-mission dataset. To ensure the accuracy and consistency of sea level measurements over time, we align all coverage and precision missions on a unified reference frame based on the reference climate altimeter series. The reference altimeters are extremely consistent with one-another thanks to their so-called tandem phases (formation flight) although we compute the static offsets between subsequent reference altimeters to reduce residual offsets (and to estimate the uncertainty of the transition between reference altimeters). Comparisons with independent in-situ tide gauges yield an agreement within 0.01 mm/year for the regions where tide gauges are located.

The DT-2024 Level-2P dataset is available to users for all the altimeter missions on AVISO+ (https://doi.org/10.24400/527896/a01-2025.004). The Level-3 and Level-4 counterparts are available on the Copernicus Marine Service catalogue (https://marine.copernicus.eu) and C3S (Climate Data Store).