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2025
Dec. 2025: Swot for ocean forecasting

Swot for ocean forecasting

Image of the Month - December 2025

Model forecast (6-day) of Sea Level Anomalies over the California Current on 2023/11/14. Left the model with no Swot assimilation, right with Swot (credit Mercator Ocean International)

Sea Level Anomalies from Swot measurements over the California Current on 2023/11/14 (credit Mercator Ocean International)

Model forecast (6 days) of vorticity over the California Current on 2023/11/14. Left the model with no Swot assimilation, right with Swot (credit Mercator Ocean International)

Swot outperforms nadir altimeters for characterizing ocean mesoscale, waves, bathymetry, internal waves, swell, and many other oceanographic and environmental applications. Its added value for one of the main applications of altimetry data: ocean modelling and forecasting is considered here.

Sea level anomaly (SLA; a, b) and sea surface temperature (SST; d, e) snapshots from the model without SWOT assimilation (a, d) and with SWOT assimilation (b, e) in the Mediterranean Sea on 11 July 2023. (c) SLA measurements from the seven nadir altimeters between 5 and 11 July 2023. (f) Satellite SST observations from the Mediterranean Ultra High Resolution L3S product on 12 July 2023. Contours represent SLA from the model without SWOT assimilation (a) and with SWOT assimilation (b–f). Black dots indicate "Adopt a crossover" (AdAC) drifter positions on 11 July 2023, and black lines show their 7-day trajectories. (credits Mercator Ocean International)

Swot is planned to be assimilated into the operational GLO12 Mercator Ocean International model (1/12° resolution) in 2026. Prior to launch, studies were conducted using simulated observations to adapt data assimilation systems to wide-swath, high-resolution altimetry. Since Swot observations became available, the system has continued to be fine-tuned and validated, with both the CalVal and Science phases.
Compared to an independent nadir altimeter (AltiKa onboard Saral, not assimilated here), results show improvements of up to 10% during the CalVal phase and to 18% during the Science phase when Swot is added to the assimilated (nadir) altimeters. Both analyses and forecasts benefit from Swot assimilation.
Overall, Swot assimilation provides a more accurate representation of mesoscale features in terms of shape and location, across all computed variables, not only at the surface but throughout all the water column. In particular, the two-dimensional sea surface height measurements from Swot improve both zonal and meridional current components, whereas nadir altimetry mainly constrains the zonal component.

Now that the Swot LR (ocean) data latency has been shortened, the models will be able to assimilate them to forecast the ocean forecasting.

References:

Benkiran, M., Ruggiero, G., Greiner, E., Le Traon, P.-Y., Rémy, E., Lellouche, J.M., Bourdallé-Badie, R., Drillet, Y., Tchonang, B., 2021. Assessing the Impact of the Assimilation of
SWOT Observations in a Global High-Resolution Analysis and Forecasting System Part 1: Methods. Front. Mar. Sci. 8. https://doi.org/10.3389/fmars.2021.691955
Fouchet E., Benkiran, M., Le Traon, P.Y., and E. Rémy (2025). Comparison of a global high-resolution ocean data assimilation system with SWOT observations Frontiers in Marine Science https://doi.org/10.3389/fmars.2025.1563934
Fouchet et al., Surface currents : Evaluating SWOT assimilation in Mercator Ocean global system: insights from AdAC campaigns surface drifters, Swot Science Team Meeting 2025, Arcachon, France.