Improving Total Surface Currents by Combining Altimetry and In-Situ Observations Using the MIOST Variational Method

Satellite altimetry provides observations of sea surface height and derived geostrophic currents through the operational DUACS/Copernicus Marine Service Sea Level Thematic Assembly (SLTAC). However, these currents represent only part of the total surface circulation, which also includes ageostrophic components such as wind-driven currents (Ekman and Near-Inertial Oscillations (NIOs)).

To address this limitation, complementary observation-based approaches have been developed to optimally combine satellite and in-situ data, improving the physical realism and temporal resolution of upper-ocean circulation fields. In this study, we use the Multiscale Inversion for Ocean Surface Topography (MIOST) variational tool (Ubelmann et al., 2020) to retrieve both geostrophic and ageostrophic currents. MIOST achieves this by decomposing the signal into representors accounting for different spatial and temporal scales (mesoscale to large-scale) and physical processes (geostrophy, wind-driven currents, NIOs).

Using MIOST, altimetry data are combined with hourly drifter velocities from the Copernicus Marine database (INSITU_GLO_PHY_UV_DISCRETE_MY_013_044, https://data.marine.copernicus.eu/product/INSITU_GLO_PHY_UV_DISCRETE_MY_013_044/description) to represent wind-driven currents, in particular the energetic NIOs, which remain a major challenge for satellite observation (SKIM mission concept, Ubelmann et al., 2021; ODYSEA mission). Previous studies within CNES-funded DUACS project have shown that combining altimetry and hourly drifter data can represent about 30–40% of NIOs in regions with sufficient drifter coverage (e.g., North Atlantic gyre). Additionally, ESA-WOC project has shown that wind reanalyses (ERA5) can predict a substantial fraction (~40%) of NIO variability (Ubelmann et al., 2025).

The objective of this work is to combine these two complementary approaches: starting from prior ageostrophic currents (ESA-WOC) and applying the MIOST variational method to reconstruct geostrophic currents and residual NIO signals contained in drifter observations. Parameter tuning of MIOST modes was required since the statistical mapping is applied to residuals rather than the full signal.

The reconstructed currents are intercompared with existing products, including Copernicus-Globcurrent total surface currents (MULTIOBS_GLO_PHY_MYNRT_015_003) and ESA WOC total surface currents.

 

Elipot, S., R. Lumpkin, R. C. Perez, J. M. Lilly, J. J. Early, and Sykulski, A.M.: A global surface drifter dataset at hourly resolution, J. Geophys. Res. Oceans, 121, doi:10.1002/2016JC011716, 2016

Ubelmann, C., Dibarboure, G., Gaultier, L., Ponte, A., Ardhuin, F., Ballarotta, M., & Faugere, Y. (2021). Reconstructing ocean surface current combining altimetry and future spaceborne Doppler data. Journal of Geophysical Research: Oceans, 126, e2020JC016560. https://doi.org/10.1029/2020JC016560

Ubelmann, C., Farrar, J. T., Chapron, B., Gaultier, L., Gomez-Navarro, L., Rio, M.-H., and Dibarboure, G.: A data-driven wind-to-current response function and application to ocean surface current estimates, Ocean Sci., 21, 2915–2928, https://doi.org/10.5194/os-21-2915-2025, 2025.