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SWOT’s new observational capacity at the ocean surface fine scales enables the reconstruction of precise vertical dynamics

SWOT’s new observational capacity at the ocean surface fine scales enables the reconstruction of precise vertical dynamics

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Dr Elisa Carli, Internal Research Fellow @Esrin Science Hub

The Surface Water and Ocean Topography (SWOT) is an innovative radar altimeter launched at the end of 2022 with a joint CNES (Centre National d’Études Spatiales)/NASA mission. Thanks to its onboard technology, it has the unprecedented ability to map ocean features at the fine scales (size of the ocean structures between 20 and 100km). These features are critical for understanding vertical velocities, which pump heat, carbon and nutrients from the surface to depth, important for biomass development.

Fig5: Eddy diagnostics on traditional altimetry (MIOST – left column), large scales SWOT (>100 km – central column) and small scales SWOT (<100 km – right column). MIOST diagnostics are on the 7th of December 2023, the SWOT swaths span the 4 days between the 5th and the 8th of December 2023 (the swaths from right to left, respectively). The diagnostics are (a-c) total geostrophic velocity, (d – f) normalized strain rate, (g – i) normalized relative vorticity, (j – l) normalized Okubo-Weiss parameter. The red contour is the PF positioning with the ARMOR3D product. The black dotted line localise the strongest structures.

In this work we use key dynamical metrics to compare the contributions of the different structures to various ocean processes. While large scales dominate the overall flow, smaller scales significantly enhance the stretching and straining of currents, contributing to the increase of the vertical exchange amplitude between the ocean surface and its interior. To validate SWOT’s accuracy, we compare its small-scale observations with high-resolution temperature data collected during a 2023 research voyage. The results align well with subsurface temperature patterns.

Fig7: Reconstruction of vertical velocities with eSQG from (a) SWOT interpolated 2D SSH, (b) MIOST NO SWOT, (c) MIOST SWOT, (d) VarDyn NO SWOT, (e) VarDyn SWOT at 250 m. Reconstructed vertical velocity (f) wavenumber spectra at 250 m depth and (g) rms. The shaded section in (g) is the average depth of the winter waters, where we do not interpret w results. Gridded altimetry products are relative to the 7th of December 2023. SWOT includes observations from December 5th to 9th.

Finally, advanced mathematical techniques allow us to estimate vertical ocean motions down to 1000 meters. SWOT shows double the amplitude of the vertical exchange compared to conventional satellites, which may incur in strong local biological processes that we will investigate in the future.

European Space Agency