Abstract's details
Assessing SWOT-Derived Surface Velocities Against HFR Data: Insights from the Ibiza Channel
Event: 2025 SWOT Science Team Meeting
Session: Oceanography: Calibration and Validation
Presentation type: Poster
The Surface Water and Ocean Topography (SWOT) mission offers an unprecedented view of Sea Surface Height (SSH) at significantly higher spatial resolution (~10 times) than conventional altimetry. This capability reveals previously unobserved submesoscale eddies and fronts. However, at these scales, the assumption of geostrophic balance, commonly used to derive current velocities from SSH, is not always valid. Consequently, it is vital to evaluate SWOT-derived velocities against independent high-resolution observations. The FaSt-SWOT project team has already contributed to this global effort by comparing the satellite measurements against a comprehensive dataset built from multi-platform in-situ data collected in the Balearic Sea throughout two oceanographic campaigns during SWOT’s fast-sampling phase, in April and May 2023.
Within this context, we now extend the evaluation by comparing geostrophic velocities derived from SWOT SSH against High-Frequency Radar (HFR) surface current measurements collected in the Ibiza Channel. This dynamically complex area, characterised by an intricate topography and significant mesoscale activity, including energetic eddies and filaments, constitutes an ideal natural laboratory for studying fine-scale processes where deviations from geostrophic balance are anticipated. In order to describe the temporal and spatial variability of the surface circulation in the area, we analysed two years of HFR-measured current velocities. Calculated Eddy Kinetic Energy (EKE) spectra present peaks at the diurnal, inertial, and semi-diurnal frequencies, which show high seasonal variability. Wavelet coherence analysis evidenced the importance of wind as a primary driver of surface currents in the area.
SWOT’s remarkable ability to detect fine-scale ocean dynamics is evidenced in a snapshot from 26 January 2025 that shows two counter-rotating submesoscale eddies with centres ~ 45 km apart. The eddies are clearly visible in the HFR-derived velocities and exhibit remarkable spatial coincidence with SWOT’s SSH Anomaly (SSHA) signal. However, the geostrophic velocities distributed with the SWOT L3 expert v2.0.1 product overestimate the filtered HFR velocities for the cyclonic eddy characterised by a SSHA of 10 cm and a ~10 km radius. Geostrophic velocities derived from SSH reached values exceeding 30 cm/s, whereas the HFR velocities extracted from 1-month time series in January 2025 and filtered using a second order 25-h Butterworth filter, remained below 20 cm/s. The Rossby number calculated from the HFR measurements over this eddy is 0.6, indicating a clear deviation from geostrophic balance. Hence, we applied the cyclogeostrophic balance approximation to SWOT observations (Penven et al., 2014, Tranchant et al., 2025), and obtained velocities which were on average 15 cm/s lower, and much closer to the HFR measurements. This empirical validation highlights the critical importance of verifying the scales and conditions for which the geostrophic balance approximation breaks down when deriving submesoscale currents from high-resolution SSH.
Ongoing work focuses on extending the comparison between SWOT and HFR-derived velocities over a longer period, covering both the Ibiza Channel and the Delta del Ebro region.
Within this context, we now extend the evaluation by comparing geostrophic velocities derived from SWOT SSH against High-Frequency Radar (HFR) surface current measurements collected in the Ibiza Channel. This dynamically complex area, characterised by an intricate topography and significant mesoscale activity, including energetic eddies and filaments, constitutes an ideal natural laboratory for studying fine-scale processes where deviations from geostrophic balance are anticipated. In order to describe the temporal and spatial variability of the surface circulation in the area, we analysed two years of HFR-measured current velocities. Calculated Eddy Kinetic Energy (EKE) spectra present peaks at the diurnal, inertial, and semi-diurnal frequencies, which show high seasonal variability. Wavelet coherence analysis evidenced the importance of wind as a primary driver of surface currents in the area.
SWOT’s remarkable ability to detect fine-scale ocean dynamics is evidenced in a snapshot from 26 January 2025 that shows two counter-rotating submesoscale eddies with centres ~ 45 km apart. The eddies are clearly visible in the HFR-derived velocities and exhibit remarkable spatial coincidence with SWOT’s SSH Anomaly (SSHA) signal. However, the geostrophic velocities distributed with the SWOT L3 expert v2.0.1 product overestimate the filtered HFR velocities for the cyclonic eddy characterised by a SSHA of 10 cm and a ~10 km radius. Geostrophic velocities derived from SSH reached values exceeding 30 cm/s, whereas the HFR velocities extracted from 1-month time series in January 2025 and filtered using a second order 25-h Butterworth filter, remained below 20 cm/s. The Rossby number calculated from the HFR measurements over this eddy is 0.6, indicating a clear deviation from geostrophic balance. Hence, we applied the cyclogeostrophic balance approximation to SWOT observations (Penven et al., 2014, Tranchant et al., 2025), and obtained velocities which were on average 15 cm/s lower, and much closer to the HFR measurements. This empirical validation highlights the critical importance of verifying the scales and conditions for which the geostrophic balance approximation breaks down when deriving submesoscale currents from high-resolution SSH.
Ongoing work focuses on extending the comparison between SWOT and HFR-derived velocities over a longer period, covering both the Ibiza Channel and the Delta del Ebro region.
Contribution: ST2025OS1-Assessing_SWOT-Derived_Surface_Velocities_Against_HFR_Data__Insights_from_the_Ibiza_Channel.pdf (pdf, 2141 ko)
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