Detection of open ocean fronts and eddies with Sentinel-3 data

Rosemary Morrow (CTOH / LEGOS, France)


Mei-Ling Dabat (CTOH/LEGOS, France); Samuel Dedoni (CTOH / LEGOS, France); Oscar Vergara (CTOH / LEGOS, France)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Science III: Mesoscale and sub-mesoscale oceanography

Presentation type: Type Forum

The future Surface Water Ocean Topography (SWOT) mission with a SAR-interferometer altimeter will be launched in 2022. SWOT aims to improve our observations of the surface ocean topographic height over two swaths of 50 km width, resolving scales of 15-40 km in wavelength, depending on the sea-state (Wang et al., 2019 ; Morrow et al., 2019). In the preparation of SWOT’s 2D swath observations, we have analysed the 1D alongtrack altimeter data at fine scales across ocean eddies and fronts, detected with today’s 2D gridded DUACS altimeter maps.

This present study will mainly concentrate on the region south of New Caledonia that will be covered by the SWOT 1-day orbit, with a planned in-situ validation campaign for SWOT. Sentinel-3 mission data provide an exceptional data set for this study. Firstly, the alongtrack altimeter data is in SAR-mode globally, with improved signal to noise compared to conventional altimetry, allowing a better resolution of small-scale SSH structures across fronts and eddies. We will colocate S3 SAR altimeter data with the AVISO+ mapped eddy tracked product and FSLE front positions, in order to quantify whether the alongtrack gradients are well colocated with the mapped gradients, and with similar or increased amplitudes, and to identify the strongest fronts. Secondly, Sentinel-3 is also flying with SST and OC sensors, providing a unique colocation of surface tracer observations with the fine-scale alongtrack altimeter data. So for small, rapidly moving eddies and fronts, there will be no offset in time. We will use surface SST and OC tracer fields, combined with the dynamic velocity parameters derived from 1D and 2D altimetry, in order to study fine-scale changes across the fronts and eddies.

When we colocate alongtrack S3 geostrophic velocity data with ocean eddies detected from DUACS mapped data, we find that the mapped eddy velocities are underestimated by 30% in the eastern Mediterranean Sea, but only by 20% south of New Caledonia; this difference is probably related to the larger Rossby radius in the S Pacific, closer to the mapped altimeter spatial scales. We identified the strongest dynamical frontal events in the South Pacific, based on indices from 2D mapped FSLE, geostrophic currents and their derived horizontal surface geostrophic strain rate. When looking at the strongest ocean front events (|FSLE| > 0.338 days-1 and surface geostrophic strain rate > 1.108e-05 s-1), the temporal and spatial distribution shows a high occurrence from September to December and near the longitude 170°E. In the New Caledonia region, a lot of the strongest ocean front events occur under the SWOT swath (1-day orbit phase), with most of them from July to December. We expect the strongest fronts to induce the largest vertical velocities. However, background Chl-A levels are weakest during July to December around New Caledonia. In contrast, when the strain/FSLEs are weaker in the austral winter (May), there are higher chlorophyll levels with a horizontal advection of cold and chlorophyll-a-rich water from the south of the region. We are currently investigating how the colocated surface tracer fields (SST, OC) vary across these strong frontal regions, and how the fine-scale surface wind and waves conditions from alongtrack SAR data are modified over these largest ocean fronts.

Rosemary Morrow