Determination of marine gravity and eddy identification: a comparison of SWOT data versus nadir-looking altimeters
The trenches and ridges on Earth’s sea- floor are shaped by tectonic processes such as seafloor spreading and plate subduction. Detailed knowledge of seafloor tectonics is lacking in many areas. The most comprehensive data come from satellite altimeters, which use the strength and waveform of the radar signal returned from the sea surface to determine the tectonic properties of the underlying seafloor. To achieve optimal accuracy, it is important to combine satellite missions of varying orbit inclinations to obtain a nearly equal accuracy for the north and east components of sea surface slope for gravity derivation. Currently, there are four high- resolution altimeter data sets to serve this need: Geosat, ERS-1, Jason-1, and CryoSat-2, with inclinations ranging from 66° to 108°.
SWOT will improve the spatial resolution and accuracy at levels that cannot be achieved by conventional altimeter missions, vastly improving the gravity field accuracy and resolution in the South China Sea for scientific, ecological and engineering applications.
In the western South China Sea region, the gravity field is rough and oceanic eddies are very active. SWOT will deliver geoid slopes evenly distributed in four critical azimuths, reducing the deficiencies of the conventional altimeter missions in resolving the north and east slope components to determine a best gravity grid and a geoid grid. The high-resolution sea surface heights here are scanned every day in the fast sampling phase and every 21 days in the science phase, allowing an automated, AI-based algorithm to process the fast-changing sea level anomalies to identify cold-cored and warm-cored oceanic eddies and their evolutions in space and time.
Gravity validation of SWOT observations will be carried out in the western South China Sea, where a SWOT track passes through a region of rough gravity field. If SWOT can observe precise SSHs, we can convert them to gravity anomalies, which can be assessed by the gravity values from conventional radar altimeters and from ship measurements. One question is how well SWOT can resolve gravity signals compared to conventional radar altimetry.
The in situ data, including ocean current velocity, salinity, sea surface heights, gravity values and data from sea gliders, will be collected on a cruise of NOR1 (Taiwan’s No. 1 research vessel) from June 7 through July 11, 2023 in the western Pacific Ocean in a region with active oceanic eddies and rough gravity anomalies. The data will be used to detect eddies and determine gravity anomalies for comparison with the results from the SWOT measurements. Gravity field and eddy field from SWOT will also be compared with conventional altimeter data.
Results from the South China Sea will be important for confirming the data quality and resolution of sea surface heights observed by SWOT. It is expected some sort of filtering will be needed to process the raw sea surface heights from SWOT to achieve a best result in gravity recovery and eddy identification. The lessons learned from the South China sea in the gravity field and eddy modeling can be a reference for researchers around the world when such modeling is performed elsewhere.
Principal investigators: Cheinway Hwang (National Yang Ming Chiao Tung University), Emmy TY Chang, (National Taiwan University), Daocheng Yu, (Liaoning Technical University)
Contact point for the study site: Cheinway Hwang (cheinway@nycu.edu.tw) (National Yang Ming Chiao Tung University)