High-frequency sea level temporal variability estimate from SWOT KaRIn and Sentinel-3A/B crossovers

The observation of sea level variability on very small time scales ranging from less than an hour to a few days is currently very limited with the constellation of nadir altimeter satellites. Calculating sea surface height at crossovers between a single mission or several nadir altimeter missions makes it possible to analyse the sea level variability on these very short timescales for a small number of measurements (a few hundred). Moreover, these observations are spatially sparsely distributed, most often at very high latitudes for crossovers of less than a few hours of time difference. Thanks to the launch of the joint CNES/NASA SWOT mission in December 2022 with swath measurements, a new paradigm for observing high-frequency temporal variability in sea level is now possible. SWOT KaRIn instrument offers 2D observations of the oceans with an unprecedented coverage and resolution. We take advantage of this new high quality dataset to estimate the sea level variability over short time scales. The crossovers of SWOT KaRIn during the 1-day orbit phase with itself and with Sentinel-3A/B nadir observations less than 1 hour to 72 hours of time difference from 17/04/2023 to 07/07/2023 are analysed. The crossovers with Sentinel-3A/B offer a very good spatial sampling of the oceans which cannot be reached with SWOT crossovers only. We analyse the variance of the sea surface height differences at the crossovers, as a function of time difference and of latitude. The variability of the sea surface height differences at the crossovers contains both the sea level variability and the instrumental errors. We attempt to disentangle the errors from the sea level variability. Sea surface height differences variance with time differences tending to 0 hours, hence free of oceanic variability, amount to ~3.2-3.5 cm. This value increases fastly with time difference up to about 4 hours as the contribution of sea level variability and errors increase. With time differences of 4 hours, the sea surface height differences variance reaches 4 cm. Then, the increase of the sea surface height differences variance slows down, as some phenomena are not correlated anymore, and roughly linearly increases to reach 5 cm for time differences of 72 hours.