SASSA Global Along-Track 1 Hz Denoised Sea Level Anomalies from Altimeter

For almost 30 years, satellite altimeters supply sea surface height (SSH) measurements which are key observations for advanced studies in ocean dynamics. However, below a wavelength of about 100 km, along-track altimeter measurements are characterized by a dramatic drop in the signal-to-noise ratio, making it very challenging to fully exploit altimeter observations for analysis of small mesoscale variations in SSH. Although various approaches have been proposed and used to filter noise from measurements, no distinctive methodology emerged to be routinely applied in operational products. Because of this unresolved issue, the Copernicus Marine Environment Monitoring Service (CMEMS) provides simple band-pass filtered data to mitigate noise contamination in the along-track SSH signals. More innovative noise filtering methods are left to users seeking to unveil small-scale altimeter signals. Here, a fully data-driven approach is applied and demonstrated to provide robust estimates of noise-free Sea Level Anomaly (SLA) signals. The method combines Empirical Mode Decomposition (EMD), known to help analyze non-stationary and non-linear processes, and an adaptive noise filtering technique inspired by Discrete Wavelet Transform (DWT) techniques. Applied, it is found to best resolve the distribution of the sea surface height variability in the 30-120 km wavelength band. A practical uncertainty variable is then attached to the denoised SLA estimates that takes into account errors in the altimeter observations as well as uncertainties in the denoising process. Here, measurements from the Jason-3, Sentinel-3 and Saral/AltiKa altimeters have been processed and analyzed, and their energy spectral and seasonal distributions characterized in the small mesoscale domain. Anticipating data from the upcoming Surface Water and Ocean Topography (SWOT) mission, these denoised SLA measurements for three reference altimeter missions provided in this dataset already yield valuable opportunities to assess global small mesoscale kinetic energy distributions.This dataset was developed within the Ocean Surface Topography Science Team (OSTST) activities. A grant was awarded to the SASSA (Satellite Altimeter Short-scale Signals Analysis) project by the TOSCA board in the framework of the CNES/EUMETSAT call CNES-DSP/OT 12-2118. Altimeter data were provided by the Copernicus Marine Environment Monitoring Service (CMEMS) and by the Sea State Climate Change Initiative (CCI) project.