Published on 1 september 2023
The dataset – presented in the Nature Research journal Scientific Data and currently available for download – provides a unique perspective on ocean surface levels, thanks to the mission’s novel orbit and its extended lifespan of more than 13 years.
It was produced and verified by the UK’s National Oceanography Centre (NOC) as part of a wider ESA-backed project to validate CryoSat ocean products.
Part of ESA’s Earth Explorer programme, CryoSat launched in 2010 on its mission to monitor changes to Earth’s icy zones, but the satellite also delivers information – such as sea level, wave height and sea wind speeds – on the global marine environment.
CryoSat carries an instrument – named the Synthetic Aperture Interferometric Radar Altimeter (SIRAL) – that provides a relatively dense coverage of observations, thanks to the mission’s novel long repeat orbit.
The new NOC Sea Level Anomaly (NOCSLA) gridded product is based on high-quality geophysical ocean products processed from SIRAL observations. This daily ¼° sea level anomaly product covers non-coastal oceans – from 60° north to 60° south – between January 2011 to October 2020.
To establish the scientific validity of NOCSLA, the product was compared against data delivered by a variety of space-borne sensors as part of a two-pronged verification process.
Firstly, NOCSLA was contrasted with observations from the international altimetry mission Jason-3, as well as with a sea level product produced by ESA’s Climate Change Initiative, and a sea surface height product produced by the Copernicus Climate Change Service (C3S).
Secondly, oceanic case studies were completed using NOCSLA and the results were compared with those from other data sources and models.
One study focused on assessing the product’s performance in observing events related to El Niño using NOAA’s ENSO (El Niño and the Southern Oscillation) index.
In another case study, NOCSLA data covering the Indian Ocean were used to investigate the occurrence of Rossby waves, also known as planetary waves, that form naturally in rotating fluids. These waves form as a result of Earth’s rotation. Rossby wave speeds determined by NOCSLA were in excellent agreement with a comparison dataset from ESA’s Climate Change Initiative.
Across both prongs of the verification process, NOCSLA outputs aligned well with ocean products from a variety of data sources, demonstrating the suitability of NOCSLA to enable a range of investigations into ocean processes.
Chris Banks, satellite oceanographer at NOC and lead author of the study, said: “Sea level anomaly products based on satellite data – such as the C3S product we employed in the NOCSLA validation – are already widely used.
“NOCSLA is unique because it is based on a single mission with a novel orbit, rather than a traditional repeat orbit every 10-30 days, and a long-term timeseries, creating a benefit for researchers and other data users.”
European Space Agency. (2023). New CryoSat data to boost understanding of the oceans. Earth Online.