Observing sea level and ocean currents

EUMETSAT collects and disseminates data from three instruments carried by the Sentinel-3 satellites.

Friday, 13 April 2018

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They are the Ocean and Land Colour Instrument (OLCI), the Sea and Land Surface Temperature Radiometer (SLSTR) and the Synthetic Aperture Radar Altimeter (SRAL).

The SRAL instrument has been designed to deliver accurate measurements of sea surface height, significant wave height and surface wind speeds over the world’s oceans.

Sea level rise is an important indicator of climate change.

Globally, the sea level has been rising by an average of just over 3mm per year for the past 20 years. However, the rise is not uniform – it varies considerably around the world.

Additionally, the rate of sea level rise is increasing by about 1mm per year each decade. This suggests that by the end of the century the sea level could have risen by as much as 65cm more than current projections.
EUMETSAT Project Scientist and Altimetry Expert Remko Scharroo said the SRAL measures an array of important data in addition to sea level.

“Sea surface height data from the satellite’s altimeter have significantly improved our capability to analyse and forecast ocean currents,” Scharroo said.

“This is essential for the applications we serve such as marine safety, ship routing and predicting the fate of marine pollution events.

“Another set of data maps significant wave height, which again is important information needed for ship safety.

“Finally, the SRAL also provides accurate topography measurements over sea ice, ice sheets, rivers and lakes.”

The successful launch of Sentinel-3B at the end of April will represent the full deployment of the Sentinel-3 mission. Sentinel-3A was launched in February 2016 and has been delivering observations for two years.

The two satellites are needed to provide the full set of measurements required by users.

"While sea surface temperature covers the total top layer of the ocean, it does not tell you much about the structure below"

This includes for use in ocean meteorology, which is all about capturing significant wave height, wind speed and sea level measurements.  Altimetry ensures the monitoring of ocean currents and eddies which carry a lot of energy throughout the ocean and interact with the atmosphere, thereby influencing the weather.

An important example is the forecasting of hurricanes. Altimeters can help derive the latent heat in oceans.

“You might argue that the SLSTR instrument does the same thing, that it measures heat in the ocean,” Scharroo said.

“But these are two different types of measurements entirely.

“While sea surface temperature covers the total top layer of the ocean, it does not tell you much about the structure below.

“For this, you need altimeter measurements. Water expands when it’s hot. The more heat penetrating the depth of the oceans, the higher the sea level rises and that will give you vital information about the column of water underneath.

“In short, the SLSTR provides information about the surface but not about the column of water underneath or the latent heat.”

Scharroo said both measurements were valid and important.

“There is no competition between the instruments, believe me,” he said. “It is in synergy that they work best.”

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