Science to Watch: SWOT

Circling above Earth at a speed of seven kilometers (four miles) per second, the SWOT satellite altimeter only needs three weeks to image the entire surface of the planet.

Its focus, however, is on the 73% part of it that is  covered with water: the ocean and large lakes and rivers. Instruments deployed off the California coast by Scripps Institution of Oceanography at UC San Diego and other research centers have been helping calibrate SWOT since February 2023. Even during this initial shakeout, the satellite delivered a goldmine of data, leaving scientists reaching for superlatives to describe how far SWOT might advance the basic understanding of how water in the ocean moves. With that might come a new way to describe climate based on the behavior of the oceans, climate’s main driver on Earth.

“It deserves the attention it’s getting,” said Scripps oceanographer Sarah Gille, who hosted one of the first international SWOT workshops at Scripps Oceanography 15 years ago.

SWOT is short for Surface Water and Ocean Topography, topography here referring to the elevation of the ever-roiling ocean surface. The instrument-packed satellite joined the constellation of human-made objects orbiting the planet on Dec. 16, 2022. It is a joint creation of NASA’s Jet Propulsion Laboratory (JPL) and its French counterpart CNES (Centre National D’Etudes Spatiales) with contributions by the Canadian and UK Space Agencies. Scripps joins six other research institutions leading ocean campaigns based on calibrating SWOT data.

Sea level sounds like a simple enough concept. The ocean is the flat thing against which elevations on land are measured and designated whatever number of feet or meters above sea level. In fact, though, sea levels on Earth are not uniform and are in constant motion in every ocean basin. Picture yourself swirling water in a shallow pan. The spin makes the water level higher around the edge of the pan while water at the center is lower in comparison. Thanks to Earth’s Coriolis force, this also takes place in the oceans.

Whatever might be simple about the process going on stops there. Myriad other forces – eddies, winds, storms, and tides among them – influence the sea level in any given place. Just in the Gulf Stream, a mix of influences makes sea level a steady meter (or yard) higher in one place than it is in another.

Until now, if scientists wanted to know what the elevation of the sea surface was at one exact spot, they could use existing satellite altimetry. This technology measures sea level at a single spot just below the satellite, but there are a lot of exact spots on the ocean surface. Measuring them in relation to each other at any ever-changing moment as swells roll and eddies pass was just never possible.

Enter SWOT. The satellite is taking an image not just of singular locations but of swaths of ocean that are 120 kilometers (74 miles) wide. Where before scientists had still photos to work with in determining the topography of the ocean, SWOT is producing the equivalent of an IMAX movie every time it passes overhead, with spatial resolution that is far superior to conventional altimetry.

“It’s going to provide us a generational shift in how we understand sea surface topography,” said Drew Lucas, also a Scripps oceanographer who spent much of 2023 deploying the instruments that could validate SWOT measurements as it flew overhead.

In oceanographic terms, SWOT’s data are exceptionally detailed, said Gille. The satellite can render the ocean surface down to two kilometers (one mile) Even if its highest resolution were more like 10 kilometers (6.2 miles), that would have been transformative enough. A traditional unit of measurement in the oceanographic community is the mesoscale, that is a middle range between 10 kilometers and several hundred kilometers in which live ocean phenomena like eddies and squalls that were previously invisible to satellite altimetry.

“We haven’t been able to bridge the gap between basin-scale sea-surface topography and what you see at the beach,” Gille said. “We’ve never had a way to do it day after day after day and do it globally.”

That high resolution allows for new kinds of reverse engineering. When oceanographers can see variations in sea surface height over large swaths down to the scale of a few kilometers, then they can try to understand what is behind what they are seeing.

That is vital, Gille said, for an understanding of a natural system that makes Earth habitable at its most essential level.

“More than 90% of the heat gained by our planet is stored in the ocean, and that has consequential impacts on our climate system,” said Gille. “Knowing the structure of the ocean is really important.”

The prospect of understanding that is why Scripps oceanographers and colleagues jumped at the chance to help calibrate SWOT. For a period of months starting in February, a stretch of the California coast was one of the few places in the world SWOT was programmed to look at specifically while being calibrated.

Lucas and Scripps oceanographer Uwe Send joined researchers from the Seattle-based Pacific Marine Environmental Laboratory (PMEL) to set out instrument-laden moorings off the central California coast, supplementing them with underwater gliders supplied by Rutgers University. The goal of the SWOT CalVal campaign, short for calibration and validation, was to ensure that satellite instruments were functioning within specifications.

The moorings were 10 kilometers (6.2 miles) apart and located where SWOT would fly overhead twice daily. For the Scripps moorings, the surface buoys were connected to an anchor on the sea floor four kilometers (2.5 miles) below by a wire with many instruments attached. The upper 500 meters (1,640 feet) were measured every half hour with a Wirewalker – an ocean wave-powered vehicle invented at Scripps Oceanography that climbs up and down the wire. Below that a set of fixed temperature-salinity sensors observed the ocean structure to the seafloor every five minutes. All instruments communicated with a satellite transmitter on the buoy at the top. The synchronized measurements not only validated SWOT but at the same time were making measurements of the whole ensemble of ocean phenomena that influence sea level moment to moment.

The satellite measured sea surface elevation directly while the moorings measured the temperature and salinity of the ocean from the surface to the seafloor, measurements that can give a second estimate of sea surface elevation.

“Nobody else has done such surface-to-bottom profiling plus fixed measurements in real-time before,” said Send. “The data are unique oceanographically and helped to validate the SWOT satellite within months.”

In support of his ship-bound colleagues, Scripps oceanographer Luc Lenain and his team at the Air-Sea Interaction Laboratory collected airborne observations of ocean topography within the SWOT swath using their MASS instrument, or Modular Aerial Sensing System, installed aboard the NASA JSC Gulfstream V jet. That was a second way to validate the satellite’s readings.

“Capturing the spatial variability of sea surface height at smaller scales than currently resolved by satellites is particularly challenging but crucial to CalVal objectives,” Lenain said. “The combination of co-located, coincident airborne and in-situ observational approaches is unique to the SWOT project.”

In all, more than a dozen Scripps researchers across various disciplines have a hand in SWOT. Scripps oceanographer Amy Waterhouse took advantage of the opportunity afforded by CalVal to add current profilers to the moorings. She is interested in the skyscraper-sized internal waves that form and break just as surface waves do on the beach. They slam into continental shelves but what happens then isn’t easy to see above the surface. The profilers measured the velocity of ocean currents as they passed underneath the moorings, providing unprecedented data.

“The data that are going to come out of SWOT are going to have so many different uses in so many different areas of research,” Waterhouse said.

Geophysicist David Sandwell, one of the world’s leading experts on seafloor bathymetry, has to go back to 1978’s Seasat to find another satellite project that has engaged so many ocean researchers at once. He and postdoctoral researcher Yao Yu plan to use SWOT data to improve the resolution of the 75% of seafloor that has not been surveyed by ships. What we know about seafloor features in those areas now comes from satellite measurements of gravity since gravity is stronger the deeper one goes on Earth. SWOT’s margin of error in measuring that is more than five times smaller than what the program’s planners thought it might be and it provides a way to see the seafloor at much higher resolution than conventional readings from satellite altimeters can provide.

“I’m really excited about this mission and will need to delay my retirement,” said Sandwell, whose 40-plus career of research created the seafloor data on apps such as Google Earth. “The gravity and bathymetry data we get will be amazing.”

New uses for the data across science disciplines continue to emerge. Polar researchers have discovered SWOT data can be used to track the deep slow swells generated when icebergs calve. Estuary researchers have a new way to watch how tides change the nature of coastal water masses.

A showing of preliminary data took place in September at a meeting of SWOT researchers in France where the unique data from the Scripps CalVal moorings were also highlighted. Researchers hope SWOT will transmit data for five years or more. 

“Early data from SWOT have already awed the ocean research community, confirming that this new capability will be a game-changer,” said Send. “The Scripps team is excited to have been a partner of the JPL mission team in this from the early stages.”