 |
| Changes in sea level height from 1993 to 2017 compared with a long-term mean of the data. Blue and purple are lower than the mean; red, yellow and white are higher. Image credit: NASA/JPL-Caltech. |
A new slideshow celebrates this important data set — a fundamental measurement for the study of the oceans and climate — and the longstanding U.S.-French collaboration that brought it about.
Topex-Poseidon
 |
| Topex-Poseidon illustration. Image credit: NASA/JPL-Caltech. |
Our planet’s oceans are too vast and complex to be fully measured by any single satellite, or even by any single nation. Topex-Poseidon and its successor Jason satellite missions are shining examples of the power of a sustained, long-term international partnership, led by the U.S. and French space agencies, NASA and CNES. For nearly three decades, NASA and CNES scientists and engineers have pooled their expertise, talents and insights to design and construct an integrated spaceborne measurement system far more powerful than the sum of its parts. NASA and CNES have worked together, applying advanced technology to collect measurements of remarkable precision and accuracy, and then making those measurements freely and openly available. With this effort, they have provided humanity with unprecedented views of the global oceans, how they change on time scales of days to decades, and how the oceans influence — and respond to — weather and climate.
“For more than a generation, NASA and CNES scientists and engineers have collaborated to make exquisitely accurate measurements of the ocean surface from space, providing insights into the workings and interactions of our planet’s two great fluid systems, the oceans and the atmosphere,” said Michael Freilich, director of NASA’s Earth Science Division in Washington.
Ocean currents
This is an animation of ocean surface currents from June 2005 to December 2007 from NASA satellites. Watch how bigger currents like the Gulf Stream in the Atlantic Ocean and the Kuroshio in the Pacific carry warm waters across thousands of miles at speeds greater than four miles per hour (six kilometers per hour); how coastal currents like the Agulhas in the Southern Hemisphere move equatorial waters toward Earth's poles; and how thousands of other ocean currents are confined to particular regions and form slow-moving, circular pools called eddies. Credit: NASA/SVS. Download video.
The Topex-Poseidon mission was the first to monitor the changing patterns of major ocean surface currents in a comprehensive way. Ocean current locations are revealed by large-scale hills and valleys on the ocean surface, which can vary by more than 6 feet (2 meters) in height. The peaks and dips defining the ocean’s topography are caused by variations in water temperature and pressure. Large-scale currents like the Gulf Stream tend to flow along contours of constant ocean height, following the sides of the hills and valleys. The steepness of a slope indicates the speed of the current. Unlike terrain on land, however, the liquid "landscape" shifts with changes in winds, temperature and other factors, causing shifts in the locations and speeds of the currents. The only way to monitor these changes over the entire surface of Earth's ocean is to make precise measurements of the height of the ocean surface from orbiting satellites.
Measuring the ocean shape over nearly the entire globe every 10 days, Topex-Poseidon gave the first quantitative view of how ocean currents change with the seasons. Topex/Poseidon and the Jason-1, Jason-2 and Jason-3 missions have provided unique insights into how ocean circulation affects climate by moving heat from place to place on our planet.
Heat storage in the ocean
 |
| NOAA's annual assessment of the heat in the upper ocean (2015 shown), a measure of global warming, draws on Topex series data. Image credit: NOAA. |
"As human-caused global warming drives sea levels higher and higher, we are literally contributing to the reshaping of the surface of our planet," said Josh Willis, NASA project scientist for Jason-3 at NASA's Jet Propulsion Laboratory in Pasadena, California. "The precision altimetric satellite missions tell us how much and how fast."
El Niño, La Niña, and more
 |
| Among Topex-Poseidon's early achievements was recording the full extent of a record El Niño in 1997 and the succeeding La Niña in 1999. Darker colors are sea levels lower than normal, lighter and white colors are higher than normal. Image credit: NASA/JPL-Caltech. |
In the last 25 years, with the help of altimetry data, scientists have pinpointed many global connections between these multi-year ocean variations and weather consequences such as drought and flooding throughout the globe. While these events have by no means yielded all their secrets, they are better understood and better forecast than before global spaceborne observations began.
Tides on the open ocean
 |
| A numerical model of daily global tides using sea level data from Topex-Poseidon. Image credit: ESR. |
Before satellite measurements, deep-ocean tide measurements were
difficult to make, expensive and sparse. Topex-Poseidon made the first
global maps of tides, which changed scientists' understanding of how
tides dissipate. The data show that a third of tidal energy dissipates
in the open ocean, playing important and previously unknown roles in
mixing water within the ocean.
Jason-1
Topex-Poseidon had a three-year prime mission, but long before that time was up, oceanographers and other Earth scientists recognized the value of continuing its measurements as long as possible. Fu explained, "Sea surface height is a fundamental measure of the Earth system, so it was a no-brainer that scientists would want to have this kind of information indefinitely." With strong community support, Jason-1 was constructed by NASA and CNES and launched in December 2001. For three years, Topex-Poseidon and Jason-1 flew in coordinated orbits that allowed scientists to cross-calibrate their measurements and then combine the data sets to observe the global oceans more frequently. Each succeeding mission has also overlapped its predecessor, ensuring a consistent data record.
So far, each of the ocean altimetry missions has proven to be long-lived. Topex-Poseidon was eventually decommissioned in 2005 after 13 years in orbit. Jason-1 survived almost 12 years, until July 2013. Nine-year-old Jason-2 and Jason-3 (launched in January 2016) are still in operation.
Jason-2
Jason-1
Topex-Poseidon had a three-year prime mission, but long before that time was up, oceanographers and other Earth scientists recognized the value of continuing its measurements as long as possible. Fu explained, "Sea surface height is a fundamental measure of the Earth system, so it was a no-brainer that scientists would want to have this kind of information indefinitely." With strong community support, Jason-1 was constructed by NASA and CNES and launched in December 2001. For three years, Topex-Poseidon and Jason-1 flew in coordinated orbits that allowed scientists to cross-calibrate their measurements and then combine the data sets to observe the global oceans more frequently. Each succeeding mission has also overlapped its predecessor, ensuring a consistent data record.
So far, each of the ocean altimetry missions has proven to be long-lived. Topex-Poseidon was eventually decommissioned in 2005 after 13 years in orbit. Jason-1 survived almost 12 years, until July 2013. Nine-year-old Jason-2 and Jason-3 (launched in January 2016) are still in operation.
Jason-2
 |
| Lee Fu (left) was the project scientist for Topex Poseidon and Jason-1 and -2. Josh Willis is the current project scientist for Jason-2 and -3. Image credit: NASA/JPL-Caltech. |
With the launch of Jason-2 in June 2008, the focus of spaceborne
ocean altimetry transitioned from research objectives to data
applications providing tangible benefits to society. Mission operations
moved from the research agencies NASA and CNES to the U.S. National
Oceanic and Atmospheric Administration (NOAA) and the European
Organisation for the Exploitation of Meteorological Satellites
(EUMETSAT); indeed, satellite altimeter measurements are used routinely
in NOAA’s El Niño forecasts. NASA and CNES continue to provide science
teams, instrument design, and science-focused, specialized data
management.
Forecasting
On smaller space and time scales, satellite altimetry measurements
provide information directly useful for marine storm prediction.
Hurricanes are fueled by heat stored in the ocean below, and since the
upper ocean expands and contracts as it heats and cools, sea level
height is a marker for water temperature and heat content. So it is
hardly surprising that ocean altimetry data are routinely used in
forecasting hurricane strength.
In 2014, an unexpected forecasting use for altimetry data became operational. Bangladesh, whose 46-year history has encompassed death-dealing river floods, uses Jason-2 measurements of river levels in its flood forecasting and warning system. Within the first year using these data, Bangladesh's system enabled the most accurate, long-lead flood warnings ever given for that nation.
Navigation
Civilian sailors and the U.S. Navy use the series' near-real-time
data on currents, eddies, winds and waves to aid surface and underwater
navigation. Information on eddy currents in the Gulf of Mexico has been
used by marine operators to schedule offshore drilling operations, with
significant cost savings.
Jason-3
Artist's rendering of Jason-3. Image credit: NASA/JPL-Caltech.
Forecasting
 |
| Jason-1 data contributed to this forecast of Hurricane Rita's track across the Gulf of Mexico in 2005. The storm track appears as a black line. Jason-1 observed a tongue of very warm water (red) in the gulf, 13-23 inches (35-60 centimeters) higher than surrounding water. Ocean heat can strengthen hurricane intensity. Image credit: NASA/JPL-Caltech/University of Colorado. |
In 2014, an unexpected forecasting use for altimetry data became operational. Bangladesh, whose 46-year history has encompassed death-dealing river floods, uses Jason-2 measurements of river levels in its flood forecasting and warning system. Within the first year using these data, Bangladesh's system enabled the most accurate, long-lead flood warnings ever given for that nation.
Navigation
 |
| The U.S. Navy uses the ocean altimetry satellites' data to aid surface and underwater navigation. Image credit: U.S. Navy. |
Jason-3
When Jason-3 launched in 2016, NASA project scientist Willis commented, "This mission has big shoes to fill. Its predecessors have built one of the clearest records we have of our changing climate." Jason-3 has performed flawlessly in continuing the global record of precise sea-surface topography measurements and is now halfway through its prime mission.
A new role for Jason-2
 |
| Jason-2's new, lower orbit will allow scientists — such as Walter H. Smith (NOAA) and David Sandwell (Scripps Institution of Oceanography), who produced this map — to improve their understanding of features on the global seafloor. Image credit: NOAA. |
Besides protecting Jason-3, the new orbit will allow Jason-2 to produce an improved, high-resolution estimate of Earth's average sea surface height. Because ocean topography is partly determined by the contours on the ocean bottom, the estimate is expected to enable scientists to improve maps of the seafloor, resolving currently unknown details of underwater features such as seamounts. These maps will permit advances in ocean modeling, tsunami wave forecasting and naval operations support.
Into the future
 |
| Illustration of the upcoming Sentinel-6 mission. Image credit: ESA. |
Satellites have already revolutionized oceanography, and soon they will do the same for hydrology -- the study of water on land. The French/U.S. Surface Water and Ocean Topography (SWOT) mission will be at the forefront, carrying an innovative interferometer dubbed KaRin that marks a break with today's technologies.
Fu notes that these changes show the value the world scientific community places on the ocean altimetry program. "The measurement is so important, and the technology is fully demonstrated," he said. "In the long haul, ocean altimetry is an international commitment."
Links
