Satellites capture first-ever gravity map of tides under Antarctic ice

December 05, 2005

Ohio State University scientists have used minute fluctuations in gravity to produce the best map yet of ocean tides that flow beneath two large Antarctic ice shelves.

They did it using the twin satellites of the Gravity Recovery and Climate Experiment (GRACE), a joint project of NASA and the German Aerospace Center.

Large tides flow along the ocean floor beneath the Larsen and Filchner-Ronne Ice Shelves. Though scientists have long known of these tides, they have not yet been modeled accurately, said C.K. Shum, professor of civil and environmental engineering and geodetic science at Ohio State. Yet the tides play a major role in scientists' efforts to measure how much the ice sheets are melting or freezing, and how the melting ice will affect global sea levels.

While the tides cause only minute fluctuations in Earth's overall gravity, they are actually composed of massive amounts of water, he explained.

The ice is a mile thick in parts, and the tides are so large that they can lift the shelves - with a combined area bigger than the state of California - as high as 15 feet.

Scientists believe that these unseen tides can carve into the ice from underneath and eventually cause pieces to break off, as part of the Larsen Ice Shelf broke off in 1995.

The tides also make the job of measuring changes in the ice more difficult. Large portions of these two ice shelves float on the water, so the rise and fall of the ice with the tides prevents scientists from making precise measurements of ice thickness.

The GRACE satellites offer a good way to track the tides, Shum said. Ocean currents slightly nudge the force of gravity higher or lower in some places around the world every day. GRACE can detect those changes.

Shin-Chan Han, a research scientist in the School of Earth Sciences at Ohio State, presented the study Wednesday in a poster session at the meeting of the American Geophysical Union in San Francisco.

Other research groups have tried to measure these tides terrestrially with sensors called tide gauges. But doing so in Antarctica means first drilling through the ice to plant the sensors on the ocean floor, and then retrieving the sensors later to download the data. Because of the equipment expense and the harsh conditions on the frozen continent, scientists have been able to plant only a handful of these sensors there.

"There were measurements of these tides before, but they were confined to very few spots," Shum said. "To get really accurate measurements - and make really accurate models of how the tides are interacting with the ice - you'd need to put tide gauges or other equipment all over the ocean bottom underneath an ice shelf, and that's not practical."

The twin GRACE satellites have circled the globe in tandem since 2002, effectively drawing a picture of the Earth's gravity field at least once a month. On-board instruments measure very precisely any minute tugs the Earth exerts on the satellites while they're in orbit.

To get a handle on the extent of the Antarctic tides, Shum and his team used GRACE to measure the change in local gravity as water flowed beneath the two ice shelves between August 2002 and June 2004.

Shum and his team, including Han and Koji Matsumoto of the National Astronomical Observatory of Japan, used the gravity variations to calculate expected tide height beneath a number of key points on the two ice shelves.

The researchers compared their data to two Antarctic tide models created by other groups. The two models - which were based on sparse data collected from tide gauges on the continent - agreed with the GRACE data to within 20 percent.

"We have reason to believe that GRACE data is more accurate because the other models are based on substantially less data," Shum said. "So we think that people who incorporate our GRACE data into their own data are going to get better results. We also hope to help glaciologists measure changes in the ice flow much more accurately."

Not knowing precisely where the tides are - and how big they are - creates two kinds of errors in scientists' measurements, he said.

As an ice sheet rises with the tide, part of the grounded portion that normally rests on the ocean floor raises up. Researchers call the point of contact between the ice and the rock the grounding line, and the rise of the ice effectively moves the grounding line inland. That means that while the tide is high, more of the ice bottom is contacting the water than when the tide is low.

Scientists need to know where the grounding line is in order to determine what portion of the ice shelf is being affected by direct contact with the water. Based on the data from GRACE, Shum suspects that previous estimates of where the grounding line is located on these two ice shelves could be off by hundreds of meters (thousands of feet) in different locations.

Meanwhile, the rise and fall of the ice can make the surface appear to be higher or lower than it really is when satellites measure the ice sheets' topography. Depending on the time of day they record their elevation measurements, scientists who use that data to calculate the mass of an ice sheet can get totally different answers.

While people have studied the tides at Earth's middle latitudes for centuries, tides at the poles have presented scientists with serious obstacles, Shum said. Where harsh conditions prevent detailed study, the terrain of the ocean bottom is not well known. The interaction between the water and polar ice sheets isn't fully understood, either.
-end-
NASA and the National Science Foundation funded this work. Shum and Han are continuing the project, and will focus on tides beneath Arctic ice shelves next.

These findings will be presented on Wednesday, Dec. 7, 2005, at 1:40 p.m. PT (4:40 p.m. ET) at the American Geophysical Union meeting in San Francisco, (Session G33B-0042, MCC Level 1).

Contact: C.K. Shum, 614-292-7118; Shum.3@osu.edu

Written by Pam Frost Gorder, 614-292-9475; Gorder.1@osu.edu

Ohio State University

Related Ice Sheet Articles from Brightsurf:

Greenland ice sheet shows losses in 2019
The Greenland Ice Sheet recorded a new record loss of mass in 2019.

Warming Greenland ice sheet passes point of no return
Nearly 40 years of satellite data from Greenland shows that glaciers on the island have shrunk so much that even if global warming were to stop today, the ice sheet would continue shrinking.

Greenland ice sheet meltwater can flow in winter, too
Liquid meltwater can sometimes flow deep below the Greenland Ice Sheet in winter, not just in the summer, according to CIRES-led work published in the AGU journal Geophysical Research Letters today.

Ice sheet melting: Estimates still uncertain, experts warn
Estimates used by climate scientists to predict the rate at which the world's ice sheets will melt are still uncertain despite advancements in technology, new research shows.

Thousands of meltwater lakes mapped on the east Antarctic ice sheet
The number of meltwater lakes on the surface of the East Antarctic Ice Sheet is more significant than previously thought, according to new research.

Researchers discover ice is sliding toward edges off Greenland Ice Sheet
They found that ice slides over the bedrock much more than previous theories predicted of how ice on the Greenland Ice Sheet moves.

A clearer picture of global ice sheet mass
Fluctuations in the masses of the world's largest ice sheets carry important consequences for future sea level rise, but understanding the complicated interplay of atmospheric conditions, snowfall input and melting processes has never been easy to measure due to the sheer size and remoteness inherent to glacial landscapes.

Researchers discover more than 50 lakes beneath the Greenland Ice Sheet
Researchers have discovered 56 previously uncharted subglacial lakes beneath the Greenland Ice Sheet bringing the total known number of lakes to 60.

Ice-sheet variability during the last ice age from the perspective of marine sediment
By using marine sediment cores from Northwestern Australia, a Japanese team led by National Institute of Polar Research (NIPR) and the University of Tokyo revealed that the global ice sheet during the last ice age had changed in shorter time scale than previously thought.

Novel hypothesis goes underground to predict future of Greenland ice sheet
The Greenland ice sheet melted a little more easily in the past than it does today because of geological changes, and most of Greenland's ice can be saved from melting if warming is controlled, says a team of Penn State researchers.

Read More: Ice Sheet News and Ice Sheet Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.