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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.
Source: Ohio State University
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