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Rob Gutro
Goddard Space Flight Center, Greenbelt, Md.     March 4, 2003
rgutro{at}pop900.gsfc.nasa.gov
(Phone: 301/286-4044)

Stephanie Kenitzer
American Meteorological Society
Kenitzer{at}dc.ametsoc.org
(Phone: 425/432-2192)

RELEASE: 03-21

CHANGES IN THE EARTH'S ROTATION ARE IN THE WIND

Because of Earth's dynamic climate, winds and atmospheric pressure 
systems experience constant change. These fluctuations may affect how 
our planet rotates on its axis, according to NASA-funded research that 
used wind and satellite data.

NASA's Earth Science Enterprise (ESE) mission is to understand the 
Earth system and its response to natural and human-induced changes for 
better prediction of climate, weather and natural hazards, such as
atmospheric changes or El Nino events that may have contributed to the 
affect on Earth's rotation.

"Changes in the atmosphere, specifically atmospheric pressure around 
the world, and the motions of the winds that may be related to such 
climate signals as El Nino are strong enough that their effect is 
observed in the Earth's rotation signal," said David A. Salstein, an
atmospheric scientist from Atmospheric and Environmental Research, 
Inc., of Lexington, Mass.who led a recent study. 

From year to year, winds and air pressure patterns change, causing 
different forces to act on the solid Earth.  During El Nino years, for 
example, the rotation of the Earth may slow ever so slightly because 
of stronger winds, increasing the length of a day by a fraction of a 
millisecond (thousandth of a second).

Issac Newton's laws of motion explain how those quantities are related 
to the Earth's rotation rate (leading to a change in the length of 
day) as well as the exact position in which the North Pole points in
the heavens (known also as polar motion, or Earth wobble).

To understand the concept of angular momentum, visualize the Earth 
spinning in space. Given Earth's overall mass and its rotation, it 
contains a certain amount of angular momentum. When an additional 
force acting at a distance from the Earth's rotational axis occurs, 
referred to as a torque, such as changes in surface winds, or the
distribution of high and low pressure patterns, especially near 
mountains, it can act to change the rate of the Earth's rotation or 
even the direction of the rotational axis.

Because of the law of "conservation of angular momentum," small but 
detectable changes in the Earth's rotation and those in the rotation 
of the atmosphere are linked.  The conservation of angular momentum is 
a law of physics that states the total angular momentum of a rotating
object with no outside force remains constant regardless of changes 
within the system. 

An example of this principle occurs when a skater pulls his or her 
arms inward during a spin (changing the mass distribution to one 
nearer the rotation axis, reducing the "moment of inertia," and speeds 
up (increasing the skater's spin); because the moment of inertia goes 
down, the spin rate must increase to keep the total angular momentum 
of the system unchanged.

"The key is that the sum of the angular momentum (push) of the solid 
Earth plus atmosphere system must stay constant unless an outside 
force (torque) is applied," Salstein said."So if the atmosphere speeds 
up (stronger westerly winds) then the solid Earth must slow down
(length-of-day increases). Also if more atmosphere moves to a lower 
latitude (further from the axis of rotation), and atmospheric pressure 
increases, it also gains angular momentum and the Earth would slow 
down as well." 

Other motions of the atmosphere such as larger mass in one hemisphere 
than the other can lead to a wobble (like a washing machine with 
clothes off-balance) and the poles move, in accordance to the law of 
the conservation of angular momentum.

Salstein looked at wind and pressure measurements from a National 
Weather Service analysis that makes use of a combination of 
ground-based, aircraft, and space-based observations.The measurements 
for the Earth's motions come from a variety of space-based 
measurements including satellites, like those in the Global
Positioning System (GPS), the geodetic satellites that included 
records from NASA's older LAGEOS satellite, and observations of 
distant astronomical objects using a technique known as Very Long 
Baseline Interferometry.  Understanding the atmospheric pressure 
patterns, moreover, is essential to interpret results from NASA's
Gravity Recovery and Climate Experiment (GRACE).

The fact that the two vastly different systems, namely the 
meteorological and the astronomical, are in good agreement according 
to the conservation of angular momentum gives us assurance that both 
these types of measurements must be accurate. It shows, moreover, that
changes in climate signals can have global implications on Earth's 
overall rotation.

NASA's ESE research focuses on the changes and variability in the 
Earth system, including atmospheric, oceanic, and geodetic areas. This 
research was recently presented at the annual meeting of the American
Meteorological Society in Long Beach, Calif.

For more information and images, see:

     http://www.gsfc.nasa.gov/topstory/2003/0210rotation.html

Special Bureau for the Atmosphere of the International Earth Rotation 
Service (IERS), at Atmospheric and Environmental Research, Inc.:

     http://www.aer.com/groups/diag/sb.html

The LAGEOS 1 & 2 Satellites:

     http://msl.jpl.nasa.gov/QuickLooks/lageosQL.html

Gravity Recovery and Climate Experiment (GRACE) satellite:

     http://www.csr.utexas.edu/grace/

NASA's Earth Science Enterprise:

     http://earth.nasa.gov

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