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March 4, 2003
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 Niño 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 Niño 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 Niño 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.
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Contacts:
Rob Gutro
NASA Goddard Space Flight Center
(Phone: 301/286-4044)
Stephanie Kenitzer
American Meteorological Society
(Phone: 425/432-2192)
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 The
Spinning Earth is Affected by Winds
Angular momentum describes the rotation of the Earth
around its axis as depicted in this animation. A normal, 24-hour, day
is based on the mean speed of the rotation of the whole Earth, including
its atmosphere and ocean. When the motions in these fluids move mass
to different positions, the angular momentum changes in them and will
affect the solid Earth rotation. Given the rotation rate, fluid mass
and distance from the Earth's radius, If one variable is changed, at
least one other variable also must also change. For example, a spinning
ice skater spins slower with extended arms and faster when his arms are
pulled in. Similarly, the spinning Earth is affected by many factors,
including changes in the way the winds blow or currents in the ocean.
Some of these factors can act to speed the planet up, while others literally
drag it down. Of course these effects are very small, but observable
by advanced scientific techniques. CREDIT: NASA SVS, Jim Strong and Horace
Mitchell
 Atmospheric
Angular Momentum and Length of Day
This graph displays the close connection between the
global angular momentum of the atmosphere derived from wind analyses
(darker red curve, scale on right) and the observed changes in the length
of day (lighter green curve, scale on left), after removing low frequencies,
for the years 2000/2001. The remarkable agreement demonstrates that a
close coupling exists between motions of the atmosphere and the solid
Earth. CREDIT: Atmospheric and Environmental Research, Inc.
 Momentum
of a Bowling Ball
This image shows the momentum of a bowling ball. Credit:
Microsoft Office
 Conservation
of Angular Momentum in Skating
What is Momentum, Angular Momentum, and Conservation
of Momentum? Momentum is a measure of the motion of a body equal to the
product of its mass and velocity, such as the weight of a bowling ball,
and how fast it's moving after being thrown. Angular momentum, such as
the Earth rotating on its axis, is the rotational equivalent of linear
momentum (moving in a straight line). Angular momentum is conserved (not
changed) when there is no outside force (torques) such as winds on its
rotation. Conservation of angular momentum allows ice skaters to spin
faster or slower (such as pictured). It's a law of physics that says
the total momentum of a rotating object with no outside force remains
constant regardless of changes within the system. Therefore, when skaters
draw in their arms, it acts as an "outside force" that changes their
momentum, causing them to spin faster. CREDIT: ScienceJoyWagon.com
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