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Wherefore the world's wobble?

Wherefore the world's wobble?

A triumphant team of explorers reaches the precise South Pole (the rotational pole) and plants a flag into the ice. Seven months later, a second group arrives at the pole and sets its own flag. Will the two banners touch?

No. Because of a puzzling phenomenon called the Chandler wobble, they will probably sit about 10 meters apart. A study using supercomputer simulations is now raising some tantalizing ideas about the force that powers this wobble.

The term Chandler wobble refers to Earth's motion around its axis of rotation. If this axis were visible, a person standing near the South Pole would observe that the pole actually moves around, forming a sprial-like pattern with a period of 14.2 months. Among other things, this motion means that a city's latitude is always changing ever so slightly with respect to the axis of rotation.

Scientists say they can explain at least one aspect of the Chandler wobble. Mathematicians have proposed for centuries that because of Earth's squashed shape, the planet naturally tends to wobble. But friction from both the oceans and the elastic inner earth should grind this motion to a halt relatively quickly. The real problem, say scientists, is identifying the force behind the wobble.

The usual lineup of suspects includes earthquakes, motion in the fluid outer core of the planet and some sort of source in the atmosphere. While previous research has shown that the atmosphere could produce a small part of the wobble, Sultan Hameed and Robert G. Currie from the State University of New York at Stony Brook suggest in the March GEOPHYSICAL RESEARCH LETTERS that the atmosphere plays the dominant role in powering the motion. As evidence, they present the results of a supercomputer experiment with a global climate model. This climate program uses laws of atmospheric physics to simulate the behavior of global air temperatures, pressures and other variables.

While examining the changes in simulated air pressure over 23 years, Hameed and Currie detected a cycle with a period of 14.7 months, which is very close to the period of the Chandler wobble. According to the researchers, the cycle is significant because the climate model simulates only the atmosphere and oceans. The equations in the model include no mention of any activity in the solid earth. "It follows that neither earthquakes nor the motions of the core fluid are need to explain the [Chandler] wobble of the solid earth; instead it appears it is produced by atmospheric excitation," the researchers say. This means the atmosphere might drive the wobble by sloshing back and forth across the globe.

Other scientists, however, remain unconvinced. Meteorologist Richard Rosen from Atmospheric and Environmental Research Inc., in Cambridge, Mass., calls the results interesting but says the cause of the 14-month-cycle in the model remains mysterious. He wonders wether the cycle, if it exists in nature, has enough power to drive the wobble.

Thomas A. Herring of the Harvard-Smithsonian Center for Astrophysics, also in Cambridge, says an interesting test on the Chandler wobble will come during the next earthquake above magnitude 8 or so. Measuring the techniques may be accurate enough to resolve whether a quake can affect the wobble, he says.
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Title Annotation:study of Chandler's wobble using supercomputer simulation
Publication:Science News
Date:Apr 8, 1989
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