Solving one mystery of polar wander

Astronomers have long known that the Earth wobbles as it spins. Several irregularities in rotation—small oscillations superimposed upon larger wobbles atop even larger waggles—cause the location of the true North Pole, about which the Earth rotates, to meander across the Arctic landscape.

The causes of some components of the pole’s overall movement are well understood, but the driving force for one element—the so-called Chandler wobble—has remained a mystery.

Seth Carlos Chandler Jr., a businessman turned astronomer, discovered this phenomenon in 1891. By itself, the Chandler wobble would cause the pole to move back and forth about 20 feet every 14 months. Scientists have calculated that the wobble would die out within 68 years if there weren’t a constant source of energy to reinvigorate it.

Over the past century, some researchers suggested that interactions between Earth’s core and the mantle that surrounds it cause the wobble. Others blamed annual changes in water distribution among the continents.

Although recent studies concluded that regular changes of the oceans and atmosphere probably have enough power to drive the wobble, the findings didn’t differentiate among those potential causes. Now, an analysis by Richard S. Gross, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., points a finger at long-term fluctuations in pressure at the ocean’s bottom.

Using improved models of the oceans and the atmosphere, Gross calculated variations in the overall amount of torque on Earth. He then compared these figures with the power needed to produce the Chandler wobble that scientists measured between 1985 and 1995. Gross presents his findings in the Aug. 1 Geophysical Research Letters.

He found that ocean currents and winds play only a minor role in driving the wobble. About two-thirds of the power driving the Chandler wobble probably comes from pressure changes on the ocean floor. Variations in the salinity and temperature of ocean water underlie such fluctuations, in part. Another third of the power likely comes from changes in atmospheric pressure, he adds.

Measurements of wobble may someday substitute for undersea data used for monitoring global variations in the oceans, suggests Clark R. Wilson, professor of geophysics at the University of Texas at Austin. “This finding could eventually help us better understand the role of the oceans as a driver for climate change,” he says. “We don’t have a lot of weather stations on the bottom of the oceans.”