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Stratospheric winds alter day's length.

Stratospheric winds alter day's length

Scientists have long known that all days are not created equal. Over the millennia, the pull from a gradually receding moon is slowing the Earth's spin and each century adding about two milliseconds to the day. Over shorter periods, different forces cause the day to lengthen or contract by amounts also on the order of milliseconds. While researchers believe that month-to-month changes in day length are driven by shifting wind patterns, the longer year-to-year variations have resisted explanations. Now, a new study suggests that most of these interannual changes appear to result from a combination of two forces, one within Earth's lower atmosphere and another within the upper atmosphere.

In recent years, several researchers have proposed that the climate phenomenon called the El Nino-southern Oscillation (ENSO) might influence the Earth's angular momentum and thus its rate of rotation. The ENSO is a warming in the Pacific Ocean that accompanies shifts in wind over broad areas of the globe. In a previous study, B. Fong Chao from the NASA Goddard Space Flight Center in Greenbelt, Md., found that ENSOs can change the day's length over periods of several years, but they cannot explain all interannual variation.

Chao now reports in the Feb. 17 SCIENCE that the ENSO in combination with another factor can account for most, if not all, of the year-to-year shifts in day length. This second factor is a pattern of winds in Earth's stratosphere, called the Quasi-Biennial Oscillation (QBO). Like a doughnut around the equator, the winds of the QBO circle tropical regions within the lower stratosphere. Roughly every two years, these winds reverse direction.

It is well known that tropospheric winds can subtly speed or slow Earth's rotation through friction on the planet's surface and through torque on mountain ranges. But Chao says it is not yet clear how winds in the stratosphere can alter the planet's spin. A study of the period 1964-1987 shows that the ENSO has twice the strength of the QBO in affecting the length of day.

Richard Rosen, a meteorologist at Atmospheric and Environmental Research, Inc., in Cambridge, Mass., says the new study presents convincing evidence that the QBO and ENSO cause much of the observed interannual changes in day length. Yet he thinks there might be other factors, possibly ocean currents, that also affect the rotation rate on time scales of a year to several years.
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Author:Monastersky, Richard
Publication:Science News
Date:Feb 18, 1989
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