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  Geotimes - December 2007 - ENSO lengthens the days of our lives
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Geophysics
ENSO lengthens the days of our lives

The linked atmosphere-ocean phenomenon known as the El Niño-Southern Oscillation (ENSO) is well-known for its powerful effect on climate around the world every three to eight years. But, as atmospheric scientists have known for several decades, ENSO also has an even more global effect: It makes the days a little longer. However, the effect only shows up several months after an ENSO event. Now, a new study suggests an answer to this puzzle.

The ENSO-related increase in the length of a day is slight — even during a powerful ENSO year such as 1982-1983 or 1997-1998, the length of a day only increases by a millisecond at most, says Jean Dickey, a physicist at the Jet Propulsion Laboratory at Caltech in Pasadena, Calif. Still, scientists are eager to better understand how the system works. What is known is that the angular momentum of Earth spinning on its axis is affected not only by the mass of the solid planet, but also by changes in wind. During a strong El Niño event, the winds within the upper troposphere become powerful enough to increase Earth’s atmospheric angular momentum; so, to keep the total angular momentum of the system unchanged, the planet responds by spinning a little more slowly, and the days get a little longer (the reverse happens during a La Niña year).

One mystery, however, has been this time lag of several months between the increased sea-surface temperatures driving an ENSO event and the corresponding increase in day length. “We’ve never quite figured out the reason for it,” Dickey says. To isolate only the changes due to interannual events such as ENSO, she and her colleagues studied atmospheric data from 1979 to 2004, observing time series of atmospheric angular momentum changes and removing seasonal cycles. “It’s a fact that what produces the length of day variations are thermal winds, driven by the change in temperature,” caused by heat transferred upward from the warmer sea-surface temperatures, she says. So the team analyzed not only the measured temperatures in the atmosphere, but also the temperature gradients between different layers and altitudes in the atmosphere at different latitudes. Those gradients, she says, drive the winds.

“And bingo, there it was,” Dickey says. Indeed, the largest gradients occurred about one to two months after the warm sea-surface temperatures, the team reported Sept. 1 in Geophysical Research Letters. “It fills in a lot of the gaps.”

“It’s a good clue, a good first step of why this is happening,” says David Salstein, an atmospheric scientist from Atmospheric and Environmental Research, Inc., of Lexington, Mass. This mechanism shows how it works, he says, “yet one still needs to look a little more closely into the transport of heat itself through the whole ocean-atmosphere system.” ENSO is a big, complicated system, he says, and while Dickey’s team focused on the time and atmospheric effects correlating to one particular indicator of the ENSO phenomenon — specifically, warmer sea-surface temperatures — a more detailed model that looks at the whole interaction, including how the ocean is sending the heat into the atmosphere, might provide even more answers.

Carolyn Gramling

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