Did it feel like time flew in November 2009? It turns out the days were actually going a wee bit faster for part of that month, according to a team of NASA and European scientists.
The planet's speedier spin appears to have been due to a slowdown in an ocean current that whips around Antarctica.
"The Earth speeding up is just like a [twirling ice] skater pulling in her arms," he explained. When the skater does this, she spins faster, because the laws of physics dictate that her body must conserve what's called angular momentum.
"When [the skater] sticks out her arms, they move pretty fast, because there's a big circle. When she pulls in her arms, the circle is smaller, so in order to have the same angular momentum, she has to speed up," Marcus said.
"It is the same with the Earth," in the sense that if an ocean current slows down, the planet's spin must speed up to conserve angular momentum.
Scientists have long known that changes in the speed of ocean and atmospheric currents can—and do—slightly affect the rate of Earth's rotation and, hence, the length of a day.
"The thing is, with the ocean the effect is a lot weaker, since the ocean flows a lot slower than the atmosphere," Marcus said.
But in November 2009, he said, a slowdown in the Antarctic Circumpolar Current "seemed to be a lot stronger than usual, and that's probably what made it large enough to be detected in the Earth's spin data."
Ocean's "Weird Behavior"
The paper on Earth's brief speedup—which was recently accepted for publication in the journal Geophysical Research Letters—is the third study so far by NASA scientists to note unusual conditions in the Southern Ocean in the final months of 2009.
The earlier papers had reported record high ocean-surface temperatures in the southeast Pacific and record high ocean-bottom pressures in the same area.
In a separate paper by JPL's Tong Lee, published last August in GRL, researchers suggest this weirdness could be due to an unusual El Niño in 2009.
In a typical El Niño, the surface waters near the northwest coast of South America get warmer than normal. But in 2009 the warmer waters were concentrated in the central Pacific, in a type of El Niño called a Modoki.
Marcus's new paper now adds this "strange behavior" of the Antarctic Circumpolar Current to the mix, he said.
"The El Niño (central Pacific or otherwise) can link to the ACC through the atmosphere, on a time scale of days (much more rapidly than ocean currents). This is called the atmospheric 'bridge' ... between different parts of the globe," Marcus said in an email.
Faster Times Ahead?
Scientists are still debating whether Modokis are becoming more common, but "it looks like they might be, based on observations of the past few decades," Samantha Stevenson, an oceanographer at the University of Hawaii, said in an email.
Experts also lack sufficient data to say whether a trend toward more Modokis would be due to natural variability or human-induced global warming.
If the latter, then months when time flies by faster could become more common.
"An El Niño or La Niña event generates an atmospheric wave that propagates down into the Southern Hemisphere and can affect the circulation there," Stevenson said.
"If the atmospheric circulation changes, then so will the ocean, so you could get an Antarctic influence from [El Niño/La Niña] that way that might potentially change due to global warming.
"But as far as I know, no one has figured out exactly how much climate change will affect the ocean's response to El Niño in the Antarctic."