Ocean current slowdown made Earth spin faster

IT SOMETIMES feels as though some months go by faster than others, but November 2009 really did. Events in the Southern Ocean conspired to make the Earth spin ever-so-slightly faster, shortening half of the days in the month by 0.1 milliseconds each.

Different factors affect how fast the Earth spins. For instance, if the winds that whip around the planet slow down, the Earth spins faster to conserve angular momentum.

There was a more down-to-earth cause in November 2009, however. The Antarctic Circumpolar Current is a powerful ocean current that rings the continent. Stephen Marcus and his colleagues at NASA's Jet Propulsion Laboratory (JPL) in California and at the Institute of Earth Physics of Paris in France noticed that it slowed abruptly on 8 November 2009, only to speed up two weeks later.

Precise day-length data revealed that the changes immediately caused the Earth to spin faster, shortening each day by 0.1 milliseconds. Like the currents, day length returned to normal on 20 November (Geophysical Research Letters, DOI: 10.1029/2011gl050671).

This is the first time we have seen a rapid change in the oceans that is large enough to affect the Earth's rotation, says Marcus. The event is worth noting as the Antarctic currents directly impact the health of the ice sheets.

No one knows for sure why the currents slowed, but Marcus and his colleagues note that it happened in lockstep with atmospheric changes. Two days before the currents slowed, regional winds that move in the same direction slowed too. Two days after the winds went back to normal, so did the currents. Winds help drive currents, so that may not seem surprising. But it's unusual to see such a large response, says Marcus.

Tong Lee, also at JPL, believes that a slightly shifted El Ni?o may be to blame for the drop in wind speed. That, in turn, could come back to the environmental zeitgeist: models suggest that such shifts will happen more frequently as a result of climate change.

This isn't the only way that climate change may affect Earth's spin. Models suggest that rising sea levels will shift water towards the poles, drawing mass in closer to the Earth's axis and making it spin faster.

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