Getting warm. The Western Pacific Warm Pool before (top) and during (bottom) the 1997-98 El Niño.

Longer Days--Brought to You by El Niño

The rogue climate pattern known as El Niño has been blamed for a host of unwelcome events, from torrential rains and droughts to failed crops. Now, a study suggests that by increasing the size of a warm body of water in the Pacific, El Niño might even slow down Earth's rotation, resulting in slightly longer days.

Scientists have linked the periodic changes in our planet's rotation rate (which determines day length) to various factors, such as tides and interactions between Earth's inner layers, but a good deal of the variation remains unexplained. Oceanographer Xiao-Hai Yan of the University of Delaware in Newark and Ocean University of China in Qingdao has studied the Western Pacific Warm Pool--a body of water, warmer and less dense than the surrounding seas, that greatly expands and moves around the Pacific during an El Niño. Yan suspected this wandering could shift the distribution of ocean mass enough to brake Earth's spinning, thus slightly stretching day length.

To study this hypothesis, Yan and his research team used 30 years of data on sea-surface temperatures to estimate the size of the warm pool. At the peak of an El Niño, it extends an average of 29 million square kilometers across the tropical Pacific. They also determined the volume and geometric center to calculate its effect on Earth's rotation. Subtracting known influences and comparing their results with satellite records of Earth's day length, they found that elevated temperatures in the pool during El Niño years correlated with days that were a few microseconds longer than in other years. The effect is especially pronounced during strong El Niños, Yan says.

Still, Yan admits that other phenomena might also contribute to the observations, which the team reports in the current issue of Geophysical Research Letters. For instance, long-period ocean waves called Kelvin waves, which operate on the same time scale as El Niños, could also be a factor.

Experts disagree about the strength of the finding. Richard Rosen, a meteorologist at Atmospheric and Environmental Research Inc. in Lexington, Massachusetts, is skeptical about the interpretation of the data in the paper and is not convinced that the pool actually plays a role in day-length variations. But oceanographer Laurence Breaker of Moss Landing Marine Laboratories in California disagrees: "This is a significant new finding that helps us to better understand the subtle motions of our planet."

Related sites
Xiao-Hai Yan's Web page
More on the geodynamic effects of the ocean
More on length-of-day variations