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El Niño gives Earth new spin

Researchers in the United States and China recently reported that a mass of warm water wandering in the tropical Pacific might be affecting Earth’s rotation rate and altering the lengths of days, particularly during strong El Niño years. The water mass, called the western Pacific warm pool (WPWP), is closely associated with the early stages of El Niño/Southern Oscillation (ENSO) events.

Xiao-Hai Yan and Jiayi Pan, of the Graduate College of Marine Studies at the University of Delaware, and Yonghong Zhou, of the Shanghai Astronomical Observatory, reported their findings in the Nov. 12 Geophysical Research Letters.

Earth’s rotation rate, and hence length of day, is linked to the angular momentum of the solid earth. Combining the solid earth’s angular momentum with the angular momentum of the atmosphere and ocean equals a constant for the angular momentum of the whole Earth system. Because angular momentum of the whole system is conserved, a change in the angular momentum of one component (earth, ocean or atmosphere) forces the opposite change in another. If the atmosphere or ocean speeds up, the solid earth slows down, and Earth experiences a longer day.

Changes in day length are measured in milliseconds and wouldn’t be evident to people. However, length of day can be an important indicator of global change, especially because recent advances in geodetic techniques have made precise monitoring of it possible.

Over long time periods the rotation rate is affected by tides, core-mantle interactions and glacial rebound. But year to year, the bulk of the variation, though not all, is attributed to changes in the atmosphere. “The atmospheric angular momentum accounts for about 80 percent of the interannual length of day change,” Yan says. However, he adds, “of the remaining length of day change, about 10 percent can be explained by the western Pacific warm pool.” Yan and his colleagues suggest that the redistribution of mass caused by the warm pool migration influences Earth’s rotation rate.

ENSO’s atmospheric component, the Southern Oscillation, has previously been linked to changes in day length. However, isolated parcels of ocean water were thought to have too little mass relative to the solid earth.

On average, the WPWP covers an area roughly the size of Africa. It has surface temperatures of 28 degrees Celsius or higher, two to five degrees warmer than most of the equatorial ocean.

Using historical sea surface temperatures, the researchers tracked the movement of the WPWP over the past 50 years and calculated its angular momentum. They compared their calculations with historical variations in day length. After removing atmospheric and tidal effects, they found a strong positive correlation between the angular momentum of the warm pool and variation in day length. The association was most robust during the strong El Niño years of 1975 to 1976, 1986 to 1987 and 1997 to 1998.

“The WPWP is the most active water body in the global oceans during the ENSO event period,” Pan says. During an El Niño, the angular momentum of the WPWP increases. This increase slightly decreases the solid-earth angular momentum, which results in a longer day, the team reports.

Pan adds that while further studies are needed, the new finding emphasizes the important role of the WPWP in length-of-day changes.

Not everyone agrees. Richard Rosen of Atmospheric and Environmental Research Inc., Lexington, Mass., one of the earliest researchers to identify an El Niño signal in the atmospheric angular momentum, suggests focusing on oceanic contribution overall, rather than the WPWP alone.

“Their results clearly suggest that changes in the western Pacific warm pool do not, in themselves, make a significant contribution to these imbalances and point, therefore, to the need to consider more of the entire system of ocean currents to explain the [remaining variation],” Rosen says.

Sara Pratt
Geotimes contributing write

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