NOAA 2002-017
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An ocean circulation system that brings cool water from ocean depths to the surface has been slowing down since the mid-1970s, causing an increase in sea surface temperatures along the Equator in the Pacific Ocean and a decrease of carbon dioxide (CO2) released into the atmosphere, say scientists from the Commerce Department's National Oceanic and Atmospheric Administration.

"Looking back at records for the past 50 years, we found that the ocean currents flowing in the north-south direction have been slowing down in the tropical Pacific Ocean since the mid-1970s," said Michael McPhaden, a senior research scientist at NOAA's Pacific Marine Environmental Laboratory in Seattle.

His findings, which were co-authored by Dongxiao Zhang from NOAA's Joint Institute for the Study of Atmosphere and Ocean at the University of Washington, will be published in the Feb. 7 issue of the science journal Nature.

"Cool water hundreds of feet below the surface typically flows from the mid-latitudes to the tropics, and these waters are eventually upwelled - or brought up - to the surface along the equator," McPhaden explained. "When the circulation slows down, the supply of cool water for equatorial upwelling decreases. In our study we found a reduction of 25 percent, causing the sea surface temperatures in a band about 600 miles on either side of the Equator to rise about 1.4 degrees Fahrenheit since the mid-1970s."

Not only does the temperature rise, but the amount of carbon dioxide released by the equatorial Pacific Ocean decreases. At present, the equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere.

These circulation changes are linked to the shifts in the climate of western North America, and can affect Pacific fisheries. They may be part of the naturally occurring ocean and atmosphere phenomenon called the Pacific Decadal Oscillation, which has a roughly 50-year cycle. But they also could be influenced by greenhouse gases.

McPhaden and Zhang studied historical ocean data sets and wind records going back a half century, when sufficiently large numbers of reliable observations began. They focused their attention on this region as an outgrowth of their interest in the El Niño/La Niña cycle, which, over the past 25 years, has favored stronger El Niños, and more frequent El Niños than La Niñas.

McPhaden notes that the system is driven by the trade winds, which have weakened since the 1970s. The trade winds in turn have weakened because the sea surface temperatures have risen.

"It's the same chicken-and-egg riddle we encounter with El Niño," he said.

The Commerce Department's National Oceanic and Atmospheric Administration is dedicated to enhancing economic security and national safety through the prediction and research of weather and climate-related events and providing environmental stewardship of our nation's coastal and marine resources.

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