Study challenges ocean-flow data

A massive ocean circulation pattern that plays a crucial role in global warming might not have been slowing down over the past few decades as scientists previously believed, according to a study released Thursday.

In a single year of measurements, scientists found enough normal variation in the pattern's flow to suggest that previous studies were premature in asserting a long-term trend.

"We can't strictly say they are wrong, but we can have an alternative explanation," said co-author Torsten Kanzow, an oceanographer at the National Oceanography Centre in Southampton, England.

The ocean flow, known as the meridional overturning circulation (MOC), has been a focal point of global-warming debates because of its influence in shaping the world's climate.

"Direct measurements of the MOC have always been lacking," said Ronald Stouffer, a climate modeler at the National Oceanic and Atmospheric Administration, who was not involved in the study.

Often called the "great ocean conveyor belt," the pattern carries tropical Atlantic water northward. Along the way, it gradually emits its heat, warming North America and Europe.

The water, laden with carbon dioxide absorbed from the atmosphere, sinks as it begins to cool, sequestering the greenhouse gas in the depths of the ocean. Eventually, the cool water heads south to start the cycle again.

With higher temperatures caused by global warming, the surface water would be not only warmer but also less salty, because of increases in precipitation and glacier melting. Warmer, less salty water sinks more slowly.

Current theories hold that a slowing of the circulation pattern would reduce the ocean's uptake of carbon dioxide and impair its transport of heat from the tropics.

A previous study on the MOC was based on data collected by ships in 1957, 1981, 1992, 1998 and 2004 that showed a gradually declining flow. But because the data points were snapshots in time and not part of a complete record, oceanographers tended to view the analysis with skepticism.

"You could convince yourself that this indicated a trend," said William Johns, an oceanographer at the University of Miami and a co-author of the new study. "But it could also be a lot of wiggles that happened on shorter time scales."

Johns and his colleagues installed a dozen moorings across the Atlantic Ocean, from the tip of Florida to the coast of Africa, to collect daily measurements of temperature and salinity at various depths.

That information was used in combination with data from satellites and a trans-Atlantic cable to help calculate the strength of the circulation pattern.

Over a 12-month period starting in March 2004, researchers found that the flow of water varied from 4.4 million tons per second to as much as 35.3 million tons per second. The average was 18.7 million tons per second.

The data the researchers collected spanned the entire range of values that previously had been used to construct the downward trend.

"This really hammers home the importance of making continuous observations," Johns said.

The researchers had no definite explanation for the wide variation in flow, but Kanzow surmised that the dynamics of wave movements, eddies and currents that interfere with the circulation pattern could have more influence than previously thought.

The instruments have been left in place and will continue to collect data.