The Log | School News
North Atlantic Warming Tied to Natural Variability
But Global Warming May be at Play Elsewhere in the World’s Oceans, Scientists Surmise
A Duke University–led analysis of available records shows that while the North Atlantic Ocean’s surface waters warmed in the 50 years between 1950 and 2000, the change was not uniform. In fact, the subpolar regions cooled at the same time that subtropical and tropical waters warmed.
This striking pattern can be explained largely by the influence of a natural and cyclical wind circulation pattern called the North Atlantic Oscillation (NAO), wrote authors of a study published Thursday, Jan. 3, in Science Express, the online edition of the journal Science.
Winds that power the NAO are driven by atmospheric pressure differences between areas around Iceland and the Azores. “The winds have a tremendous impact on the underlying ocean,” said Susan Lozier, professor of physical oceanography at Duke’s Nicholas School, who is the study’s first author.
[for more]Other studies cited in the Science Express report suggest human-caused global warming may be affecting recent ocean heating trends. But Lozier and her coauthors found their data can’t support that view for the North Atlantic. “It is premature to conclusively attribute these regional patterns of heat gain to greenhouse warming,” they wrote.
“The take-home message is that the NAO produces strong natural variability,” said Lozier in an interview. “The simplistic view of global warming is that everything forward in time will warm uniformly. But this very strong natural variability is superimposed on human-caused warming. So researchers will need to unravel that natural variability to get at the part humans are responsible for.”
In research supported by the National Science Foundation in the United States and the Natural Environment Research Council in the United Kingdom, her international team analyzed 50 years of North Atlantic temperature records collected at the National Oceanic Data Center in Washington, D.C.
To piece together the mechanisms involved in the observed changes, their analysis employed an ocean circulation model that predicts how winds, evaporation, precipitation and the exchange of heat with the atmosphere influences the North Atlantic’s heat content over time. They also compared those computer predictions to real observations “to test the model’s skill,” the authors wrote.
Other authors of the report included Richard Williams and Vassil Roussenov of Liverpool University; Susan Leadbetter, previously at Liverpool University but now a postdoctoral researcher with Lozier; Mark Reed, a computational scientist who also works with Lozier at Duke; and Nathan Moore, a former Duke graduate student now at Michigan State University.