A new report has claimed that the warming of the North Atlantic Ocean's surface waters is more attributable to natural variability, than global warming.
The report suggests that though human-caused global warming may be affecting recent ocean heating trends, the data collected by researchers can't support that view for the North Atlantic.
"It is premature to conclusively attribute these regional patterns of heat gain to greenhouse warming," said the researchers.
An 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).
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, a professor of physical oceanography at Duke's Nicholas School of the Environment and Earth Sciences who is the study's first author.
To piece together the mechanisms involved in the observed changes, the analysis by Lozier's team 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 group's analysis showed that water in the sub-polar ocean - roughly between 45 degrees North latitude and the Arctic Circle - became cooler as the water directly exchanged heat with the air above it.
"By contrast, NOA-driven winds served to pile up sun-warmed waters in parts of the subtropical and tropical North Atlantic south of 45 degrees, " said Lozier. "That retained and distributed heat at the surface while pushing underlying cooler water further down," she added.
According to Lozier, "The take-home message is that the NAO produces strong natural variability."
This suggests that these large-scale, decadal changes associated with the NAO, are primarily responsible for the ocean heat content changes in the North Atlantic over the past 50 years.
"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," said Lozier.