Another piece of the large jigsaw puzzle that is current-day climate changes has simply made it harder to understand just what the finished jigsaw picture actually is. A Duke University-led analysis of records has shown that though the North Atlantic Ocean’s surface waters warmed between 1950 and 2000, the change wasn’t uniform.
In fact, to further distance Mother Nature from any semblance of logic, the sub-polar regions cooled while at the same time the subtropical and tropical waters warmed. Nevertheless, this pattern is not unexpected, and is as a result of a natural and cyclical wind circulation pattern called the North Atlantic Oscillation.
Wikipedia definition; the North Atlantic Oscillation (NAO) is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of sea-level pressure between the Icelandic Low* and the Azores high**.
“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 first author of the study published in January 3’s Science Express (the online version of the journal Science).
And though other studies cited in the Science Express issue suggest that human-caused global warming may be affecting recent ocean heating trends, Lozier and her coauthors couldn’t support such a view for the North Atlantic, saying that “It is premature to conclusively attribute these regional patterns of heat gain to greenhouse warming.”
“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.”
Their analysis of 50 years worth of North Atlantic temperature records collected at the National Oceanic Data Center in Washington, D.C., employed an ocean circulation model that predicts how winds, evaporation, precipitation and the exchange of heat with the atmosphere influences the North Atlantic’s level of heat over time. This model was not only used to predict, but was tested using real observations, “to test the model’s skill,” the authors wrote.
The results of the analysis are eyebrow raising for those who attempt to blame the entirety of the world’s problems on man-made greenhouse gas emissions (which we of the Green Options network are not).
Waters in the sub-polar region – approximately between 45 degrees North latitude and the Arctic Circle – grew cooler as the water directly exchanged heat with the air above it. Conversely, NOA-driven winds in the North Atlantic south of 45 degrees built up sun-warmed waters in the tropics and subtropics. As a result the heat was not only retained but redistributed at the surface, while simultaneously pushing cooler waters further down.
These findings eventually predicted that warmer sea surfaces in the tropics and subtropics, and colder readings within the sub-polar regions were likely whenever the NAO is in an elevated state of activity.
Just such a high NAO has been the norm since 1980 to 2000 (the analysis did not venture past 2000).
“We suggest that the 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 authors concluded.
The authors of this study note that their work should not be viewed in isolation. They note that as a result of reported heat content gains in other ocean basins across the planet, alongside rising air temperatures, other parts of the world’s ocean systems may have taken up the excess heat produced as a result of man-made global warming.
“But in the North Atlantic, any anthropogenic warming would presently be masked (hidden) by such strong natural variability,” they wrote.
So to both sides of this argument and to the multitude sitting on the fence; no study should be taken in isolation. While the North Atlantic may have suffered from natural and cyclical variations, it would be hard pressed to foist the results of this study on to studies focused within the Arctic Circle or down at the South Pole.
*The Icelandic Low is a semi-permanent centre of low atmospheric pressure found between Iceland and southern Greenland and extending in the Northern Hemisphere winter into the Barents Sea.
**The Azores High, also known as the Bermuda High in the United States, is a large subtropical semi-permanent centre of high atmospheric pressure found near the Azores in the Atlantic Ocean, at the Horse latitudes.
Duke University via PhysOrg – North Atlantic warming tied to natural variability; but global warming may be at play elsewhere
Photo Courtesy of desi.italy via Flickr