June 29th, 2006 03:39 EST
NOAA Scientists Demonstrate Southern Ocean Divide
The complex relationships between atmospheric carbon dioxide, biological productivity and the role of the Southern Ocean in carbon sequestration have been demonstrated by scientists at the NOAA Geophysical Fluid Dynamics Laboratory and Princeton University in Princeton, N.J.
Ocean waters that move toward the Antarctic continent sink as their temperatures drop. Once this occurs, these waters then move northwards. New research demonstrates that water at greater depths have a significantly different impact than the high-nutrient waters that flow northwards at intermediate depths. The circulation in the regions around Antarctica where water sinks to depths greater than 1.5 km was shown to be largely responsible for controlling the air-sea balance of carbon dioxide (CO2). The circulation in the Subantarctic regions that feed water to depths between 0.5 km and 1.5 km controls biological productivity. This research builds on recent studies showing that different parts of the Southern Ocean have responded differently to climate change.
Scientists looked at atmospheric CO2 and tiny marine plants known as phytoplankton, which remove almost 60 billion tons of carbon from the surface ocean each year. Much of the carbon is recycled by other organisms within the surface layer of the ocean, but about 10 billion tons of this carbon sinks into the deep ocean. This "biological pump" in the Southern Ocean is known to play a central role in the how much CO2 is contained in the atmosphere, as well as the global nutrient cycle.
"Even though it's a long way from occurring, we've long known that the Southern Ocean could change Northern Hemisphere climate by changing atmospheric carbon dioxide" said Anand Gnanadesikan, research oceanographer at GFDL and a co-author of the paper, which was published in the journal Nature. "What we didn't understand was the importance of the details of circulation."
Gnanadesikan worked with lead author Irina Marinov (currently a fellow in the NOAA Postdoctoral Program in Climate and Global Change), and with GFDL oceanographer J.R. Toggweiler and Jorge Sarmiento of Princeton University. The team used a series of ocean model simulations to investigate which areas of the Southern Ocean control atmospheric carbon dioxide levels, which areas control biological production and whether there is a link between the two.
Using GFDL's Modular Ocean Model coupled with a biogeochemistry model, researchers increased biological productivity in different parts of the Southern Ocean, depleting surface nutrients and forcing more CO2 into the ocean. They were able to discern which location determined the impact on air-sea carbon exchange. Depleting nutrients in the Antarctic zone was up to 12 times more effective at reducing atmospheric carbon dioxide as depleting nutrients in the Subantarctic zone.
"The work we had done previously considered the Southern Ocean as a single, relatively uniform ecosystem," Gnanadesikan said, "but J.R. Toggweiler had the idea that this was too simplistic—that more attention needed to be paid to the Antarctic. We decided to put his idea to the test." The research confirmed that the key region for understanding atmospheric carbon dioxide is not the relatively large area to the north of the polar front, but the much smaller, poorly sampled region around the Antarctic margin. This biological divide separating the Antarctic from the Subantarctic suggests that one area could be modified—by climate change or human intervention—without greatly altering the other. These results have important implications for understanding both past and future variations in atmospheric carbon dioxide and biological production.
The NOAA Geophysical Fluid Dynamics Laboratory advances NOAA's expert assessments of changes in national and global climate through research, improved models and products. The goal of GFDL's research is to understand and predict the Earth's climate and weather, including the impact of human activities.
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