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Saltier North Atlantic should give currents a boost

  • 13:12 23 August 2007
  • NewScientist.com news service
  • Catherine Brahic
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The surface waters of the North Atlantic are getting saltier, suggests a new study of records spanning over 50 years. And this might actually be good news for the effects of climate change on global ocean currents in the short-term, say the study's researchers.

This is because saltier waters in the upper levels of the North Atlantic ocean may mean that the global ocean conveyor belt the vital piece of planetary plumbing which some scientists fear may slow down because of global warming will remain stable.

The global ocean conveyor belt is the crucial circulation of ocean waters around the Earth. It helps drive the Gulf Stream and keeps Europe warm. The density of waters which drives the flow of ocean currents is dependent on temperature and salinity, so any change in saltiness may have an impact.

Tim Boyer of the US National Oceanographic Data Center and colleagues compiled salinity data gathered by fisheries, navy and research ships travelling across the North Atlantic between 1955 and 2006. They found that during this time, the layer of water that makes up the top 400 metres has gradually become saltier.

The seawater is probably becoming saltier due to global warming, Boyer says. "We know that upper ocean is warming in the North Atlantic, so it stands to reason that there should be more evaporation, making waters more salty," he says.

Polar 'pulse'

The global ocean conveyor belt is in part driven by salty and relatively dense subpolar waters sinking and flowing south to the equator.

So when a huge "pulse" of less dense freshwater was found to have been dumped into the sub-polar waters of the North Atlantic in the mid-1960s, researchers speculated the sub-polar waters might just stay floating where they were and cause circulation to stagnate.

The freshwater pulse probably came from a combination of increased rainfall and melting ice, as well as big chunks of ice suddenly pushing through the Fram Straight into the Atlantic.

When in their recent study Boyer and his colleagues zoomed in on the subarctic Atlantic, they found that the waters there became much less salty in the 1960s, as expected. But since the 1990s, they have been getting saltier again, and are now about as salty as they were in the 1970s.

Backing up this finding, when the team looked at the salinity of deeper waters, those flowing more than 1300 metres beneath the surface, they found that these have been getting less salty since the late 1980s. They see this as a sign that the pulse of freshwater has been slowly making its way south.

It takes roughly 10 to 15 years for subpolar water to move away from the Arctic and down to the equator.

Rapid changes

While Boyer admits that it looks from his study like there is currently less danger of the ocean conveyor belt shutting down, this could be short-lived relief only.

"Things change rapidly," he told New Scientist. "Just a few years ago, when we had only seen the increase in freshwater, there was speculation that it might shut down. Now we've seen that things have stabilised. But in five years things could change again."

Another team of researchers recently deployed a network of sensors that will be keeping a close eye on the progress of the North Atlantic Drift, the segment of the global ocean conveyor belt which pushes warm water northward from the Equator (see Fickle ocean current foils climate modellers).

Stuart Cunningham of Southampton University, UK, who led that study, says it is too early to say for sure but he believes there is a chance the North Atlantic Drift could still be slowing down in spite of Boyer's findings.

He points out that the conveyor belt depends on temperature as well as salinity. A reassuring signal from one of those factors does not necessarily mean problems with the other will be overcome.

Journal reference: Geophysical Research Letters (DOI: 10.1029/2007GL030126)

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There are 7 comments on 2 pages    1 | 2 | Next | Most recent

ocean currents

By david dunn

Thu Aug 23 18:04:54 BST 2007

are we really sure that we know the exact movemments of these currents in enough detail or is it extrapaltion from sat lte data and not actual measurements of the movement of water. i have not seen any real documentary proof that this is the case. i understand fully that it is very difficult to track this physical movement because labeling the water is difficult. please can someone allay my fears that the movement of water and at what speedsis correct in all the simulations

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REALLY?

By Paul Lawrenson. UK.

Fri Aug 24 09:28:59 BST 2007

Interesting that this apparent u turn in the desalination of the global conveyor is an indication through research from the U.S. I wonder what scientists from the rest of the world have to say about this? Before i even got to the end of the story i said to myself that this is likely to be a blip in the curve and likely due to greater oceanic agitation by strong weather patterns mixing the differing saline saturations in the North Atlantic, something which does not indicate a trend and does not mean that all is fine and dandy and the conveyor is not closing down after all. Wishful thinking methinks! The only thing which is going to stop the lowering of salinity at the top end of the global conveyor in the North Atlantic is if the polar icecaps stop melting at the accelerating rate they are currently and that isn't about to happen anytime soon, in fact that acceleration will continue to rise. The icecaps are disappearing into the ocean and there is no amount of wishful thinking based on short term anomalies is going to change that and the consequence that all the freshwater added to the region is affecting the conveyor. Come on! Be serious! Some people may find hope in such hypothesis but i think the majority of people who have half an inkling about what is happening will see this sort of attempt to obfuscate the truth for what it is, just like other attempts to hide what is happening by some elements of the American scientific community, this is all it is. Obfuscation of reality to attempt to gloss over the effects of global warming and try to tell everybody that it isn't really happening as has been the case for years now. Methiunks the Union of Concerned Scientists would be in fits of laughter at this story if it wasn't so darned serious in nature. Notice in the UK researchers are being wholly more realistic. this is what is called "Bunkum", the conveyor is severely affected, will not stop being affected just because of a short period of stability and this manifestation of glabal warming will get worse in years to come, end of story and there's nothing apart from reversing the melting of the icecaps and reversing the amount of fresh water leeching into the ocean will slow and stop this phenomena. In my opinion.

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REALLY? 2

By Paul Lawrenson. UK.

Fri Aug 24 09:39:44 BST 2007

.... Which we all know is impossible. It took millions of years for nature to build the icecaps and a century to melt them as much as they are today and this will not stop happening.

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