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Oceanic Conveyor belt diagram
 

Saltier North Atlantic should give currents a boost

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.
 
 
 

Oceanic Conveyor Belt

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The Oceanic Conveyor Belt

The global oceanic conveyer belt, is a unifying concept that connects the ocean's surface and thermohaline (deep mass) circulation regimes, transporting heat and salt on a planetary scale.

The conveyor belt system can be thought of as beginning near Greenland and Iceland in the North Atlantic where dry, cold winds blowing from northern Canada chill surface waters. The combined chilling of surface waters, evaporation, and sea-ice formation produces cold, salty North Atlantic Deep Water (NADW). The newly formed NADW sinks and flows southward along the continental slope of North and South America toward Antarctica where the water mass then flows eastward around the Antarctic continent (in the Antarctic Circumpolar Current). There the NADW mixes with Antarctic waters (i.e., AABW and AADW). The resulting Common Water, also called Antarctic Circumpolar water, flows northward at depth into the three ocean basins (primarily the Pacific and Indian Oceans).

These bottom waters gradually warm and mix with overlying waters as they flow northward. They move to the surface at a rate of only a few meters per year. After rising to the surface in the Pacific, the surface waters flow through the many passages between the Indonesian islands into the Indian Ocean. Eventually they flow into the Agulhas Current, the Indian Ocean boundary current that flows around southern Africa. After entering the Atlantic Ocean, the surface waters join the wind-driven currents in the Atlantic, becoming saltier by evaporation under the intense tropical sun. Trade winds transport some of this water vapor out of the Atlantic Ocean basin, across the Isthmus of Panama, and into the Pacific Ocean basin. Atlantic surface waters eventually return northward to the Labrador and Greenland seas in the North Atlantic.
 


This animation first depicts thermohaline surface flows over surface density, and illustrates the sinking of water in the dense ocean near Iceland and Greenland. The surface of the ocean then fades away and the animation pulls back to show the global thermohaline circulation.
 
   

Continued operation of the oceanic conveyor belt is important to northern Europe's moderate climate because of northward transport of heat in the Gulf Stream and North Atlantic Current. The system can weaken or shut down entirely if the North Atlantic surface-water salinity somehow drops too low to allow the formation of deep-ocean water masses. This apparently happened during the Little Ice Age (about 1400 to 1850 AD). The conveyer system shut down and northern Europe's climate became markedly colder.

Old paintings from this era show Dutch skaters on frozen canals-something that would not occur during today's climatic regime. Cores extracted from deep-sea sediment deposits contain evidence of earlier cold periods.

Plastic Ducks help model Ocean Currents

Some very famous plastic bath toys have, since 1992, been used to study ocean currents in the Pacific. On January 10, 1992, 28,800 plastic toy turtles, ducks, beavers and frogs bound for the US company 'The First Years' fell into the mid Pacific. Unlike many bath toys, these ones had no holes in them so did not take on water. They have been floating around ever since, and beachcombers' ongoing reports of their landings have provided information which has helped US oceanographer Curtis C. Ebbesmeyer and collaborators to construct a model of the north Pacific currents.

Ebbesmeyer, after christening the toys the 'Friendly Floatees', managed to recruit beachcombers across the world to feed him data in a splendid example of public involvement in real science. He made several correct predictions about the toys' route, and correctly predicted that thousands of the toys would get frozen into Arctic ice near Alaska, and then, moving at a rate of about a mile a day, would work their way around their very own North-West Passage to the Atlantic. In 2003, the first plastic toys from 'The First Years' cargo began to wash up on the eastern seaboard of the U.S. and Canada.

Ocean currents are expected to sweep them southwards down the coast and then off north-east towards Europe. So now, after 18 years and up to 17,000 miles afloat, the toys are expected to start turning up on the south-west and western British coasts. They look a bit different to how they started off - some have bleached white, whilst others still retain their bright colours.

Click here to view a powerpoint presentation baised on chapter 6 (Ocean Circulation) from Invitation to Oceanography by Paul R. Pinet.

 
 
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