Wednesday Afternoon: Oceanic Accounting - Water In Equals Water Out

What Happens If The Books Don't Balance

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The path of 6 drifting RAFOS floats off the Grand Banks
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Float tracks from export pathways showing no water moving westward around the Grand Banks (Amy Bower & Heather Fury, WHOI)

When describing thermohaline circulation of the ocean, an often used analogy is that of an oceanic conveyor belt moving warm water from lower latitudes to higher latitudes where it cools, sinks and returns to the equator. One element in this three-dimensional journey of the seas is the Gulf Stream, a warm surface current that tempers the winters of Europe and helps determine the path of Atlantic hurricanes.

Given the importance of thermohaline circulation, it’s no wonder that scientists are eager to find out how it works and what the consequences would be if it shuts down. Amy Bower, of WHOI’s Physical Oceanography department, is one such scientist. Her presentation focused on a small, but important element of the global circulation picture: the North Atlantic Deep Western Boundary Current (NADWBC). For years, the understanding was that water from the Labrador Sea formed the NADWBC as it flowed back along the western edge of the Atlantic towards the equator, replacing water flowing northwards in currents such as the Gulf Stream.

To test this theory, neutrally buoyant, sub-surface drifting floats  were deployed off the coast of Newfoundland in a southward flow of Labrador Sea water, which lies at a depth of 700 to 1,500 meters (2,300 to 5,000 feet). The goal was to map the path of this Labrador Sea water and see how much of it returned southward. The experiment remains on-going, but preliminary results were surprising, showing almost no transport of water further south than the southern tail of the Grand Banks. Assuming there is a fixed amount of sea water on Earth, if water is displaced in one area it must be replaced by water from another area.

Where did the water go? It appears that almost all of it went east, not southwest as expected. This would indicate that some other force is blocking the flow or that it is taking an undiscovered path southward. Another scenario is that the thermohaline circulation may be slowing as a result of global warming and that the trickle of Labrador Sea water in the NADWBC is a precursor to more serious changes in Earth’s ocean circulation.


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Last updated February 24, 2006
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