Ocean circulation in North Atlantic at its weakest
University of Hong Kong study finds dramatic weakening of the circulation during the 20th century that is interpreted to be a direct consequence of global warming and associated melt of the Greenland Ice-Sheet.
The research co-led by Drs. Christelle Not and Benoit Thibodeau from the Department of Earth Sciences and the Swire Institute of Marine Science,
This is important for near-future climate as slower circulation in the North Atlantic can yield profound change on both the North American and European climate but also on the African and Asian summer monsoon rainfall. The findings were recently published in the prestigious journal Geophysical Research Letters.
The Atlantic Meridional Overturning Circulation (AMOC) is the branch of the North Atlantic circulation that brings warm surface water toward the Arctic and cold deep water toward the equator.
This transfer of heat and energy not only has direct influence on climate over Europe and North American but can impact the African and Asian monsoon system through its effect on sea surface temperature, hydrological cycle, atmospheric circulation and variation in the intertropical convergence zone.
Many climate models predicted a weakening, or even a collapse of this branch of the circulation under global warming, partly due to the release of freshwater from Greenland Ice-Sheet. This freshwater has lower density than salty water and thus prevents the formation of deep water, slowing down the whole circulation.
However, this weakening is still vigorously debated because of the scarcity of long-term record of the AMOC. Drs. Not and Thibodeau used microfossils, called foraminifer, found in a sediment core to estimate the past temperature of the Ocean. The sediment core used is located in the Laurentian Channel, on the coast of Canada, where two important currents meet.
Thus, the strength of these currents will control the temperature of the water at the coring site which implies that the temperature reconstructed from this core is indicative of the strength of the North Atlantic circulation. With their collaborators from the United-States of America, they validated their results using instrumental data and two numerical models that can simulate the climate and the ocean.
“The AMOC plays a crucial role in regulating global climate, but scientists are struggling to find reliable indicators of its intensity in the past. The discovery of this new record of AMOC will enhance our understanding of its drivers and ultimately help us better comprehend potential near-future change under global warming” said Dr. Thibodeau.
Interestingly, the research team also found a weak signal during a period called the Little Ice Age (a cold spell observed between about 1600 and 1850 AD). While not as pronounced as the 20th century trend, the signal might confirm that this period was also characterized by a weaker circulation in the North Atlantic, which implies a decrease in the transfer of heat toward Europe, contributing to the cold temperature of this period. However, more work is needed to validate this hypothesis.
“While we could ground-truth our temperature reconstruction for the 20th century against instrumental measurement it is not possible to do so for the Little Ice Age period. Therefore, we need to conduct more analysis to consolidate this hypothesis” said Dr. Not.
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About the journal paper
“Last century warming over the Canadian Atlantic shelves linked to weak Atlantic Meridional Overturning Circulation”, Geophysical Research Letters.
About the research team
Drs Not and Thibodeau are leading the Environmental Geochemistry and Oceanography research group at The University of Hong Kong and are member of the department of Earth Sciences and the Swire Institute of Marine Science.
For media enquiries, please contact Ms Cindy Chan, Senior Communication Manager of HKU Faculty of Science (tel: 3917 5286/6703 0212; email: [email protected]) or Dr. Thibodeau.
More at www.eurekalert.org
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Joseph A Olson
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Wiki/Planet_Popsicle > what happens when 2500°F deep sea rifts shut down.
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Sabin Colton
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Some of the above does not make sense. First, covering a century and not looking at the up and down of the temperature during that century skips a lot of important detail.
Sediment cores off the tip of Florida indicate the rise and fall of the the Florida Current, which becomes the Gulf Stream. The findings were that, when warm, the current flows faster and, when cold, it flows slower. This is completely in tune with water viscosity and, thus, when warm, Europe gets warmer as more heat reaches it more rapidly and, when cold, less heat is delivered more slowly.
Thus, the detected weakening of the current indicates an overall cooling of the Gulf Stream during the 20th Century. Indeed, we never got to as hot as it was in 1938 when we were at the current peak in 1998. Warming and cooling alternately, but cooling in the long term. Perhaps the Laurentian site was not the best, as the collision point of two currents might not be a very stable place in the short and long term.
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Herb Rose
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The equatorial Pacific and Atlantic currents are a result of the rotational energy of the Earth not heat. They both transfer heat west, but this is not what causes them. The waters in these currents are moving east but not as fast as the 1000 mph that the Earth is turning, which is due to the inertia of the water. Global warming or cooling may affect the amount of heat they are transferring but I don’t see how this would change the rate of the current flow unless it changes the rotational energy of the Earth.
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