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| Photo: The North Atlantic Current (NAC), the Irminger Current (IC), the East Greenland Current (EGC) and the Labrador Current (LC) form the Atlantic subpolar gyre. Credit: A. Born, Bjerknes Centre |
Jump-start for the ocean
Increased freshwater inflow into the North Atlantic is regarded as a brake on deep water formation that drives the Atlantic meridional overturning circulation. Because freshwater is less dense than seawater, it remains in the upper ocean, blocking deep convection. However, geologic evidence shows that the drainage of a lake seven times the size of the Great American Lakes combined could have jump-started deep water formation and the modern ocean circulation 8,200 years ago.
A new view
“A transition of the Atlantic surface circulation of the subpolar gyre to the modern, enhanced state can explain how these apparently contradictory events are related,” says first author of the study, Andreas Born of the Bjerknes Centre for Climate Research. By considering this transition Born and Anders Levermann of the Potsdam Institute for Climate Impact Research were able to combine a wealth of different paleoclimatic data to give a consistent picture of the 8.2 ka event. This event is named after the date it occurred, and is known as the largest cooling event on Greenland in the past 11,000 years.
Coeval to this spike in Greenland ice core records, surface temperature in the Western North Atlantic dropped abruptly and persistently and deep water formation commenced in the Labrador Sea. Proxy data from the Reykjanes Ridge to the southwest of Iceland, however, show a similarly abrupt and persistent warming. Until now it has not been clear how these incidents are related.
Subpolar Gyre is a 'tipped' element
“The changes in North Atlantic surface temperatures match the picture of a flip of the gyre into a stronger mode,” Anders Levermann explains. The transition was triggered by the intense freshwater pulse. “In its enhanced state, the Atlantic subpolar gyre qualifies as a ‘tipped’ element,” says Levermann.
Tipping elements have been identified as the most vulnerable large-scale components of the Earth System. Pushed past a critical threshold, these components may “tip” into qualitatively different modes of operation. As a direct consequence of the subpolar gyre’s strengthening, deep water formation around its centre intensifies, establishing the modern flow regime.
This newly established causal relationship also puts the 8.2 ka event into a new light, commonly regarded as a touchstone for the vulnerability of the Atlantic meridional overturning circulation to freshwater. While a temporary weakening of the overturning is beyond doubt, it might not have been as severe as previously thought, the authors suggest.
Article:
Born, A., and A. Levermann (2010), The 8.2 ka event: Abrupt transition of the subpolar gyre toward a modern North Atlantic circulation, Geochem. Geophys. Geosyst., 11, Q06011, doi:10.1029/2009GC003024.