The North Atlantic Current (NAC), the Irminger Current (IC), the East Greenland Current (EGC) and the Labrador Current (LC) form the Atlantic subpolar gyre (SPG). Credit: A. Born, Bjerknes Centre
June 25, 2010 - An abrupt and persistent strengthening of the ocean currents forming the Atlantic subpolar gyre could have resulted from a large freshwater flooding event 8,200 years ago. New climate model simulations resolve the contradiction of increased freshwater inflow and enhanced deep water formation in the North Atlantic. The enhanced surface circulation could contribute to the climatic stability since the last ice age, the researchers suggest in the American Geophysical Union's electronic journal "G-Cubed".
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 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 the 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. So far it was not clear how these incidents are related.
"The changes in North Atlantic surface temperatures match the picture of a flip of the gyre into a stronger mode," explains Anders Levermann. 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.
The abrupt transition of the subpolar gyre is due to two positive feedbacks. First, a stronger gyre accumulates saline water in the subpolar North Atlantic. Increased surface salinity enhances deep convection. Additionally, a stronger gyre results in more efficient removal of heat from the gyre's centre. Both effects, salt accumulation and removal of heat, increase the core density of the gyre compared to the relatively light exterior, the sea surface drops and the cyclonic circulation of the gyre is strengthened.
In addition to these self-sustaining internal feedbacks, there exists an interaction with the flow over the Greenland-Scotland ridge. The freshwater perturbation reduces sinking in the Nordic Seas and thus the supply of dense overflow waters to the northern rim of the subpolar gyre. Consequently, its outer rim gets lighter and the circulation intensifies slightly. This triggers the two internal feedbacks mentioned above and yields the stronger state of the gyre.
The 8.2 ka event with widespread cooling in the North Atlantic region coincides with a meltwater outburst from Lake Agassiz in North America. The flooding is believed to have weakened the Atlantic meridional overturning circulation and heat transport from the tropical to the Northern Atlantic. After a few centuries, the circulation recovered and temperatures in the North Atlantic region rose again to the former levels. The study by Born and Levermann shows why the ocean systems, however, changed persistently.
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. http://www.agu.org/pubs/crossref/2010/2009GC003024.shtml
Further reading:
Background article: A. Levermann and A. Born (2007): Bistability of the Atlantic subpolar gyre in a coarse resolution climate model. Geophysical Research Letters, 34 (2007), L24605. http://www.agu.org/pubs/crossref/2007/2007GL031732.shtml
