Records from ice cores show that around 8,200 years ago the Northern Hemisphere's climate abruptly cooled. Many scientists link this event to the final drainage of Lake Agassiz, a large glacial lake covering much of central Canada that formed at the foot of North America's continental glaciers. This drainage is thought to have freshened waters in the northern Atlantic Ocean, slowing down the density-driven oceanic circulation that helps to distribute heat.
Noting that an actual chronology of events must be established before scientists can speculate on causes of this cooling, Hillaire-Marcel et al. study oceanic records downstream from Lake Agassiz's flood discharge route. They find that the lake's drainage occurred between 8,500 and 8,350 years ago but that sea-surface and deep-current conditions, derived from oceanic sediment cores, lack significant concurrent changes in the northern Atlantic.
Instead, the data shows that the 8,200-year-old cooling event was generated by several factors, including melting of North American continental glaciers and subsequent rapid sea level rise which induced a large-scale reorganization of broad oceanic circulation patterns.
Reference for first article: David J. W. Piper: Atlantic Division, Geological Survey of Canada, Dartmouth, Nova Scotia, Canada and C. Hillaire-Marcel and Anne de Vernal: Geochemistry and Geodynamics Research Centre, University of Quebec at Montréal and McGill University (GEOTOP-UQAM-McGill), Montreal, Quebec, Canada; "The ~8.4 ka Lake Agassiz drainage event in the northwest North Atlantic", Geophysical Research Letters (GRL) paper 10.1029/2007GL030396, 2007
Warmer surface temperatures in the North Atlantic may affect the mighty current that encircles Antarctica
In the northern Atlantic Ocean, cold salty water sinks, forming the North Atlantic Deep Water, a southward moving water mass centered around the depth of 2.5 kilometers (1.6 miles). This sunken water is replaced by water essentially originating in the Antarctic Circumpolar Current and flowing across the equator northward through surface currents such as the Gulf Stream and the North Atlantic Current.
In a sensitivity study using a coarse-resolution ocean general circulation model in an idealized single-basin configuration with a circumpolar channel, Fu kar and Vallis find that deep water production diminishes as surface temperature increases in the north, affecting the basin's overturning circulation and stratification.
This induces a change in water mass properties in the southern circumpolar region, causing a substantially higher volume transport around Antarctica. The authors note that significant variations in certain critical model parameters do not change this result. If their model holds true, a change of surface buoyancy in the Northern Hemisphere may significantly influence the stratification and transport of the Antarctic Circumpolar Current.
Reference for second article: Neven S. Fu kar: Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, New Jersey, U.S.A.; Geoffrey K. Vallis: Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, New Jersey, U.S.A.; also at Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, New Jersey, U.S.A.,"Interhemispheric influence of surface buoyancy conditions on a circumpolar current" Geophysical Research Letters (GRL) paper 10.1029/2007GL030379, 2007
Materials provided by American Geophysical Union. Note: Content may be edited for style and length.
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