Potential for Atlantic current collapse hinted by complex global circulation model
The Atlantic meridional overturning circulation (AMOC) is a gigantic heat, salt, and nutrient mixer that spans the length of the Atlantic Ocean. Drawing warming surface waters up from the south through the Gulf Stream and along the North Atlantic Current, the system has a large amount of control over the climate of western Europe. Once in the North Atlantic the water cools, becoming more dense and sinking to between 3,000 and 5,000 meters (between 1.9 and 3.1 miles) in depth before commencing a return journey south. Both paleoclimate evidence and simplified ocean circulation models suggest that the AMOC may have two stable states (either its current behavior or an "off" mode), and this has left some researchers worried that the system may experience a sudden and drastic cessation.
While threshold behavior for the AMOC has been readily observed in simple climate models, Hawkins et al. reproduce the dynamic in a much more complex atmosphere-ocean coupled general circulation model (AOGCM) for the first time. Re-creating more than 56,000 years of ocean activity in the Fast Met Office/Universities Simulator (FAMOUS) AOGCM, the authors find that by progressively adding freshwater into the North Atlantic they are able to trigger the transition from a healthy functioning AMOC to a nonexistent one, which does not recover when the freshwater addition is subsequently decreased. Further, on the basis of the FAMOUS simulations and recent observations, the authors suggest that measurements of the direction of the net flux of freshwater at the southern edge of the Atlantic could serve as indicators that the AMOC actually has two stable states and thus has the potential to exhibit a threshold transition.
Geophysical Research Letters, doi:10.1029/2011GL047208, 2011
"Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport"
- E. Hawkins, R. S. Smith, L. C. Allison and T. J. Woollings
- NCAS-Climate, University of Reading, Reading, UK;
- J. M. Gregory
- NCAS-Climate, University of Reading, Reading, UK; and Met Office, Exeter, UK;
- H. Pohlmann
- Met Office, Exeter, UK;
- B. de Cuevas
- National Oceanography Centre, Southampton, UK