Assessing model sensitivity to North Atlantic freshwater perturbations using past abrupt climate events

One part of the coupled ocean-atmosphere system that could change considerably in the future is the Atlantic Meridional Overturning Circulation (AMOC). Based on results from comprehensive coupled climate models, a recent CCSP assessment found that it is very likely that the strength of the AMOC will decrease over the 21st century in response to increasing greenhouse gases, with a best estimate decrease of 25-30%. Representations of the AMOC in these models are imperfect, but we have limited observational data for model-data comparisons that could help to improve the models. The paleoclimate record, with its longer-term perspective on climate variability, may help to fill this gap.

We will present results from a series of experiments using a state-of-the-art coupled climate model, the NCAR CCSM3, to test its sensitivity to a known freshwater perturbation in the North Atlantic that occurred 8,200 years ago in a climate state not too different from present-day. This “8.2 ka” event occurred when a large lake drained into the Labrador Sea, decreasing the strength of the AMOC. Climate impacts, including cooling over large parts of the North Hemisphere and a southward shift of the Intertropical Convergence Zone, are documented and quantified in a variety of marine and terrestrial paleoclimate records. Our initial results indicate a model sensitivity that is potentially less than that observed in the records. We will discuss possible reasons for this and their implications.