Spatial analysis of early-warning signals for a North Atlantic climate transition in a coupled GCM
The climate system can potentially switch from one stable state to another. The closer a system is to a bifurcation point (i.e., ‘tipping point’), the more likely it is that even small perturbations can force the system to experience a state shift, e.g., a collapsing Atlantic meridional overturning circulation (AMOC) and associated cooling in parts of the North Atlantic. Here, we present an abrupt state transition from a warm to a cold North Atlantic climate state with expanded sea ice during an orbitally forced transient Holocene simulation performed with the Community Climate System Model version 3. The state transition is associated with a weakening of the AMOC by about 33% in this simulation. The changing background climate induced by slow external orbital forcing plays an important role for the abrupt climate shift. The model allows the identification of regions and variables that play a key role for a potential climate transition and show early-warning signals. Increase in autocorrelation and standard deviation as well as trends in skewness especially for sea-surface salinity in the northern North Atlantic are identified as robust early-warning signals, whereas no early-warning signals are found in the time series of the AMOC stream function.
KeywordsEarly-warning signals Climate transition North Atlantic AMOC
We greatly appreciate the constructive comments by three anonymous reviewers, which substantially improved the presentation of our findings. This project was supported by the Deutsche Forschungsgemeinschaft (DFG) through the International Research Training Group “Processes and impacts of climate change in the North Atlantic Ocean and the Canadian Arctic” (IRTG 1904 ArcTrain) and the German climate modeling initiative PalMod. The authors would like to thank Ute Merkel for providing the restart files of the pre-industrial control run. A special thanks goes to Vasilis Dakos for making available the earlywarnings package. The CCSM3 experiments were performed with resources provided by the North-German Supercomputing Alliance (HLRN).
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