Geological data reveal that there have been abrupt changes in the past climate. Examples are the terminations of Pleistocene glaciations, the sudden cooling after the last deglaciation (Younger Dryas), and the formation of ice sheets. Model studies suggest the importance of multiple equilibria of the climate system. Especially the role of the oceanic thermohaline circulation has been discussed recently.

The oceanic thermohaline circulation occupies a central position in the understanding of climate variability and predictability because of its link to long-term variability and climate changes. In addition to paleoclimatic shifts, interdecadal climate variability may originate from changes of North Atlantic Deep Water formation. The changes in distribution of winter convective activity in the North Atlantic are highly correlated with the dominant pattern of atmospheric variability, the North Atlantic Oscillation.

Models with different levels of complexity (conceptual models, box models, coupled atmosphere-ocean circulation models) have been used to examine stability, variability, and predictability of the climate in the North Atlantic region. In this sensitive part of the global thermohaline circulation complicated feedback mechanisms act on many different timescales. The interactions involve different climatic components and processes such as the atmosphere, oceanic deep water formation, gyre circulation, and sea ice.

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