Multi-Model Large Ensemble projections of the North Atlantic Oscillation
during the 21st century
There is large spread in projections of the winter North Atlantic
circulation. Coupled Model Intercomparison Project archives typically
provide a few ensemble members per model, rendering it difficult to
quantify the contributions of reducible model structural uncertainty and
irreducible internal variability (IV) to the spread in projections. We
use the Multi-Model Large Ensemble Archive to estimate that model
structural differences explain two-thirds of the spread in late 21st
century North Atlantic Oscillation (NAO) projections. This estimate is
biased by systematic model errors in the forced NAO response and IV.
Across the North Atlantic, the NAO explains a substantial fraction of
the spread in circulation projections due to IV except in the central
North Atlantic. Conversely, spread in North Atlantic circulation
projections due to model differences is largely unexplained by the NAO.
Therefore, improving understanding of the NAO may not help to constrain
the reducible uncertainty in North Atlantic circulation projections.