Sources of Uncertainty in Multimodel Large Ensemble Projections of the Winter North Atlantic Oscillation

Projections of the winter North Atlantic circulation exhibit large spread. Coupled Model Intercomparison Project archives typically provide only a few ensemble members per model, rendering it difficult to quantify reducible model structural uncertainty and irreducible internal variability (IV) in projections. We estimate using Multimodel Large Ensemble Archive that differences explain two-thirds spread late 21st century (2080–2099) projections Oscillation (NAO). This is biased by systematic errors forced NAO response IV. Across Atlantic, explains substantial fraction mean sea level pressure (MSLP) due IV, except central Atlantic. Conversely, MSLP associated with largely unexplained NAO. Therefore, improving understanding alone may not constrain