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.