The North Atlantic Oscillation in coupled climate models: a CMIP1 evaluation

This study investigates the North Atlantic Oscillation (NAO) simulated by 17 global coupled ocean-atmosphere models participating in the Coupled Model Intercomparison Project (CMIP). Robust NAO indices are defined by calculating the leading principal components of winter time mean surface temperatures (land and sea) in the North Atlantic region (120°W-60°E, 20-80°N). Encouragingly, 13 out of 17 of the models capture the NAO surface temperature quadrupole pattern with centres of action over Northwest Europe, the northwest Atlantic, the southeastern USA, and the Middle East. The northern dipole is better captured than the southern dipole which is often simulated too far eastwards over the Atlantic Ocean. Out of the 17 models, ten models produce NAO indices that vary similar to the observations as stationary "weakly red noise" with only small correlations between successive winters (r < 0.3). Another five models drift monotonically towards warmer conditions, and two models exhibit long-term stochastic trends. Several of the models significantly overestimate the teleconnection between NAO and the tropical ENSO phenomenon.