Evaluation of the North Atlantic Oscillation as simulated by a coupled climate model

The realism of the Hadley Centre's coupled climate model (HadCM2) is evalua ted in terms of its simulation of the winter North Atlantic Oscillation (NA O), a major natural mode of the Northern Hemisphere atmosphere that is curr ently the subject of considerable scientific interest. During 1400 y of a c ontrol integration with present-day radiative forcing levels, HadCM2 exhibi ts a realistic NAO associated with spatial patterns of sea level pressure, synoptic activity, temperature and precipitation anomalies that are very si milar to those observed. Spatially, the main model deficiency is that the s imulated NAO has a teleconnection with the North Pacific that is stronger t han observed. In a temporal sense the simulation is compatible with the obs ervations if the recent observed trend (from low values in the 1960s to hig h values in the early 1990s) in the winter NAO index (the pressure differen ce between Gibraltar and Iceland) is ignored. This recent trend is, however , outside the range of variability simulated by the control integration of HadCM2, implying that either the model is deficient or that external forcin g is responsible for the variation. It is shown, by analysing two ensembles , each of four HadCM2 integrations that were forced with historic and possi ble future changes in greenhouse gas and sulphate aerosol concentrations, t hat a small part of the recent observed variation may be a result of anthro pogenic forcing. If so, then the HadCM2 experiments indicate that the anthr opogenic effect should reverse early next century, weakening the winter pre ssure gradient between Gibraltar and Iceland. Even combining this anthropog enic forcing and internal variability cannot explain all of the recent obse rved variations, indicating either some model deficiency or that some other external forcing is partly responsible.