Abstract
The Northern Hemisphere atmospheric circulation change from the 1960s to the 1990s shows a strong positive North Atlantic Oscillation (NAO) and a deepening of the Aleutian low The issue regarding the contributions of external forcings and Internal atmospheric variability to this circulation change has not been resolved satisfactorily Previous studies have found the importance of tropical SST forcing Here this hypothesis is examined again using relatively large ensembles of atmospheric general circulation model simulations of the twentieth century climate forced only by historically varying SST The resulting ensemble mean amplitude underestimates the observed change by at least 70%, although the spatial pattern is reproduced well qualitatively Furthermore, AGCM experiments are performed to investigate other driving factors such as the greenhouse gases sea ice the stratospheric ozone as well as the contribution from atmospheric internal variability The increase in ensemble mean trend amplitude induced by these additional drivers was not enough to substantially improve the agreement with the observed trend However the full distribution of simulated trends reveals that the ensemble members at the upper tall are much closer to the observed amplitude In the best ensemble the 95th percentile of the simulated NAO trend amplitude remains at about 80% of the observed trend amplitude, with nearly equal contributions from external forcings and internal variability The results also indicate that a complete set of driving factors and a correct simulation of stratospheric trends are important in bridging the gap between observed and modeled interdecadal variability in the North Atlantic winter circulation