Oceanic influence on the North Atlantic Oscillation and associated Northern Hemisphere climate variations: 1959-1993

The North Atlantic Oscillation (NAO) exhibits variations at interannual to multidecadal time scales and is associated with climate variations over eas tern North America, the North Atlantic, Europe, and North Africa. Therefore , it is very important to understand causes of these NAO variations and ass ess their predictability. It has been hypothesized, based on observations, that sea surface temperature (SST) and sea-ice variations in the North Atla ntic Ocean influence the NAG. We describe results of an ensemble of Sixteen experiments with an atmospheric general circulation model in which we used observed SST and sea-ice boundary conditions globally during 1949-1993. We show that multiyear NAO and associated climate variations can be simulated reasonably accurately if results from a large number of experiments are av eraged. We also show that the ambiguous results of previous NAO modeling st udies were strongly influenced by the ensemble size, which was much smaller than that in the present study. The implications of these results for unde rstanding and predictability of the NAO are discussed.