Influence of the Ocean on North Atlantic Climate Variability 1871–1999

Abstract

The influence of changing ocean conditions on the variability of climate in the North Atlantic region is studied by analyzing ensemble simulations with an atmospheric GCM forced with reconstructed sea surface temperature (SST) data for the period 1871?1999. The ocean influence on multidecadal variability is analyzed separately from the influence on interannual variability. SST-forced variability on multidecadal timescales is shown to be dominated by a single mode that, in wintertime, resembles the North Atlantic Oscillation. The principal forcing for this mode is from variations in North Atlantic SST. In addition, however, evidence is found that SST variations in other ocean basins were influential during some sections of the time period studied, in particular during the most recent 50 yr. Variations in North Atlantic climate on interannual timescales are influenced by the Pacific ENSO phenomenon and also by Atlantic SST. There appears to be competition, with differing outcomes in different regions, between these two influences. Furthermore, it is shown that during the period studied the relative importance of these influences varied; that is, the oceanic influence on North Atlantic climate was nonstationary. The consequences of these results for seasonal forecasting efforts are discussed.