Long-Term Modulations in North-Pacific Decadal Climate Variability as Revealed in Observations and a Coupled Model Simulation

Observations for the post-war period suggest that (quasi-) decadal wintertime climate variability over the North Pacific may have undergone notable modulations. A 20-year segmented EOF analysis of 3-year running-mean anomalies of the North Pacific SST reveals that until the 1980s the primary center of action of the extratropical decadal SST variability was the subarcitic oceanic frontal zone, where the ocean variability has the potential to exert thermodynamic forcing onto the atmosphere via modulating storm-track activity. Corresponding to slow displacement of the frontal axis, the particular SST variability exhibits high correlation with the decadal variability of the surface Aleutian Low and a PNA-like pattern aloft but no significant simultaneous correlation with tropical SST variability. Though extracted in the second EOF, however, this extratropical ocean-atmosphere coupled variability has lost its predominance in the 1990s and 2000s. Instead, the primary center of action has shifted to the subtropical oceanic frontal zone, where the decadal SST variability that accompanies the variability of the subtropical anticyclone is strongly anti-correlated with the tropical SST variability that has enhanced since the 1980s in the presence of background warming. Interestingly, a 150-year CGCM integration is found to simulate similar long-term modulations both in the decadal variability over the extratropical North Pacific SST and in the associated atmospheric variability.