Mechanisms and Predictability of the June to August Summer North Atlantic Oscillation

We extend the definition of the summer North Atlantic Oscillation (NAO) to include the three summer months (June to August, JJA) and use recent reanalyses to include more of the Arctic explicitly within the SNAO pattern. The leading eigenvector of pressure at mean sea level (PMSL) in the North Atlantic region is still the SNAO with a pattern similar to that for July and August used widely in the literature. A similar pattern is also the leading eigenvector in this region in the BCC Climate System Model of China Meteorological Administration. Lagged maximum covariance analysis between sea surface temperature (SST) and PMSL shows that there appears to be predictive patterns of spring SST in the reanalyses and perhaps the model. The two key observed SST patterns involve a spring North Atlantic SST pattern identifiable as that of the Atlantic Multidecadal Oscillation, and a second one that is fairly like the spring North Atlantic SST tripole that influences the following winter NAO through re-emergence. These SST patterns also link to the related Arctic sea ice variations in the Atlantic sector, though the sea ice is probably not a primary forcing agent. Part of the source of the SST influences may come from correlated tropical Atlantic precipitation and thus diabatic heating anomalies. We also find some evidence of forcing of the southern node of the SNAO from spring tropical northwestern Pacific precipitation and thus diabatic heating anomalies. An initial estimate of the predictability of the JJA SNAO using these three spring predictors suggests significant skill on decadal and interannual time scales between 1979 and 2014.