Interannual Variability and Seasonal Climate Predictability

A twenty-year integration of an atmospheric general circulation model with identically evolving prescribed surface boundary conditions each year is employed to provide a measure of the interannual variability obtainable from internal atmospheric dynamics alone. In particular, the variability of seasonal mean sea level pressure and 700-mb geopotential height is considered by means of a sampled climate ensemble approach. This model-generated internal dynamics variability is assumed to be identical to that resulting from the real atmosphere if it operated without anomalous boundary conditions and is considered unpredictable since the time scales involved are beyond the traditional limits of deterministic predictability. By means of objective statistical tests, sampled model variances for these fields are compared to sampled variances of observed seasonal means (which have contributions from anomalous boundary conditions as well) for all four seasons in order to ascertain if, in an infinite population sense, the range of possible climate states is reduced without interannual external variations. These tests are applied primarily for the continental United States and secondarily to the rest of the Northern Hemisphere (from field significance considerations). Indications of potential predictability from some as yet unspecified anomalous boundary conditions for the former region and possible predictability for the latter are inferred when grid point values of the sampled model variance are declared significantly less than the observed. Within this framework, it is found that no potential predictability exists in the primary area of interest for the mean sea level pressure for any season but that some potential predictability of the mean 700-mb geopotential height is obtained for limited sections of the United States for summer and winter only. Regions of possible predictability vary with both field and season, but at least one subtropical region is usually found. The ultimate predictability of such regions has to be verified by an appropriate field significance test. Results from additional numerical experiments and analyses of observed data are generally consistent with the conclusions from the original basic internal dynamics experiment, especially over the continental United States.