Patterns of temperature variability on multidecadal to centennial timescales

The spatio-temporal structure of low-frequency temperature variability, as reflected in a paleo network covering the North American and European domains, is studied using a combination of statistical techniques including principal component analysis and uni- and multivariate singular spectrum analysis. On timescales longer than 50 years two statistically significant modes of temperature variability are identified, one on multidecadal and one on centennial timescales. The first mode is oscillatory, with a timescale in the narrow range 60-80 yr. The spatial pattern of this mode implies coherent oscillations over Europe and over north-eastern North America, with maximum amplitudes in Europe; over north-western North America this mode is absent. Its geographic shape suggests a connection to the North Atlantic Oscillation; a relation with solar forcing could not be detected.

The second mode, which dominates low-frequency variability at high latitudes, describes temperature variations in a wide interval of timescales (longer than 100 yr); in the longest records a weak ~120 yr oscillation can be identified with confidence. The temporal pattern of centennial mode shows a multiple-phase 'Little Ice Age' and a prolonged 'Medieval Warm Period'. The interpretation of this mode is difficult given its rather variable large-scale pattern and the uncertainty associated with centennial timescales in the data.

The analysis of seasonality of the identified low-frequency modes is limited due to lack of seasonally-resolved paleo data. The present dataset gives indications that in North America the centennial variability is largely independent of season, while the multidecadal mode might exhibit some season-dependent features. In Europe, where longer seasonal series are available, indications are found that both low-frequency modes of temperature variability are season-specific.