Patterns of climate variability of the Northern Hemisphere wintertime circulation

The principal patterns of variability of the extratropical Northern Hemisphere (NH) wintertime circulation are examined. The two-dimensional phase space defined by the two leading PCs of the monthly-mean sea-level pressure (SLP) field poleward of 20°N is used as a basis for surveying the structure of the geopotential height and other fields. The leading EOF corresponds to the NH annular mode (NAM) and the second EOF resembles the Pacific-North American (PNA) pattern. Together these two patterns account for roughly half the variance of SLP on interannual time scales and longer, and virtually all the planetary-scale SLP trends over the 42 year period of record. Also, all the most important NH patterns of variability can be described in terms of our two basis functions; therefore, several of the pattern definitions found in the climate literature are shown to be redundant.; The relationship between the low frequency variability and time scales closer to the synoptic time scale are investigated by studying the variability observed within four different subsets of the climatology as defined by the high-index and low-index polarities of the 10-day mean projections of the two SLP PCs. The variability of the flow is substantially different within those different mean states. Within the individual subsets of the climatology there are suggestions of multiple circulation regimes; teleconnection patterns for the subsets of the climatology are also discernibly different; cold temperature anomalies associated with low polarities of both PCs are observed more frequently than expected on the basis of linear correlation analysis. These results constitute evidence of non-normal or non-linear behavior of 10-day mean fields, and provide indications that the intraseasonal variability depends on the mean state of the flow in which it is embedded.; The latter idea is extended to investigate the effects of changes in the winter mean background flow observed in association with the ENSO cycle upon the extratropical month-to-month variability. The structure of the NAM is shown to be significantly different during warm and cold winters of the ENSO cycle. Furthermore, the observed NH surface trends of the period 1958--99 were much stronger during the years characterized by warm ENSO events.