The NAM and PNA-like patterns, as defined by the two leading EOF's of the monthly sea-level pressure field, together define a two-dimensional phase space that provides a framework for interrelating some of the more important patterns of Northern Hemisphere wintertime low-frequency variability.

We show that these two modes have a significant influence on the observed atmospheric variability over a range of frequencies. Together, they explain substantial amounts of variance of monthly temperature and precipitation variability over the whole hemisphere, they modulate the frequency of extreme cold 5- and 10-day mean events, and also dominate the longer term variability, accounting for most of the hemispheric sea level pressure and surface temperature trends observed in the last 40 years.

Many of the modes of variability that have been reported in the literature project strongly onto the two-dimension phase space defined by these modes, and are therefore largely redundant with them.

Non linear interactions between these modes, as well as their dependence on the mean state are also considered.