Observed atmospheric circulation over the North Atlantic and European (NAE) region is examined using cluster analysis. Clustering is applied to daily mean sea level pressure (MSLP) fields to derive a set of circulation types for six two-month seasons.

It is shown that a range of classifications can be produced which have almost identical within-cluster variance but which partition the days quite differently. This lack of a unique set of circulation types suggests that distinct weather regimes in NAE circulation do not exist, or are very weak. Furthermore, the stability of the clustering solution to removal of data is not maximised by a suitable choice of the number of clusters. Indeed, there does not appear to be any robust way of choosing an optimum number of circulation types.

Despite the apparent lack of preferred circulation types, cluster analysis can usefully be applied to generate a set of patterns that fully characterise the different circulation types appearing in each season. These patterns can then be used to analyse NAE climate variability. Ten clusters per season are chosen to ensure that a range of distinct circulation types that span the variability is produced.

Using this classification, the effect of forcing of NAE circulation by tropical Pacific sea-surface temperature (SST) anomalies is analysed. This shows a significant influence of SST in this region on certain circulation types in almost all seasons. A tendency for a negative correlation between El Niņo and an anomaly pattern resembling the positive winter North Atlantic Oscillation (NAO) emerges in a number of seasons. A notable exception is November-December, which shows the opposite relationship, with positive NAO-like patterns correlated with El Niņo.