This study examines inconsistencies in the Pacific/North American (PNA) index relative to atmospheric circulation over North America. Two types of atmospheric circulation were found to be associated with high PNA values. The first type is the true PNA pattern characterized by an amplified ridge-trough system. It appears to be related to the leading mode of sea surface temperature variability in the North Pacific. The second type is observed during El Ni¿o events. It is characterized by a flattening of the polar jet stream and southward shift of the subtropical jet stream. The recognition of these two types of the PNA index improves our understanding of the relative role of to El Ni¿o /Southern Oscillation events and sea surface temperatures in the North Pacific in affecting winter atmospheric circulation over North America. ¿ 2001 American Geophysical Union