The North Pacific Oscillation (NPO) may be a significant factor in how El Niño and La Niña affect North American weather. A cold NPO phase indeed may have been partially responsible for the uncommon strength and stability of the El Niño-induced North American climate anomalies of early 1998. On the other hand, the latest La Niña excursion, if NPO persists in its cold phase, would likely produce weaker, less stable, and less predictable climate anomalies.
It is well known that the effects of interannual tropical forcing, or El Niño-Southern Oscillation (ENSO) influences, penetrate into middle latitudes to produce particular forms of climate anomalies, such as the relatively well-predicted temperature and precipitation patterns over North America during the great El Niño of 1997–1998. Evidence is now mounting that this ENSO effect accentuates certain types of synoptic scale events, so that the likelihood of extreme events is biased above or below its climatological normal over broad regions. Observations show that these ENSO effects over the United States are also affected by the phases of decadal-scale climate states such as the North Pacific Oscillation (NPO).The NPO influence can be seen in both the seasonal aggregate of various ENSO patterns and the distribution of extreme daily events.