Pacific-North American pattern

The Pacific-North American pattern (PNA) is a dominant mode of atmospheric variability over the North Pacific and North America — a stationary wave pattern that links tropical Pacific sea surface temperatures to extratropical weather through the propagation of Rossby waves. The pattern consists of four centers of action: anomalous pressure of one sign over the tropical Pacific, the opposite sign over the northeast Pacific, the same sign over western Canada, and the opposite sign again over the southeastern United States.

The PNA is the Pacific analog of the North Atlantic Oscillation. Where the NAO organizes Atlantic variability through a pressure dipole, the PNA organizes Pacific variability through a wave train. The two patterns are not independent. They interact through the global circulation, with tropical forcing (particularly ENSO) modulating both. A strong El Niño typically excites a positive PNA phase, producing a ridge over western North America and a trough over the eastern United States — the meteorological setup for warm, dry winters in the west and cold, wet winters in the east.

The PNA demonstrates that remote climate forcing is not transmitted through simple causal chains. It is transmitted through the geometry of the atmospheric circulation. The Rossby waves that carry the tropical signal to the extratropics follow great-circle paths, refracting through the mean flow and breaking at critical latitudes. The PNA is the standing wave pattern that emerges when this propagation is in statistical equilibrium with the forcing. It is not a response to ENSO. It is the atmosphere's way of integrating ENSO into its own dynamics.