365 Days of Climate Awareness 113 – The North Pacific Oscillation

 

The North Pacific Oscillation (NPO) is analogous to the North Atlantic Oscillation (NAO), being a variation in surface air pressure between more northerly and more southerly latitudes. One such index uses pressure anomalies at the cities of St. Paul, Alaska (57°N) and Hawaii (md-20's). There are two modes (Aleutian Above Normal—AA; Aleutian Below Normal—AB) which describe the seesaw in pressure anomaly difference between the two locations. This difference is diagnostic of large-scale atmospheric and, by extension, oceanic conditions throughout the northeastern Pacific.

Aleutian low pressure and Pacific subtropical high pressure zones.

In the oscillation’s positive (AB) phase, eastern Pacific Ocean air pressure at the midlatitudes (30°-40°N) is elevated, and air pressure at higher (55°-65°N)  latitudes is reduced. This affects weather patterns in eastern Russia, and the western portion of North America.  Subpolar northerly winds come to the United States. Siberia and the United States southwest are colder than usual, while the west coast of North America has mild weather, with above-average precipitation over Canada and the Great Plains.

NPO index.

The negative (AA) phase means the opposite: reduced east Pacific midlatitude (30°-40°N) air pressure, and elevated air pressures farther north (55°-65°N). Warmer, southerly winds come to North America. Siberia and the southwest US are warmer than usual, the North American west coast is cooler, and with dry weather in Canada and the Great Plains.

There appears to be no driving cause of the NPO except for random variation.

Tomorrow: the North Pacific Gyre Oscillation

Be brave, and be well.