Pacific North American Index (PNA)

The Pacific North American (PNA) is one of the most important modes of low-frequency variability in the extratropics of the Northern Hemisphere, appearing in almost every month except June and July. The PNA loading pattern reflects a quadripolar pattern of 500-millibar height anomalies, with same-sign anomalies located south of the Aleutian Islands and southeast of the United States. Opposite-sign anomalies to the Aleutian center are located near Hawaii and in the North American mountain region (central Canada) during winter and autumn.

The spatial scale of the PNA pattern is largest during Northern Hemisphere winter. (See variability) During this period, the Aleutian center spans most northern latitudes of the North Pacific. In spring, the Aleutian center contracts and is mainly confined to the Gulf of Alaska. However, the subtropical center near Hawaii reaches its maximum amplitude during spring. The PNA pattern disappears in June and July but reappears in late summer and autumn. During this period, the mid-latitude centers become dominant and appear as a wave pattern emanating from the eastern North Pacific. The subtropical center near Hawaii is weakest during this period.

The positive phase of the PNA is characterized by above-average atmospheric pressure near Hawaii and over the mountainous region in western North America, and below-average pressure in southern Alaska and the southeastern United States. This phase is associated with an upper-level meridional flow.

The PNA index tends to be more positive in El Niño years, although not all PNA variability is explained by ENSO — studies show that about 29% of PNA variance is linearly linked to ENSO, while the remaining 71% depends on internal atmospheric processes and North Pacific surface anomalies. (Li, Hu & Liang, 2019)

The positive PNA phase is associated with above-average temperatures on the U.S. West Coast and western Canada, but below-average temperatures in the south-central and southeastern states. The circulation favors a reinforcement of the East Asian jet stream extending into the Pacific and influences the distribution of cold/warm air over North America. (NOAA CPC, PNA documentation)

Precipitation anomalies during the positive phase include above-average totals in the Gulf of Alaska and the U.S. Northwest, while some central regions experience below-average precipitation. (NOAA CPC, PNA precipitation)

During winter, the positive PNA phase is associated with higher surface aerosol concentrations in the U.S., including PM₂.₅, sulfate, and organic carbon, due to reinforced atmospheric circulation. (Feng, Liao & Li, 2016)

The Asian-Bering-North American (ABNA) teleconnection can strongly modify temperature anomalies associated with the PNA, especially in the Canadian prairies. (Chen et al., 2018)
See diagram.

The negative phase of the PNA is characterized by above-average atmospheric pressure along the Aleutian Islands coast and in the southeastern United States, and below-average pressure in the Rocky Mountains west of the U.S., as well as near Hawaii and the North American interior. This phase is associated with an upper-level zonal flow.

The PNA index tends to be more negative in La Niña years, although the relationship is not rigid and can vary according to other atmospheric factors.

During the negative phase, the western U.S. may experience relatively low temperatures, while the east shows above-average temperatures. Studies show that the negative PNA phase precedes 2 to 12 days of stronger low-level jets in the Great Plains, favoring moisture transport and heavy rainfall in the north-central U.S. (Harding & Snyder, 2015)

This negative phase is associated with drought on the U.S. West Coast and increased snow in the Midwest due to circulation favoring cold air transport and atmospheric blocking.
See diagram.

Paleoclimate studies show that during the Last Glacial Maximum, the PNA pattern was altered by the enormous Laurentide Ice Sheet, weakening the influence of Pacific teleconnections on North America. (Hu et al., 2020)

The PNA interacts with other teleconnections, such as the North Atlantic Oscillation (NAO), and can amplify or dampen regional climate effects depending on the phase combination. (Link Springer, 2010)

Loading Pattern

PNA Positive Phase

PNA Negative Phase