Abstract:
[Source: NOAA Climate Prediction Center,
http://www.cpc.ncep.noaa.gov/ ]
The daily Pacific-North America Patter (PNA) index is constructed by properly projecting the daily (00Z) 500mb height anomalies over the Northern Hemsiphere onto the loading pattern of the PNA.
The daily PNA index and its forecasts using MRF and Ensemble mean forecast data are available for the previous 120 days and they
... are normalized by standard deviation of the monthly PNA index from 1950 to 2000. A 3-day running mean is applied for the forecast indices.
Monthly and seasonal mean (JFM) PNA Index data are also
available. See: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/pna_index.html
The PNA pattern is one of the most prominent modes of low-frequency variability in the Northern Hemisphere extratropics, appearing in all months except June and July. The PNA pattern reflects a quadripole pattern of height anomalies, with anomalies of similar sign located south of the Aleutian Islands and over the southeastern United States. Anomalies with sign opposite to the Aleutian center are located in the vicinity of Hawaii, and over the intermountain region of North America (central Canada) during the Winter and Fall (Spring).
The spatial scale of the PNA pattern is most expansive in Winter. During this period, the Aleutian center spans most of the northern latitudes of the North Pacific. In Spring, the Aleutian center contracts and becomes confined primarily to the Gulf of Alaska. However, the subtropical center near Hawaii reaches maximum amplitude during the spring. The PNA pattern then disappears during June and July, but reappears in the late summer and fall. During this period, the midlatitude 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 time series of the PNA pattern also indicates substantial interseasonal, interannual and interdecadal variability. For example, a negative phase of the pattern dominated the period from 1964-1967, while a positive phase of the pattern tended to dominate from 1976-1988. A negative phase of the PNA then dominated during the 1989-1990 period, followed by a prolonged positive phase from fall 1991- early spring 1993.
For additional information on the PNA, see:
http://www.cpc.ncep.noaa.gov/data/teledoc/pna.shtml 
