The PNA teleconnection pattern is one of the most prominent modes of low-frequency climate variability, especially during the Northern Hemisphere winter (Wallace and Gutzler, 1981). It manifests itself as anomalies in the geopotential height fields, and is usually depicted using the 500 and 700 mb levels. The PNA is closely related to the upper-level flow patterns and surface temperature and precipitation conditions in North America (Yin, 1994a). Height anomalies of similar sign are located south of the Aleutian Islands and over the southeastern United States. Anomalies with a sign opposite to those of the Aleutian Islands are located in the vicinity of Hawaii, and over the inter-montane region of North America (Figure x). It is most prominent in the boreal winter, spring and fall.

The PNA teleconnection pattern is largest during the winter as the Aleutian centre expands to cover a large portion of the North Pacific (CPC, 1999). During the spring, the Aleutian centre shrinks and the subtropical centre near Hawaii expands to its maximum extent. The PNA is a dominant teleconnection pattern in all months except June and July (CPC, 1999).

The PNA teleconnection pattern has two phases. The positive phase is characterized by deeper than normal troughs over the Aleutian area and the eastern United States and a higher than normal ridge over the Rockies (Yin, 1994b). This phase is associated with a meridional upper-level flow (Figure x). The negative phase, or Reverse Pacific/North American pattern (RPNA), is characterized by filled troughs over the Aleutian area and the southeastern United States, and a lowered ridge over the Rockies (Yin, 1994b). This phase produces a more zonal upper-level flow.

The positive phase tends to bring above normal temperatures to the western United States, while the southeastern United States is affected by an upper-level ridge and may experience blocking and drought conditions (Yin, 1994a). Also, the eastern and especially the southeastern United States may experience intrusions of polar air masses with enhanced cyclonic activity. During the negative phase, the western United States may experience relatively cold and wet conditions, while the eastern United States is under dry and warm conditions (Yin, 1994a).

Canadian precipitation patterns are also related to the phase of the PNA. The positive phase of the PNA pattern is associated with dry conditions of the Canadian prairies and the negative phase is associated with wet conditions (Knox and Lawford, 1990). Romolo (1998) also found a weak but significant relationship between the PNA pattern and spring precipitation on the Canadian prairies. The correlations were found to be strongest in areas where synoptic systems control precipitation and weakest where orographic precipitation dominates.

The PNA index is the average of the standardized monthly 700 mb height anomalies located at three points, called centres of actions (Romolo, 1998). The three centres of action commonly chosen are the Aleutian area (47.9º N, 170.0º W), the Rockies in southwestern Canada (49.0º N, 111.0º W), and the southeastern United States (29.7º N, 86.3º W) (Leathers et al., 1991).

Because the record of continuous upper-level measurements is relatively short, the application of the PNA index in climatological research has been limited. However, the time series of the PNA pattern seems to indicate substantial inter-seasonal, inter-annual, and inter-decadal variability. For example, the negative phase of the PNA dominated the period from 1964-1967, while the positive phase tended to dominate from 1976-1988. The negative phase of the PNA then returned to dominance during the 1989-1990 period, followed by a prolonged positive phase from fall 1991 to early spring 1993 (CPC, 1999). Yin (1994a) used the winter temperature field over the United States to reconstruct the PNA pattern during 1895-1947, thereby lengthening the record of upper-level data. Yin’s model explained 89% of the variance in the PNA index and offers unbiased estimates of the past state of the PNA index. Yin also noted that generally the positive phase of the PNA tends to occur during ENSO warm events, although the association between these two major teleconnection patterns is rather weak.

Mo and Livezey (1986) showed that there are regions in the tropics where height anomalies are strongly linked to anomalies in North America. The relationship appears in the form of a large-scale wave pattern which may be a PNA pattern, or a Tropical/Northern Hemisphere (TNH) pattern, or a mixture of the two. Together, these three different patterns explain about one third of the variance of the Northern Hemisphere wintertime flow, and over North America they explain even more. The refinement of these teleconnection links is of great interest to long-range forecasters.

The PNA characterizes the 700 mb atmospheric flow and is important in order to understand the low-frequency variability of the mean tropospheric flow over North America. Although relatively little research has been done using the PNA for long-range forecasting, it has proven useful in explaining temperature patterns and precipitation patterns over North America (Yin, 1994b).

Knox and Lawford (1990) used the PNA pattern along with the NAO and NP teleconnection indices to investigate the relationship between NH circulation and pressure patterns at the 500 mb level. They established that the phase of the PNA is strongly related to prairie precipitation. Wet months are associated with the negative phase and dry conditions are associated with the positive phase. They also found that the NAO and NP are strongly related to spring precipitation on the prairies.
 
 

Knox, J.L. and Lawford, R.G., 1990. The relationship between Canadian prairie dry and wet months and circulation anomalies in the mid-troposphere.  Atmosphere-Ocean, 28(2): 189-215.

Leathers, D.J., Yarnal, B.M. and Palecki, M.A., 1991: The Pacific/North American teleconnection pattern and United States climate. Part I: Regional temperature    and precipitation associations. Journal of Climate, 4, 517-528.

Mo, K.C. and Livezey, R.E., 1986. Tropical-extratropical geopotential height teleconnections during the northern hemisphere winter. Monthly Weather Review, 114: 2488-2515.

Romolo, L., 1998. The relationship between the Pacific/North American teleconnection pattern and spring precipitation on the Canadian prairies. Master of Science Thesis, University of Saskatchewan, Saskatoon, SK, 149 pp.

Wallace, J.M. and Gutzler, D.S., 1981. Teleconnections in the geopotential height field during the northern hemisphere winter. Monthly Weather Review, 109: 784-812.

Yin, Z.-Y., 1994a. Moisture conditions in the south-eastern USA and teleconnection patterns. International Journal of Climatology, 14: 947-967.

Yin, Z.-Y., 1994b. Reconstruction of the winter Pacific-North American teleconnection pattern during 1895-1947 and its applicationin climatological studies. Climate Research, 4(2): 79-94.