The Pacific/North American teleconnection pattern (PNA) is one of the most recognized, influential climate patterns in the Northern Hemisphere mid-latitudes beyond the tropics. It consists of anomalies in the geopotential height fields (typically at 700 or 500mb) observed over the western and eastern United States. It is important to note that the PNA has been found to be strongly influenced by the El NiĆ±o-Southern Oscillation (ENSO) phenomenon. The positive phase of the PNA pattern tends to be associated with Pacific warm episodes (El NiĆ±o), and the negative phase tends to be associated with Pacific cold episodes (La NiĆ±a).
The positive phase consists of above normal geopotential heights over the western U.S. and below normal geopotential heights over the eastern U.S. This correlates to ridging over the western U.S., and deep troughing over the east. The net result of the height field pattern in this phase is that it forces cold air residing in Canada to plunge southeastward, which results in below normal temperatures over the eastern U.S. and above normal temperatures over the western U.S.
- Research at the SCO indicates that a positive PNA, especially during an El NiĆ±o year, produces an above average number of winter weather events in NC
|500mb Height Anomalies During a Positive PNA||Surface Temperature Anomalies During a Positive PNA|
In the positive phase, above average geopotential heights are seen over the western U.S., and below average geopotential heights are seen across the eastern U.S. This results in warm air moving much farther north than normal over the western U.S., while cold, Canadian air is forced southward over the eastern U.S. resulting in below normal temperatures.
The negative phase features troughing and below normal geopotential heights over the western U.S. and ridging with above normal geopotential heights over the eastern U.S. The result is below average temperatures for the western U.S., and above average temperatures over the eastern U.S.
- Research at the SCO indicates that a negative PNA typically results in a reduced potential for winter
weather in NC
|500mb Height Anomalies During a Negative PNA
||Surface Temperature Anomalies During a Negative PNA
The negative phase of the PNA pattern features below average geopotential heights over the western U.S., and above average geopotential heights across the eastern U.S. This results in deep troughing over the western U.S., which allows cold air from western Canada to drain southward into this region. In the eastern U.S., warm, moist air from the Gulf of Mexico and the Atlantic Ocean is able to travel northward, often resulting in above normal temperatures and more humid conditions.