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.
Examples of a Negative PNA & What it means!
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.
Of course if it is winter time it would be milder air and not humid conditions.
Examples of Positive Arctic Oscillation & Negative Oscillation & what they mean!
The Arctic Oscillation (AO) is a climate index of the state of the atmospheric circulation over the Arctic. It consists of a negative phase, featuring below average geopotential heights , which are also referred to as negative geopotential height anomalies , and a positive phase in which the opposite is true. In the negative phase, the polar low pressure system (also known as the polar vortex) over the Arctic is weaker, which results in weaker upper level winds (the westerlies). The result of the weaker westerlies is that cold, Arctic air is able to push farther south into the U.S., while the storm track also remains farther south. The opposite is true when the AO is positive: the polar circulation is stronger which forces cold air and storms to remain farther north. The Arctic Oscillation often shares phase with the North Atlantic Oscillation (NAO) (discussed below), and its phases directly correlate with the phases of the NAO concerning implications on weather across the U.S.
Examples of Positive North Atlantic Oscillation & What it means...
- Above average geopotential heights are observed over the eastern U.S., which correlates to above average temperatures
- The eastern U.S. often sees a wetter pattern with stronger storms during the winter season in this phase due to increased upper level winds
- Recent studies at the SCO indicate a decreased potential for wintry weather in NC due to the lack of cold air availability and above average temperatures associated with a positive NAO in this region
A negative NAO indicates weakening of both the Icelandic low and Azores high, which decreases the pressure gradient across the North Atlantic. This decreased pressure gradient results in a slackening of the westerlies. The decrease in the westerlies allows cold air to build up over Canada, and this combined with below average heights (troughing) over the eastern U.S. gives the cold air a greater chance to move south and affect the eastern United States.
- Below average geopotential heights
are often observed over the eastern U.S. during the negative phase of the NAO, which correlates to below average temperatures
- The eastern U.S. typically receives colder, drier air masses during the winter season in this phase
- Recent studies at the SCO indicate an increased potential for wintry weather in NC due to the position and availability of cold air, and a more favorable upper level pattern conducive to coastal storm tracks
Pacific Decadal Oscillation Warm & Cold Phases & What it means...
Warm PDO
The broad area of above average water temperatures off the coast of North America from Alaska to the equator is a classic feature of the warm phase of the Pacific Decadal Oscillation (PDO). The warm waters wrap in a horseshoe shape around a core of cooler-than-average water. Impacts from the PDO depend in part on how it is aligned with the ENSO cycle; if the cycles are in opposite phases, then effects will be weakened. However, when both the PDO and ENSO are in the warm phase, meaning ENSO would be in the El Niño phase, expected impacts on the southeast include:
The broad area of above average water temperatures off the coast of North America from Alaska to the equator is a classic feature of the warm phase of the Pacific Decadal Oscillation (PDO). The warm waters wrap in a horseshoe shape around a core of cooler-than-average water. Impacts from the PDO depend in part on how it is aligned with the ENSO cycle; if the cycles are in opposite phases, then effects will be weakened. However, when both the PDO and ENSO are in the warm phase, meaning ENSO would be in the El Niño phase, expected impacts on the southeast include:
- Below average winter temperatures
- Above average winter precipitation
Cold PDO
Opposite of the warm PDO, the expansive area of below average water temperatures off the coast of North America from Alaska to the equator signals the cold phase of the PDO. The area of warmer-than-average sea surface temperatures in the central Pacific are surrounded by below average temperatures near the North American continent. Expected impacts from a cold PDO and ENSO (La Nina) phase on thesoutheast include:
Opposite of the warm PDO, the expansive area of below average water temperatures off the coast of North America from Alaska to the equator signals the cold phase of the PDO. The area of warmer-than-average sea surface temperatures in the central Pacific are surrounded by below average temperatures near the North American continent. Expected impacts from a cold PDO and ENSO (La Nina) phase on thesoutheast include:
- Above average winter temperatures
- Below average winter precipitation
The best combination for winter storms is a Positive PNA & Negative NAO & Negative AO
The worst combination for winter storms is a Negative PNA + Positive NAO & Positive AO
Simplified:
Negative NAO allows cold air to be filtered into the east coast & Northeast
Negative AO allows cold air to plunge south from Canada into the conus
Positive NAO & AO results in milder air across the conus as the colder air is trapped up in canada!
Positive PNA places a trough on the east coast and a ridge on the west coast.
Negative PNA is the opposite which is a ridge across the east and trough across the west.
These teleconnections can end up in any combination but knowing what each means helps one with figuring out what the weather will be like!
