|Arctic Oscillation (AO)The Arctic Oscillation (AO) is a climate index of the state of the atmospheric circulation over the Arctic. It consists of a positive phase, featuring below average geopotential heights , which are also referred to as negative geopotential height anomalies , and a negative 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.
Images in right column From: National Geographic Magazine, March 2000; Sources: Doug Martinson, Wieslaw Maslowski, David Thompson, and John M. Wallace
North Atlantic Oscillation (NAO)
The North Atlantic Oscillation (NAO) consists of two pressure centers in the North Atlantic: one is an area of low pressure typically located near Iceland, and the other an area of high pressure over the Azores (an island chain located in the eastern Atlantic Ocean). It is important to note that these two locations are most commonly used to measure the NAO, but studies have found that the pressure centers move around on a seasonal basis, and other locations have also been used for measuring this index. Fluctuations in the strength of these features significantly alters the alignment of the jet stream, especially over the eastern U.S., and ultimately affects temperature and precipitation distributions in this area. It is also important to note that the AO and NAO are two separate indices that are ultimately describing the same phenomenon of varying pressure gradients in the northern latitudes and the resultant effects on temperature and storm tracks across the continent.
During a positive NAO there is a strengthening of the Icelandic low and Azores high. This strengthening results in an increased pressure gradient over the North Atlantic, which cause the westerlies to increase in strength. The increased westerlies allow cold air to drain off the North American continent rather than letting it build up and move south.
- 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
|500mb Height Anomalies During a Positive NAO
||Surface Temperature Anomalies During a Positive NAO
A stronger Icelandic low and Azores high translates into lower than average 500mb heights near Iceland, and above average heights near the Azores Islands. In turn, above normal temperatures are seen over the eastern U.S. as the stronger westerlies transport colder air away from North America.
When the Icelandic low and Azores high are weaker, above average 500mb heights are located near Iceland, and below average heights near the Azores Islands. This results in a phenomenon referred to as high latitude blocking, which allows cold air to drain from Alaska and Canada into the U.S. and become entrenched, resulting in cold air outbreaks and below normal temperatures.
|500mb Height Anomalies During a Negative NAO
||Surface Temperature Anomalies During a Negative NAO
A. Positional Effects
Favorable positioning of the geopotential height anomalies within the negative phase of the NAO appears to enhance snowfall potentialin the eastern U.S. When negative NAO anomalies are positioned such that an area of high pressure is located near Greenland and a polar vortex (area of low pressure) is somewhere near 50°N 50°W, we find an increased potential for winter weather along the U.S. east coast.
- Atmospheric features in a “west-based” negative NAO disrupt the polar jet stream, causing it to buckle and move southward into the Southeast
- Allows cold, Arctic air to be transported south and increases the likelihood of interaction between the northern and southern jet streams
- Jet stream interactions often result in a combining of energy (phasing), which leads to rapid, intense surface cyclogenesis over the southern U.S.