The atmosphere is one chaotic "fluid". It's constantly in motion, shifting due to dips and rises in the jet stream and this can have major influences on weather patterns across the globe. This is most pronounced in winter, where we see the greatest contrasts and clashes in air masses between the the cold air from the Arctic, and the warmer air in the tropics. Sometimes, we see these interactions occur and this can result in notable weather systems and extremes.
One way we can analyze these sensible impacts is by grouping large-scale global pattern shifts and pressure changes. This is done by using indices or "teleconnections". Two main teleconnections that we look at (especially in the winter) are the "NAO" (North Atlantic Oscillation) and the "AO" (Arctic Oscillation).
North Atlantic Oscillation (NAO)
Image courtesy of Simon H. Lee's Weather Page & Britannica
The first image represents pressure differences between Greenland and the Azores in the far Northern Atlantic. This teleconnection is "boxed" within an index. In other words, there are two main phases - a positive (+NAO) and negative (-NAO) phase. The Images above show the differences between the two phases. Here are some key points of what happens in each phase:
Negative Phase (-NAO):
- Has a strong blocking North Atlantic High (the "H" circled in red).
- Has a weak to very weak (almost non-existent) Icelandic/Greenland low (the "L" circled in blue).
- Obstructs the flow across the U.S. (particularly over the Midwest & East Coast), causing stagnation! (Think of a -NAO as an analogy to rush hour traffic on a main highway with red lights; prepare to be stopped and move very slowly at times.)
- In the Summer: can result in warmer than normal temperatures for the Eastern US.
- In the Winter: can result in colder than normal temperatures for the Midwest, Great Lakes, and Northeast as it forces the jet stream to dip, and cold air filters in.
- During Winter, this can lead to large winter events across the Midwest and the Northeast. This is one way to get a blockbuster of an event!!
Positive Phase (+NAO):
- Has a weaker and suppressed North Atlantic High (the "H" circled in red).
- Has a deeper Icelandic/Greenland low (the "L" circled in blue).
- A more zonal flow across the Eastern Seaboard and a fast jet stream across the North Atlantic. This usually does not result in widespread snowfall.
- Typically leads to mild and wet winters in the Eastern US (No blocking means everything "zips" by, not allowing cold air to become established)
Arctic Oscillation (AO)
Image courtesy of Just In Weather : +AO (L), -AO (R)
Sometimes, the AO and NAO work together, and can be in the same phase. Other times, they end up in opposite phases. Not one teleconection depends soely on the other. Similar to the NAO, the AO also has two phases; a positive and negative state. You can see how they are represented globally with the images above. Here are some key points of what happens in each phase:
Negative Phase (-AO):
- Has blocking (higher than normal pressure) across the Arctic.
- Results in the jet stream becoming "wavy", leading to the spilling of cold, arctic air down into the middle latitudes.
- This phase also helps to slow the flow of the atmosphere at higher latitudes.
- The negative phase ultimately results in colder than average temperatures for many in the US.
Positive Phase (+AO):
- Has no blocking (higher than normal pressure) across the Arctic and higher latitudes. Lower than normal pressure is dominant here.
- The Polar jet stream is confined to higher latitudes like the Arctic Circle; therefore, it "bottles" up the cold air to the north.
- This phase keeps the flow of the atmosphere moving at higher latitudes, with no slowing down.
- This negative phase ultimately keeps the Arctic air “locked” in at higher latitudes.
Image Courtesy of NOAA :variability of the NAO since 1950. + NAO (Blue) - NAO (Red)
So in reality, when these two "tag-team" in the negative phase, this not only significantly increases the odds for cold air to spill into the lower 48 and support snow/wintry precipitation; it also boosts the chances for big winter events to develop along the East Coast and in particular, the Northeast.
Did you know? Our Long Range team uses teleconnections like the NAO and AO when updating the extended outlooks in our WinterRisk® Service. We provide weekly and monthly outlooks on the upcoming winter season, breaking down potentials for snow threats and explaing the chances of having saltable/plowable types events. For more information on this service, you can email Sales@weatherworksinc.com or call us at 908-850-8600