In the last couple days, the NOAA released their winter forecast which recognized the La Nina but also called out an index named the AO that may be the 'wild card' for temperature averages this winter. I've heard quite a few people ask what the AO is.
The AO is the 'Arctic Oscillation' index. It is a varying phenomenon that exists year-round but varies the most in the cold season. Here's an image to offer a better explanation:
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Left side: Positive index. Right side: Negative index. Image courtesy of J. Wallace |
If the Arctic Oscillation is positive, the US basically sees more moderate temperatures. However, when in the negative phase, the AO often brings down colder temperatures to the US. For more advanced weather folk, below is a 500 mb heights image showing a typical AO+, AON (AO Neutral) and AO- conditions during the DJF (December, January, February) time frame.
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Colors are pressure anomalies. Blue= lower pressure= colder Orange/red=higher pressure=warmer, sunnier |
Notice the swath of blue over the eastern US when the AO index is negative (bottom image), and the opposite when positive (top image).
That said, it is anticipated that we will be seeing an AO that should end up negative over the course of this winter, according to our analogues being used for this winter.
However, the AO greatly varies over a season, and this might be completely wrong. It varies just too much to be completely sure.
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