MESONET TICKER ... MESONET TICKER ... MESONET TICKER ... MESONET TICKER ... October 22, 2012 October 22, 2012 October 22, 2012 October 22, 2012
A O, AAAAAAAAAAAAAAAAAAAAA O... AO come and me wan' go home!
It's been so warm the last few days that even this cold-blooded Associate State Climatologist finds it a bit ridiculous. Highs yesterday across western Oklahoma were better suited for September than late October. Mangum topped the Mesonet with 98 degrees for a high.
http://ticker.mesonet.org/archive/20121022/oct21-highs.png
Heck, for most of the state, the low temperatures were right where the normal highs should have been.
http://ticker.mesonet.org/archive/20121022/oct21-lows.png
Alas, all good things must end, and cold weather is once again staring us in the face starting on Thursday and through the weekend. Here are the NWS forecast temps for Friday to set the chill in early for ya.
http://ticker.mesonet.org/archive/20121022/thursday-night-lows.png http://ticker.mesonet.org/archive/20121022/friday-afternoon-highs.png
With the possible demise of El Nino, dead before it even really got started, some other climate factors are starting to get talked about as the big players for this winter, so I thought I would go over the biggie in the news recently, the Arctic Oscillation (AO). Our old friend the AOis acting all negative again, which CAN lead to arctic air plunging into the eastern half of the U.S., including the Plains states. The AO index is a measure of sea level pressure differences between the arctic and the mid-latitudes. Now this is going to get a bit sticky, but here goes.
When the AO is in positive phase, low pressure over the arctic and high pressure over the mid-latitudes act to increase the strength of the mid-latitude jet stream, and keeps it blowing predominantly from west to east. In essence, this helps trap the cold air in the polar region and more mild winter weather in the U.S. can result. Europe can be cold and snowy during this type of weather pattern. This is largely what occurred last winter, with the AO being mostly positive (or weakly negative) for most of the winter.
On the other side of the oscillation, the negative phase occurs with higher pressure over the arctic and weaker or more normal pressure in the mid-latitudes. This helps to weaken the mid-latitude jet and allow for some pretty substantial dips to the south from time to time. Within those dips, you get a full blast of air from the polar regions. The AO was in negative phase for the winters of 2009-10 and 2010-11. During the winter of 2009-10, it was more negative than it had been during the last 50 years. Some climate researchers believe the negative phase of the AO combined with El Nino to produce a cold, snowy season that year for much of the South (including Oklahoma) and the East Coast (remember the Snowmageddon storm that buried the Northeast back in December 2009?).
Here are a couple of graphical renderings from our friends at NOAA and National Geographic that can better sum up what I've clumsily tried to explain.
NCDC http://ticker.mesonet.org/archive/20121022/NCDC_Arctic_Oscillation.png
National Geographic http://ticker.mesonet.org/archive/20121022/nat-geographic-netagive-phase.jpg http://ticker.mesonet.org/archive/20121022/nat-geographic-positive-phase.jpg
Now the AO is closely related to the North Atlantic Oscillation (NAO), which some would say is a regional version of the AO. The NAO is also a measure of the pressure differences between the same areas in the North Atlantic, with similar results.
I like to think of it like this, since it can be rather confusing: the AO is the large-scale pattern that makes the cold polar air available, and the NAO is the smaller-scale blocking pattern that helps funnel the cold air into the eastern half of the country by causing that big "dip" we talked about earlier to form across the eastern U.S. So when the AO and NAO are both strongly negative and work in unison, cold weather can be the result.
Annnnnnd, as it so happens, that's exactly the type of pattern we see setting up with both indices plunging into negative territory over the next week. Here are the forecasts for the AO and NAO for the next week or so. Pay attention to the top graph in each figure. These "spaghetti plots" show the different model solutions for the index for the next couple of weeks. Also notice how the plot gets a bit wild the farther out you go. Unfortunately, these are phenomena that are very difficult to predict out much farther than that, as the solutions get chaotic.
http://ticker.mesonet.org/archive/20121022/ao.gif http://ticker.mesonet.org/archive/20121022/nao.gif
Now we have to be careful when we start blindly saying "this is because of that" and "that was caused by this." But, a lot of research is continuing to find links between climate patterns like this and regional weather. The difficulty in forecasting these types of patterns out past a couple of weeks also underlines the difficulty forecasters have in predicting seasonal temperature and precipitation patterns.
Now after all of that, let's remember we can't do anything about any of it!
Back to the banana boats.
Gary McManus Associate State Climatologist Oklahoma Climatological Survey (405) 325-2253 gmcmanus@mesonet.org
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