Wednesday, February 01, 2012

Not All -AO/-NAO's Are Created Equal

It's somewhat interesting. In a winter that has seemed to be anything but winter-like, we finally appear to have a pattern set up in the AO/NAO regions that might be favorable for delivering something more reminiscent of the last two winters. Yet we're talking 60s for highs.

While it's easy to look at index values and make determinations based on those numbers, the reality is that the real world does not always work this way. It was sexy after the last couple of winters to suggest that maybe the Arcitc Oscillation, in conjunction with the North Atlantic Oscillation holds the key to what happens during winter. Well here's the deal. The AO has turned SHARPLY negative in the last week or so, falling to about -4 as of today. That's almost as good as we saw the last two winters.

You can also see that from Ryan Maue's excellent site, the NAO is actually negative as well (despite the NCEP site showing it neutral...he uses a different method to analyze the NAO. Worth mentioning, I did attend a talk at the AMS conference in New Orleans last week dedicated to reassessing the NAO index. There's been value found in using a dual-NAO index...a smoothed NAO, which relates amplitude, but also an Angle Index, which relates the centers of pressure over Greenland, Iceland, and Mainland Europe. This might be something that gains some use in the coming years).

So with all this in mind, here's a look at last night's 00z GFS ensemble measurement (at left) for 500 mb anomalies today. Everything strong is on the other side of the North Pole. What this is doing is displacing the Arctic and Siberian air and directing into Europe and Asia. In fact, some parts of Europe will be frigid over the next several days. Unlike the last couple winters, this isn't a major cold shot for the UK into Germany. The heart of the cold is directed this time into Poland, Finland, Belarus, and the Baltic States.The forecast high is a balmy -2F in Riga, Latvia tomorrow. It was -8 in Sofia, Bulgaria this morning. Everything is displaced differently than we saw last year and in 2009-10. Here in North America, we're still being dominated by a Pacific influenced pattern. We continue (as we have for several weeks) to see mild Pacific air flood the US, and this does not bode well for cold lovers.

So the moral of this story is that despite the fact that we have favorable signals on paper, we're stuck in an unfavorable pattern still, while winter arrives with authority over in Russia and Europe. This is why looking at pure index values is oftentimes useless. One needs to appreciate the larger scale things at work to fully grasp what's going on.