The Arctic Oscillation and Winter 09-10: A Piece of the Puzzle

This latest entry is going to focus on the past winter (since we're now in meteorological spring!) and one of the main drivers in what made this one so memorable in much of the East and South.

If you'll recall a couple weeks back, I posted a lengthy blog trying to debunk people using this winter to prove or disprove the theory of anthropogenic global warming. I mentioned in that entry that one of the biggest issues about people trying to use this year as proof of anything was that this was the first time we've had both an El Nino and the Arctic Oscillation at such sustained strong levels since at least the 1960s, and even that's a loose match.


What the Heck Is It?

Here's a quick primer on the Arctic Oscillation (AO). The AO is basically a gauge of blocking in the upper levels of the atmosphere over, not surprisingly, the Arctic. Blocking is pretty much what it sounds like. The overall weather pattern in a blocking pattern becomes "blocked up," meaning certain aspects sustain themselves for days, if not weeks at a time. When the Arctic Oscillation is in its negative phase, a large area of high pressure in the upper atmosphere basically sits over the Arctic. Underneath the high, temperatures get warmer than normal and the cold air is forced to be displaced. Traditionally, it's shunted southward, right into the Central and Eastern US in North America and straight into the heart of Europe and parts of Asia. You're basically forcing the coldest air, relative to normal, to leave the Arctic and it oozes south. The further south you live in the US, generally, the colder, relative to normal, it is.

Here's a visual primer on the AO. The top part is the positive phase of the AO. The bottom part is more representative of this winter and the negative phase of the AO.

So How Odd Was This Winter?

If you click the chart above, you'll see a running graph of monthly Arctic Oscillation values since 1990. Notice how things tank at the end of the chart for this winter. So we're automatically at the most sustained negative levels of the AO that we've seen anytime in the last 20 years.

People may argue, "Well, look at the AO and how it's tanking. That's not normal. That's clearly an indication that something is changing in the Arctic." Well, the picture's a little different when you look at the AO since 1950. Click the image below to enlarge.

Notice that since 1950, we've had several instances where the AO has tanked. For my purposes, my definition of "tanked," will be when the average AO monthly value ends up under -3. It happened in the winters of 1962-63, 1965-66, 1968-69, 1976-77 and 1977-78. We came close in 1984-85 and 1985-86. Unfortunately, data only exists back to 1950 and we can't see any serious long-term trends.

But it is interesting how there was a build up in the 1950s and by the mid-1960s, the AO tanked and held low for 5-8 years, followed by a break from ~ 1971-1976. We tanked again in 76-77 and stayed generally low until 1986. We started seeing more negatives in the AO around 1995-96 and continuing periodically through this year, when it tanked. I'm wondering if we aren't entering a period of 4-8 winters where we're going to see the AO stay at a very negative level, helping to supply more cold than we've been accustomed to seeing lately.

At any rate, with the AO as negative as it was this year, we pretty much had (with a few exceptions) an almost continuous supply of cold air draining out of the Arctic and helping to supply sustained cold weather for the Plains and Midwest (contrary to typical El Nino conditions). It also helped supply enough cold in the East (especially the further south Mid-Atlantic, Southeast and Southern Plains...areas that normally would have been borderline for rain/snow) to keep many storms this winter as all snow. The plentiful amounts of moisture were primarily courtesy of El Nino. The mechanism to help keep it snow, was likely due largely in part to the pattern over the Arctic.

Remember, the AO is just one piece of a much, much larger puzzle. But the main point is, while it's unusual for us to have seen the AO this negative in recent years, it does fall within the realm of reason of the last 60 years.