The NAO: The Crossing Guard of the Atlantic

February 15, 2012

The North Atlantic Oscillation determines whether cold fronts cross the Atlantic.

Credit: Microsoft
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If you think this winter has been weird, blame the North Atlantic Oscillation.

If you find the term confusing, think of it as a crossing guard. The NAO lets cold fronts cross the Atlantic. If it is positive, it lets cold fronts whiz through the Midwest and Eastern states and dash out to sea. If it is negative, it stops them. They linger on the shore, freezing everything. If it stops them long enough, several cold fronts pile up and it gets really frigid.

When scientists discuss the North Atlantic Oscillation, they are talking about air pressure. When the low-pressure area around Iceland is similar to the high-pressure around the Azores Islands in the mid-Atlantic, they say it is negative. In their words, negative NAOs cause blocking. It blocks cold fronts and eventually steers them north. Cold front after cold front enters North America and lingers. The temperatures drop and the frozen air plunges south.

Last year’s Negative NAO blocked cold fronts and the winter was cold. SOURCE: NOAA

That is what happened last year.

When the two air pressures are very different, the NAO is positive. The cold Arctic air remains trapped to the north. That is what happened throughout most of November, all of December and early January.

This year’s Positive NAO kept early winter warm and is letting storms zip out to sea. SOURCE:NOAA

The low pressure around Iceland began to weaken in January and, by January 14, the NAO was neutral. Cold fronts could finally drop south of the Arctic area. However, nothing blocked them, so they could quickly sail out to sea.

Since the middle of January, the NAO has been positive or near neutral. When it becomes near neutral, Arctic air shoots south and then east across the Atlantic. Then warm air from the Atlantic heats everything up again.

Of course, the crossing guard NAO is not the only thing shaping weather. La Niña is making the West and the South dry. The Arctic Oscillation lets cold polar air escape. (Europe is buried in frigid weather from Siberia. They are calling the storm “The Beast from the East.”) It is a complicated and crazy mess.

However, if you have enjoyed not getting as cold as last winter, you can thank the brave little cross-guard. The NAO has kept the nasty weather from loitering and being a nuisance.

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Evelyn Browning Garriss, historical climatologist, blogger, writer for The Old Farmer's Almanac, and editor of The Browning Newsletter, has advised farmers, businesses, and investors worldwide on upcoming climate events and their economic and social impact for the past 21 years.

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Comments

I live in Lynchburg, Virginia

By WinterLightFly

I live in Lynchburg, Virginia and this has been a mild winter but I love the winter time and the cold and snow. We only had one good snow this month in February but what does this mean for summer? Will it be hot for us? and will next winter be better? Can you help me out by telling me whats coming our way? Any kind of information would be very nice! Thank You!

We live in Blaine, WA in the

By colason

We live in Blaine, WA in the extreme NW of the United States. We never get information on what to expect or why we are getting the weather we've gotten. We are 200 miles outside of Seattle so the weather they get is totally different from us. Can you let us know what we might be in store for? Thanks!

Thanks for this article...

By thayes

Thanks for this article... Here it is 80F in tidewater, va and it's just been really weird all winter! It's nice to have a logical explanation so thanks again!

this is better than watching

By v.roane

this is better than watching the weather!

i play scottish charector at

By reiever

i play scottish charector at a local renfaire circa 1533. what was the weather like in scotland from say 1530-1535? or can you tell me where to go to research it. thanks

During the 1500s, Europe was

By Evelyn Browning...

During the 1500s, Europe was in the middle of the Little Ice Age. Google "Little Ice Age" and think of furs and cozy woolens. I hope it isn't a summer production.

Evelyn, When is La Nina going

By Sue Nobody

Evelyn, When is La Nina going to go away? We really need him to go elsewhere so we can get out of this drought in Texas.

The different agencies around

By Evelyn Browning...

The different agencies around the world are mixed in their forecasts, but most agree that the cool La Nina will end in spring. However, at our latitude, we usually feel the after-effects for one or two more months.

Here's some good news, Texans (and beef-eaters). About a third of the agencies are expecting an early autumn El Nino. That warm tropical event would bring abundant rain to the South AND reduce the hurricane season.

La Nina is weakening, but

By jeltez42

La Nina is weakening, but that is what it does in the spring and summer months. Will it be back for the winter? Some are thinking that it will. The big question is will another dome of high pressure form over West Texas, Mexico, AZ, and NM like it did last spring/summer?

I am not sure where in Texas you are Sue, but from research that I did last fall, the DFW area's water problems were due to human mismanagement as well as lack of rain. DFW was more than 18 inches short of normal going into Jan 2012 and that area has not be effected as severely as the rest of the state.

Evelyn, fantastic! This is

By Heather Ferreira

Evelyn, fantastic! This is the kind of great weather article I love about the Almanac! PS, I need it to be unusually cool this spring. Will it be?

It really depends on where

By Evelyn Browning...

It really depends on where you are. Some parts of the US (hopefully including your area) will be experiencing a really cool spring.

thank you!

By qwenk

thank you!

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