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The Return of the NAO: The Crossing Guard of the Atlantic

March 3, 2013

Credit: NOAA
Your rating: None Average: 4.4 of 5 (7 votes)

It’s been cold in the South. Freezing temperatures, even snow, hit Florida. (Denver and Tampa had the same temperatures!) Another storm is pouring into the Midwest and is expected to hit the Mid-Atlantic States.

Why the storminess? Blame the North Atlantic Oscillation (NAO), the crossing guard of the Atlantic.

The North Atlantic Oscillation determines whether cold fronts cross the Atlantic or linger in the East. SOURCE: Microsoft

The NAO is a weather pattern that 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, creating colder temperatures and expansion into the Deep South.

Right now, the pattern is negative. What this means is that the air pressure is blocking the westward sweep of the cold front. It’s as difficult for the storms to move straight west as it would be for water to flow uphill. Instead, cold front after cold front enters North America and lingers. Eventually the jet stream carries the storms north, into Greenland, and they plunge south again into Europe.

A Negative NAO blocked cold fronts, and the cold weather is piling up in the South and East. SOURCE: NOAA

Winds are shaped by air pressure and a negative NAO means the semi-permanent low pressure near Iceland and the high pressure in the middle Atlantic are both weak. If they were strong, a positive NAO, they would create strong winds that would zip the storms across the Atlantic. Currently, however, the pattern has weakened the westerly winds and the storms are moving very slowly. Think of the weather pattern as a crossing guard encouraging the storms to cross the Atlantic or making them stop and wait.

This month’s Negative NAO is keeping cold fronts stuck on this side of the Atlantic, where they are piling up and plunging south. SOURCE: NOAA

Last year, the NAO was one of the most positive on record. Arctic storms zipped across the Atlantic before they hit the US. Now we are experiencing a more normal winter, with more blocks. This means colder, stormier weather. Not all of the white stuff in the South is cotton. Fruit and vegetables are being frozen outside the refrigerator. And all of you who missed winter last year are finally getting some cold weather and snow.

You can thank the North Atlantic Oscillation, the crossing guard of the Atlantic.  


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Evelyn Browning Garriss, historical climatologist, is a longtime writer for The Old Farmer's Almanac. She is also editor of The Browning World Climate Bulletin and 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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Has anyone noticed how

By Debbie R

Has anyone noticed how significant the snow cover was in the lower 48 compared to last year? In January I believe almost 70% of the nation was snow packed. This has been an active winter season. Here in North Carolina we finally had normal snowfall. This will really help our water tables. Last summer we had a -NAO but yet it was so hot. I read that a -NAO in the summer has a complete opposite cause, instead hot air is being pulled into the US rather than cool. Are there signs of another hot summer? I think we could be looking at another hot summer. Summers are always hot, so what can I say?

Good observation! At the

By Evelyn Browning...

Good observation!

At the moment, there is the potential for another hot summer, but not as hot as last year. Yes, the -NAO causes hot summers as well as cold winters. The crossing guard blocks weather patterns so they stop and become more extreme.

Where is the early spring

By marielynn

Where is the early spring Phil promised us? Here in Virginia, I see no signs of spring. All I see is 40s and 50s then going back down after that. We are expecting 6-10" of snow. Im ready for spring :( This is delaying our spring season. March 2013 looks grey & cold. When does it look like spring arrives? I need March to come out like a lamb so my trees will start flowering. Im from Mississippi so I dont know the Virginia climate too well. I guess thats why I'm irritated, you know how we southerners are LOL but back in my home state we didnt have this mess. Virginia is a beautiful state though, as I have been here for a year.. summers here are GRAND! Thanks Evelyn

Poor Phil. He really messed

By Evelyn Browning...

Poor Phil. He really messed up and will now have to brood about it while his home is buried in Pennsylvania snow.

Those storms that finally

By Sil in Corea

Those storms that finally make it to Europe then come stumbling across Asia and land over on the east side about 7 days later. When my friends have gales in Wales, I can mark my calendar here in South Korea.

Sil, you're right. It took

By Evelyn Browning...

Sil, you're right. It took scientists over a century to recognize that Europe/Asian connection. By the way, my daughter was born in your beautiful nation.

So what causes the North

By JP Mo

So what causes the North Atlantic Oscillation? BTW, the Pacific also has an oscillation, 180 degrees out of phase with the NAO. Don't know what causes that one, either.

It's the relationship between

By Evelyn Browning...

It's the relationship between the air pressure around Iceland and the air pressure in the middle latitudes. The differences in those air pressures shape wind strength and direction.

Interesting article. Thank

By CharlieNY

Interesting article.

Thank you for explaining why the weather is the way it is this winter.

What happens if this

By Talbert McMullin

What happens if this condition persists through the spring and summer...or is that possible?

A negative NAO and a warm

By Evelyn Browning...

A negative NAO and a warm Atlantic in late summer and fall helps create a more active hurricane season.

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