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.
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.