Every few years, and more often in recent years, meteorologists start talking about a something called El Niño, or its opposite effect La Niña. Collectively known as Southern Oscillation, these two phenomena can have a dramatic effect on global weather patterns.
Less well-known, and even less understood than its southern cousins, but with just as much impact on the weather in North America, Europe an Asia, is a pattern called “Arctic Oscillation.” An atmospheric phenomenon, unlike El Niño and La Niña, which are caused by temperature shifts in the Pacific Ocean, AO refers to shifts in atmospheric pressure between the Arctic Circle and much of the Northern Hemisphere. In the “positive phase” of AO, atmospheric pressure lessens over the Arctic Circle and increases in southern latitudes. In its negative phase, it’s just the opposite. Atmospheric pressure is higher over the Arctic Circle and lower in the south. A related phenomenon, North Atlantic Oscillation, refers to shifts in pressure over the northern Atlantic Ocean.
What that means for our weather is that, when AO or NAO are in a positive phase, low-pressure systems — which cause cold, stormy weather — stay trapped in the extreme north. In a negative phase, those low-pressure systems are forced southward, bringing frigid air from the polar region down with them.
Over the last two winters, when cold temperatures and snow pounded much of the U.S. and Canada, AO was in an extremely negative phase. Combined with the effects of La Niña, which magnifies normal weather patterns, we’ve seen some extreme storms, prompting names like Snomageddon and Snowpocalypse.
We’re expecting the upcoming winter to once again be dominated by AO’s negative phase! On top of that, many meteorologists believe last year’s La Niña has returned for an encore performance. All I can say is, fasten your “sleet belts,” friends. As I said in the 2012 Farmers’ Almanac, we’re in for a “wet, wild winter!”