Given the acronym-resistant name of NAO, it has been known since about 1770 and studied extensively for over a century. Many or most meteorologists believe that the warmer temperatures here in America and Europe the past few decades have been mainly or entirely due to more winters than usual exhibiting a positive NAO.
You may never have heard of the NAO. Other big weather-influencers like El Nino get far more press, so that most people know that El Nino is a warming of eastern Pacific tropical waters that creates far-reaching weather consequences. On a year like this one without a pronounced El Nino, we were expected to have a normal winter or maybe even a slightly cold one.
Despite its lack of media coverage, most weather experts now believe that the NAO influences our winter weather more than any other factor. It determines whether a particular period of time - several weeks or months in a row - is warm or cold, snowy, clear or rainy. When people groan that we seem "stuck in a pattern" - well, the NAO is that pattern.
It's simple to understand: A permanent high-pressure area sits over the Azores Islands, west of Portugal, and a low-pressure area hovers over Greenland and Iceland, which is what gives them their chronic dismal, let's-end-it-all-now weather. But sometimes that low is deeper at the same time that the Azores high is stronger, and this is called a positive NAO. This setup channels warm air from the Gulf rapidly up the East Coast, giving us mild winter conditions; plus, it's fast-moving enough to keep cloudy weather systems from ever lingering very long. Such a positive winter NAO was rare 60 to 100 years ago, but started becoming commonplace in the 1980s and 1990s - which is why our winters have gotten so much milder than what the old-timers remember.
This winter, a negative NAO has bucked the trend and has persisted - which means that the Azores high is weak, and the Greenland low is weak too. That allowed cold polar air to pour into the Northeast, across the North Atlantic and across northern Europe, which set the stage for periodic snowstorms here and record snows in Holland, France and Italy. It has rarely let precip fall as rain here in the Northeast this winter, and made itself powerfully evident in New York, Philly, Washington and points south. So whether you know it or not, you don't like a negative NAO one bit - even if it's good for the ski resorts.
Nobody knows why this oscillation occurs, only that it steers our weather and provides weeks or months at a time of either agony or ecstasy. You can go on the Web, look at the Albany forecast discussion and often read what the meteorologist says about the current and projected changes to the NAO. We will indeed be "stuck" in this winter's familiar gloomy pattern until the NAO changes. A chart of how the NAO has shifted during the past 60 years is available here: www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.timeseries.gif.
Could the melting Greenland glaciers and warmer North Atlantic temps be due to the anomalously positive NAO of the past couple of decades (which, again, did not occur this winter) - rather than anthropogenic global warming? Sure. But it's also possible that human-created atmospheric meddling is what's somehow creating the odd NAO in the first place. Some astronomers blame the sun. They believe it's the sun's more powerful sunspot cycles that have tilted the NAO positive. This makes sense, since this year's negative NAO could indeed be due to the weirdly low sunspot minimum we've observed the past two years.
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