This winter has been warm and snow-free, and the reason is simple: We’ve had a positive North Atlantic Oscillation – which is forecast to continue, at least for a while. It resembles the winters of 2000 and 2007, which were also pleasant for the same reason.
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.
In fact, 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 notice that we seem “stuck in a pattern,” either for good or for bad – well, the NAO is that pattern. In 2010 and 2011 the NAO was negative, and those winters were snowy and miserable.
The NAO is simple to understand. A permanent high-pressure area sits over the Azores, west of Portugal. And a low-pressure area hovers over Greenland and Iceland, which is what gives them their chronic dismal 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. It stops Nor’easters from forming. It encourages a zonal (horizontal) northern jet stream, which traps cold weather north of the border in Canada.
Such a positive winter NAO was rare 60 to 100 years ago, but started becoming commonplace in the 1980s and especially the 1990s – which is why our winters have gotten so much milder than what the old-timers remember.
Two winters ago, a negative NAO bucked the trend and persisted – meaning that the Azores high was weak, and the Greenland low was 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 rarely let precip fall as rain here in the Northeast that winter, and made itself powerfully evident in New York, Philly, Washington and points south, with repeated deep snowstorms in the Northeast corridor. As to the cause, some astronomers think that the very low sunspot minimum from 2006 to 2010 is what caused the NAO to shift negative most of those winters.
It should also be noted that climate-change deniers point to the NAO as explaining the warm temperatures in the 1990s, rather than increased carbon dioxide levels from anthropogenic sources. My rebuttal would be that the NAO probably did influence winters here in the Eastern US, but not global temperatures, which continue to climb regardless of the NAO oscillation. Whatever the reason for its existence, this oscillation 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. If you want to study it further, a chart of how the NAO has shifted during the past 60 years is available at this site: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.timeseries.gif.
If only there were a way to preserve it, so we could enjoy having Carolina-winters-in-Saugerties always, and not just NAO!
Astronomy tip: Watch the sky this weekend, just as night falls. The Crescent Moon will beautifully hover next to Venus on Saturday evening, and next to Jupiter the next night.