What is the NAO?

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By Richard Sparago on December 24, 2012, 11:40am Last modified: December 26, 2012, 12:22pm

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Now that we've officially entered winter, we're hearing about the various factors that influence our weather. One such factor that is mentioned quite often is the NAO, or North Atlantic Oscillation. The term has significant meaning to meteorologists, but as a non-meteorologist, I was never quite clear on what the NAO is, and how it impacts weather in the northeast. If you're in the same state of understanding (or misunderstanding) as I am, here's a layman's attempt at explaining the NAO.

The NAO is a winter phenomenon that is a large-scale seesaw in atmospheric mass between the subtropical high (near the Azores) and the polar low (near Iceland). The interplay between these two atmospheric giants affects the jet stream, which ultimately affects our temperatures and storm track. The relative positioning and strengths of these two forces make up the NAO.  A large difference in the pressure between the two systems (denoted NAO+) leads to increased south-westerly winds for us, that can "pull up" mild air from the south to the northeastern U.S. This can influence our winter to be mild, by blocking significant outbreaks of cold air. In contrast, if the index is low (NAO-), meaning not such a dramatic contrast between the pressures of the subtropical high and polar low, the effect on our winter is the opposite. Thus, the eastern seaboard and southeastern United States can incur cold outbreaks more often than normal, with associated snowstorms and sub-freezing conditions. Thinking about recent winters, the above verifies quite nicely. The winter of 2010-2011 was characterized by a negative NAO, while last winter was dominated by a positive NAO.

Historically, NAO patterns have shown to last many years, with interval anomalies. From the Climate Prediction Center, the negative phase of the NAO dominated the circulation from the mid-1950s through the 1978/79 winter. During this approximately 24-year period, there were four prominent periods of at least three years each in which the negative phase was dominant and the positive phase was notably absent. An abrupt transition to recurring positive phases of the NAO then occurred during the 1979/80 winter, with the atmosphere remaining locked into this mode through the 1994/95 winter season. During this 15-year interval, a substantial negative phase of the pattern appeared only twice, in the winters of 1984/85 and 1985/ 86. However, November 1995-February 1996 was characterized by a return to the strong negative phase of the NAO. 

For those of us who have lived through some of the winters referenced above, we can attest to the NAO's impact. I recall the winters in the mid 1970s as being quite harsh, bearing strong similarity to the winter of 2010-2011. Now the big question...what phase will the NAO be in this winter? The data I've seen suggest a negative NAO, beginning shortly after Christmas and lasting through at least mid January. While forecasting the NAO is best left to scientists, the rest of us should keep an eye on their predictions for the NAO. Its impact on our weather is clearly quite significant.


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Richard Sparago

Town: Milford, CT  

Reporting for WXedge since February 2012.

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