Thursday, October 20, 2011

Laying The Grounds For Winter: The NAO

The North Atlantic Oscillation (NAO) is typically the most-looked at factor besides Nino and Nina regarding winter weather. The NAO is essentially a numeric reflection of a pattern in the atmosphere over the Atlantic Ocean.  In the winter months, a positive reflection of the NAO reflects an atmospheric pattern where a trough of low pressure in the atmosphere resides over Greenland, with a stronger than usual subtropical ridge of high pressure in the Atlantic.  This results in milder conditions on the US East Coast and in Europe.

On the flip side, a -NAO pattern means that the high pressure ridge near Bermuda is weaker and that the ridge is stronger in higher latitudes (Greenland or thereabouts), which results in a trough over the Eastern US and colder conditions...and if timing and jet stream placement are right, the potential for storms along the East Coast.   -NAO conditions in winter do not magically equate to winter snowfall though but it does increase the odds that a storm or storms will spit out snowfall.

Even in the summer, the NAO can be a useful indicator of the following winter weather.  If you have a stronger blocking pattern in the higher latitudes during the summer months, the tendency over the last 60 years leans slightly towards that same blocking regime holding serve into winter (8/14 strongest -NAO's in summer were followed by -NAO winters).  Given that we've been in a rather strong blocking regime the last four years (NAO values have been strongly negative each of the last four summers) and that those -NAO summers since 2008 have all correlated into a -NAO winter the following year, the odds do lean in favor of a -NAO winter in the coming year.    Research suggests a correlation between solar activity with the NAO, with a quiet period of solar activity (lessening of solar wind from a quieter sun) correlating to -NAO.  Given the quietness of the sun over the last few years, one could see the potential relationship between these solar factors and what happens in our own atmosphere.

This past summer featured the second strongest -NAO factor since 1950.   The five other strongest -NAO's on record, without regard to Nino or Nina state, are shown above.   The tendency of these five years suggests colder-than-average weather in the following winter with three of the five winters featuring above average snowfall.   Two years, one Nina (1998-1999) and one Nino (1958-1959) brought less snow than average.   If we break things down by Nina years only, the average snowfall decreases somewhat and the average temperature for the following winter runs about a degree cooler than average.   Three of the five winters feature more than 20" of snow, with all of the colder-than-average winters featuring at least 20" of snow

You'll notice that a couple of the years on the list are recent -- 2008 and 2010.   Both of these years featured that mix of strong -NAO in the summer and a La Nina state in the Pacific.   Both winters were colder than average, with 2010-2011's winter featuring a five week bonanza of snow from late December through late January and 2008-2009's winter featuring its snows in two hits in February and March, but including a very cold January for good measure.   2008 also featured a negative Arctic Oscillation value  although not as significant as this summer's -AO.

The trends do suggest a colder-than-typical winter overall.   The question is when do we time the cold shots and warm shots for the coming winter.   We'll tackle that next week when we release our winter forecast.