Saturday, October 20, 2007

Weather 101: The NAO

The North Atlantic Oscillation (NAO) is one of the many factors that drives Philadelphia's weather. It is not just a winter-time event that is good for cold and snow, as many perceive it to be, but it is something that helps shape our weather throughout the year.

The NAO is a measurement of the relationship between low pressure that situates itself over Northern Climates in the Atlantic Ocean in comparison to high pressure that sets up shop farther to the south over the subtropics. The NAO indexes the relationship between the pressure of the Icelandic low and the subtropical high, with 0 being used to define 'neutral' conditions meaning the pressure difference between the two is 'normal' over time.

A positive NAO means either that the low pressure near Iceland is stronger, the subtropical high is stronger, or both apply. Typically when you have a strong subtropical high pressure system you see warmer take place in the atmosphere over the Southeastern US via the infamous the Southeast Ridge. This ridge nudges the main storm track farther north, typically through the Northeast United States or Southeast Canada. The Icelandic low will cause a trough over Greenland and allow for colder weather in Northeastern Canada and the Northern Atlantic.
A negative NAO does the opposite, meaning that weaker low pressure over Iceland and/or a weak subtropical high are the main players in the Atlantic. Because the low over Iceland is weaker than average, systems can get 'blocked' and can spin in place near Greenland for several days. The '50-50 low' is in reference to a low pressure system that gets stuck near 50 degrees North latitude and 50 degrees West longitude, near the Newfoundland coast. The resulting low may drive a trough into the Eastern US and assuming that there is no strong zonal jet pattern from the Pacific Ocean, would result in lower temperatures for the East.

The NAO CAN influence our weather but it is not the only factor in influencing our weather in the Eastern US. From 1950 on, there have been 25 winters that have featured a -NAO on average for December, January, and February. Those winters did go colder and snowier on average without any account of ENSO state but there is a wide variation in snowfall over the course of those winters.

35.5 4.6 LA NEG 1950
38.5 16.8 NEU NEG 1952
37.4 22.6 NEU NEG 1953
34.9 12.1 LA NEG 1954
33.4 23.0 LA NEG 1955
33.1 41.8 EL NEG 1957
31.4 5.1 NEU NEG 1958
35.9 21.8 NEU NEG 1959
30.5 29.2 NEU NEG 1961
28.3 20.5 LA NEG 1962
30.9 32.9 EL NEG 1963
33.3 26.2 LA NEG 1964
32.5 27.4 EL NEG 1965
32.6 15.9 LA NEG 1967
31.4 23.7 EL NEG 1968
30.4 20.3 EL NEG 1969
33.2 18.3 LA NEG 1970
28.0 18.7 EL NEG 1976
28.4 54.9 EL
NEG 1977
31.4 40.2 NEU
NEG 1978
34.8 16.5 LA NEG 1984
34.1 25.7 EL NEG 1986
37.6 12.9 NEU NEG 1996
40.4 0.8 EL NEG 1997
31.3 46.3 EL NEG 2002
33.2 23.1

The average -NAO winter since 1950 in Philadelphia brings you temperatures that are a degree and a half below normal (normal 34.8 degrees) and snowfall about three inches above normal. However, if you account for Nino, the Pacific, and other factors, you see the WIDE range in snowfall, from as low as 0.8" in 1997-1998, a winter with a very strong El Nino, to 54.9" of snow in 1977-1978, a borderline weak Nino.

If we factor in a La Nina state and a -NAO average for the winter, the results are colder but not that snowy.

35.5 4.6 LA NEG 1950
34.9 12.1 LA NEG 1954
33.4 23.0 LA NEG 1955
28.3 20.5 LA NEG 1962
33.3 26.2 LA NEG 1964
32.6 15.9 LA NEG 1967
33.2 18.3 LA NEG 1970
34.8 16.5 LA NEG 1984
33.3 17.1

The average among 8 La Nina/-NAO winters is about a degree and a half below normal on temperatures but about three inches below normal on snow. The range in snowfall over these winters is not as wide...4.6" is the low in 1950-1951 and the high is 26.2 in 1964-1965.

36.1 7.9 LA POS 1956
36.4 12.2 LA POS 1971
35.4 20.8 LA POS 1973
37.5 13.6 LA POS 1974
35.5 17.5 LA POS 1975
35.6 11.2 LA POS 1988
31.9 65.5 LA POS 1995
38.3 12.5 LA POS 1998
36.5 21.0 LA POS 1999
36.9 19.5 LA POS 2005
36.0 20.2

When the NAO is positive for winter, the average snowfall actually is higher (it's around normal) for Philadelphia despite warmer than normal temperatures, thanks to one special winter in particular! When we factor out the winter of 1995-1996 anomaly from the equation, the average snowfall is lowered to 15.1", two inches below the -NAO state in La Nina winters. In essence we get a warmer winter with slightly less snowfall but regardless of NAO state we see slightly less snowfall than normal in Philadelphia in the winter.

The graphic below illustrates what the NAO can do in a La Nina winter based on a positive or negative state.
As you can see, the North Atlantic Oscillation alone does drive some of the weather around here and can lead us to colder or warmer weather on average. However, given the wide ranges in snowfall that we can see annually, one cannot look to the NAO alone to determine what will happen in the coming winter. There are many other factors at play on the table and in studying weather one needs to look beyond just one piece of information and look at the whole puzzle before they make their conclusion regarding what will happen.