Winter '09 / '10 - No Negative NAO

Eastern snow crows are often heard aching for a negative North Atlantic Oscillation (NAO) come winter b/c of its negative correlation to season-total snowfall. Winters where the NAO is...on average...negative (positive)...higher (lower) snowfalls and colder (milder) temperature are more likely.

Buzz-killing evidence is beginning to mount that is sure to disappoint those pining for predictions of a coming negative NAO.

The Developing Case Against Negative NAO
1) The British MET Office's 'positive NAO' forecast is based on SST anomalies in the Northern Atlantic during the month of May. The technique's correlation skill is about 0.45 with the correct sign of the NAO predicted for 66% of all winters. Details here.

2) Trend analysis supports a positive NAO this winter. NAO/s yearly 'winter' average (D-J-F-M) has been positive 21 out of the last 26 years. NAO/s five-year moving-average for the same four months has been positive every year since 1983. Data.

3) Analog years point to positive NAO this winter. NAO has been negative five months this year...three of which occured during meteorological summer. This was also the case the past two summers. July observed its third lowest value (-2.15) since 1950.

Analog years selected using a 'least-squares' regression technique (shown below) are 2007...2000... 1956...1999...and 1974. Monthly NAO values for the analog years are displayed through year/s end.



When monthly NAO index values for the analog years are extended through the winter months of D-J-F...three of the five analog years had a positive monthly NAO index. One analog year/s NAO monthly index was positive during J-F. Another analog year/s NAO monthly index was positive during D-J. NAO analog analysis does not support a forecast for negative NAO this winter. Similar story from the Arctic Oscillation (AO).



4) Contravening evidence is found in the Quasi-biennial oscillation (QBO)...which is beginning its negative phase and will persist throughout the coming winter. QBO/s east phase and solar minimums are correlated with positive high-latitude height anomalies. More about this winter/s QBO here.

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Negative NAO is not a be all and end all for decent season-total snows...nor is it required for severe winter storms along the eastern seaboard. NAO was positive when The 'Storm of the Century' raked the east coast in mid-March of 1993.

Positive NAO image courtesy Lamont-Doherty Earth Observatory...Columbia U.