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Tuesday, December 22, 2009

Winter '09 / '10 - Arctic Oscillation


The Arctic Oscillation (AO) was headed toward near-record levels during the past weekend while an historic storm buried many locations in the mid-Atlantic and southern New England with record snowfalls. 

A minor stratospheric warming event preceded the east coast snow storm by three-to-four weeks. The polar vortex (PV) weakened after splitting in two during late NOV and early DEC...as reported on the NEWxSFC web log here...here...and here.

The PV/s stronger half shifted from the eHEMI to wHEMI...where opposing flow between the two circulations created weak ridging near the pole.  With time...the ridging aloft made its way to the surface where an arctic anti-cyclone pushed the AO to levels seldom seen.

The last time the AO was less than or equal to 21-DEC-09/s -5.668 value was over 20 years ago on 01-JAN-85.

The AO has been this low...or lower...on just 15 days since modern record-keeping began in 1950  (NOV-59, JUN-64, JUL-64, MAR-70, JAN-77, JAN-85).  The odds of an AO index value at or below -5.668 are 1 in 1460 (0.07%).

The record negative value of -7.433 was set 15-JAN-77 during a borderline +ENSO (MEI = 0.484)... strong +PDO (1.65)...and an east QBO (-14.91).



3 comments:

amazingsnow said...

Stunning!! AO index, ... with its clear repercussions here, in Europe, too.

TQ said...

Welcome AmazingSnow!

Your blog is a joy to visit. Only wish I spoke the language.

amazingsnow said...

no problem for this TQ, ... "amazingsnow" basically is for "watching" , and enjoying the gifts that cold always gives. For me it's a pleasure visit your blog, too, ... always interesting.

Regards from Catalonia!