|NYC - East 46th St. near 2nd Ave.|
The polar vortex (PV) has been wicked strong until recently when it weaken slightly during the on-going minor stratospheric warming. Minor warmings are not known for their association with PV-reversals...so there's little expectation this event will be reflected at the surface in the form of a negative AO.
What a difference a year makes. Last December/s monthly AO was the second lowest (-2.631) during the period of record which began in 1950. JAN-11 didn't stand out in the historical record (ranked 14th) but it was strongly negative. This winter...December was the polar opposite at +2.221. January is on-track to finish...not unexpectedly positive...around +1.
Below is a time-series of the AO on winter's cusp throughout the 60+ year period of record (1950 - 2011). The blue line is a trace of the daily AO index on 15-JAN. The dark red line is a a trace of the same data after applying a nine-point binomial filter. The filter removes low frequency noise and highlights decadal trends.
Not sure what this says about the AO on 15-JAN other than there is a discernible cycle every nine years or so. It does have an interesting comparison to the cycles found in the 30-DEC data.