Positive Arctic Oscillation Increasingly Likely To Start Off Winter

We are seeing a pretty strong -AAM anomaly propagating equatorward currently which helps support a +AO/+NAO pattern. The GEFS consensus keeps the index positive all the way into mid november.

Reduced 36% 1000 x 500 (70.69K)

This type of pattern means a stronger vortex and a northerly displaced jet. Luckily a strong piece of the polar vortex is expected to park over eastern canada through early november. This may be caused, in part, by the neutral enso/+QBO regime which favours a cooler late oct into early nov period for the east. We have seen a wave 1 response across the bering sea area which has also helped bring the chill, along with an active east asian jet.

 Reduced: 90% of original size [ 565 x 437 ] - Click to view full image

What this also means is a recovery of the vortex is likely to occur by mid november, with a potentially positive AO regime setting up through early winter. The SAI/OPI has been the opposite of last year with a sharp negative trend in snow cover since early october. Ridging is expected to continue south of 60N across siberia so I would expect this trend to continue with snow cover ending off at near normal levels.

Reduced 20% 800 x 618 (10.8K)

If we look at the 10 most positive AO winters we find the following SLP setup near the north pole. Notice the -MSLP anomalies near siberia.

 Reduced: 82% of original size [ 621 x 480 ] - Click to view full image

This year we have seen below normal MSLP across northern siberia as well.

 Reduced: 90% of original size [ 565 x 437 ] - Click to view full image

The likely scenario IMO is the AO index remaining on the positive side through much of november and december with a SE ridge in the means and the core of cold/ winter storms aimed towards the midwest. This does not mean we cannot see a wintry pattern along the east coast, especially for new england, and the northern tier. Dec 07 is a good example of this. The +AO game is risky though as we will need to depend on a -EPO and favourable pacific setup to deliver the goods. The pattern does look ripe for a SSW event by jan/feb with a more poleward 0 line in the mean wind department as well as a large ozone build up from previous ENSO warming. This will likely play a role in hampering the AO state by Jan-March.