Importance of the North Pacific Pattern in Autumn
As we progress deeper into the autumn season, many are beginning to wonder if the upcoming cold pattern is a precursor to the type of winter that we will see. Unfortunately, it is not as easy as cold October yields cold winter or warm October yields warm winter. If that were the case, we’d have long range forecasting figured out by now and there would be no need for winter outlooks, as the winter would be set in stone by the end of October. The best we have are correlations, analogs, physical pattern drivers, and pattern persistence. When added together, the summation of these various factors provide a higher probability of success in medium to long range forecasting, but these methods are far from perfect. With that being said, the evolution of different atmospheric oscillations during the autumn season can give us a substantial clue as to the ensuing winter’s dominant index modalities.
In this particular case, let’s examine the autumn north pacific pattern of years since 1990, giving us a sample size of 22 years (we would have to wait through this winter to see if 2013′s correlation holds up). The north pacific pattern is often represented as the East Pacific Oscillation (EPO). Like the North Atlantic Oscillation, the EPO has certain, generally predictable outcomes when it oscillates from positive to negative phases. The positive phase of the EPO is usually associated with lower than normal heights across the northeastern Pacific, much of Alaska, and northwestern Canada. Conversely, the negative phase of the EPO generally produces higher than normal heights over the region just described. The positive (+EPO) tends to flood the United States with maritime pacific air due to the low pressure around Alaska, while the negative (-EPO) acts as a block to maritime pacific air due to high pressure over the NE Pacific/Alaska. The -EPO often sets up a cross polar flow with high pressure centered over Alaska and the flow oriented across the north pole from Siberia into Canada. The -EPO pattern has yielded some of most impressive, severe arctic outbreaks on record. The upcoming 10-15 days, for example, will be colder than normal in the East primarily due to the forcing from the negative EPO signal in the northeast Pacific. There’s not much blocking in the north Atlantic or arctic regions, so the main impetus for this cold through November 5th is the -EPO.