Winter '11 / '12 - Trending: The North Atlantic Oscillation

The North Atlantic Oscillation (NAO) often plays an important role in defining the character of sensible weather during east coast winters.  It and its godfather the Arctic Oscillation (AO) are also two of the harder features to forecast beyond a week or ten days which leaves seasonal forecasts vulnerable to considerable error.

The developing consensus for Winter '11 / '12 leans heavily on a strengthening la Nina fortified by the trending cool phase of the Pacific Decadal Oscillation (PDO).  These conditions in the eastern Pacific ocean favor a predominantly northern stream storm track passing west of the Appalachian Mountains into northern New England, the result of a blocking ridge axis over the SE CONUS...especially during a positive phase of the Northern Annular oscillations (NAO and AO).

Given the recent trend shown below...how probable is it the phase of NHEMI annular oscillations will be positive this winter?

The time-series above shows the total number of months the NAO was positive or negative during each year.  These data are quite noisy but do show most years during the '50s had greater than or equal number of negative months than positive months and the late '80s to the mid-'90s were dominated by years with more positive months than negative.

Summing the positive and negative months over five years produces a smoother plot and reveals clearly the negative dominance during most of the '50s...the mid-'60s to early '70s...and ever since 2000.  The 'even' line for the period is 30 months.

Positive months held sway between the mid-'80s and much of the '90s during the peak of the AGW war.

Summing the positive and negative months over a decade removes all the noise and shows the positive phase has been the dominant state of the NAO for thirty years (1970 - 2000).  The 'even' line for this period is 60 months.

Coupled with conditions in the Pacific -- evidenced by the initial years of the PDO's cool phase -- the NAO's trending decadal data could be signaling an impending era of colder than 'normal' winters.

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More later about the correlation between NAO and west Atlantic ocean SST anomalies along with how a la Nina winter can be affected by negative NH annular oscillations..