PDO/PNA. The PDO (Pacific Decadal Oscillation) is an index derived from North Pacific sea surface temperature anomalies and it has a high correlation to the PNA (Pacific North America) pattern.
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THE PDO (Pacific Decadal Oscillation) and PNA (Pacific/North American Index)
As you can see, we are clearly heading into, and staying in a negative PDO Fall as the SSTs right now could make the PDO index severely negative.To get a good idea as to where we are headed for the winter, I looked at years where we saw a +PDO Spring and Summer, followed by a -PDO Fall. Out of the years mentioned above (years where we saw a +PDO Jan-July), 9 years saw a -PDO Fall in that the PDO averaged out to be below 0.0 for the Sept/Oct/Nov period. Out of the 9 years that saw a -PDO Fall after a +PDO Jan July period, 7 of these years (78%) saw a drastic rise in the PDO through the early winter making it positive for the winter as a whole (DJFM average). Since the PDO will most likely be rising from negative to positive through the late Fall and Winter, I believe that the odds are in strong favor of a +PNA.
Below are the PDO readings for Jan-July 2006
Jan: +1.03; Feb: +0.66; Mar: +0.05; Apr: +0.40;May: +0.48; June: +1.04; July: +0.35.Since 1900, there have been 26 years where each month from January to July saw a positive PDO reading. (1900, 1902, 1906, 08, 28, 31, 34, 36, 40, 41, 42, 58, 77, 81, 83, 85, 86, 87, 88, 92, 94, 96, 97, 2003, 2004, 2005) Of these 26 years, 21 of the following winters (81%) saw a +PDO in that the DJFM PDO average was > 0.0.
Interpretation of PNA and NAO indices on computer forecasting models.
In addition, the PDO directly affects the ability of the El Nino or La Nina event to reach the STRONG category. When the PDO is in a negative phase the El Nino events tend to be weaker while La Nina events tend to be stronger. On the other hand when the PDO is in the positive or warm phase the El Nino events tend to be longer lasting and stronger while La Nina events tend to be weaker.