Until last week I knew that the PDO was an acronym for the Pacific Decadal Oscillation but little else. I now know a little more thanks to the Wikipedia and this very informative article from the North Carolina Climate Office of all places (fig 1).
Two data sets
I had got a little confused when looking for monthly data to graph the series with. I now realise that there are in fact two versions of PDO series one maintained by the people who discovered the relationship in 1997 called the Mantua PDO, and the other maintained by NOAA known as the NCEI PDO and based on the Matua PDO:
“The NCEI PDO index is based on NOAA’s extended reconstruction of SSTs. It is constructed by regressing the ERSST anomalies against the Mantua PDO index for their overlap period, to compute a PDO regression map for the North Pacific ERSST anomalies. The ERSST anomalies are then projected onto that map to compute the NCEI index. The NCEI PDO index closely follows the Mantua PDO index“
That’s why I decided to write an application to download, parse and plot both the Mantua PDO series which started in 1900, and the NCEI PDO series which started in 1854. This is how the two series have behaved over the last 30 years (fig 2).
The third cycle
As the North Carolina Climate Office says (see above), researchers have found evidence for just two full PDO cycles in the past century. A cold PDO regime prevailed from 1890 to 1924 and from 1947 to 1976, whilst a warm PDO regime dominated from 1925 to 1946 and again from 1977 through to 1998. That suggests to me that in 2018 we are in the cold part of a third cycle at the moment (fig 3).
This flurry of development on my part was brought about reading a recent post about the PDO to the uk.sci.weather newsgroup from Graham Davis who reported: “The Pacific Decadal Oscillation Index has been positive for 50 straight months. This was the longest period that the index has been in either positive or negative territory. However, last month, the index slipped to -0.05. To put this value into some perspective, the highest monthly value of the index on record has been +3.5 and the lowest -3.6“. How significant this negative index in March is anyone’s guess at the moment. The Mantua PDO has been oscillating positive negative since 1998, but it has been in positive territory since January 2014.