The Chicken or The Egg: PDO or ENSO?

Does the Pacific Decadal Oscillation drive the frequency and amplitude of El Nino and La Nina events, or does the frequency and amplitude of ENSO drive the PDO?

I won’t offer an answer, but I will post a number of graphs of NINO3.4 and PDO data derived from the same data sets, which illustrate there is no simple answer available by visually inspecting graphs. At times, NINO3.4 signal changes lead the PDO. At others, the PDO signal varies first.

The NINO3.4 and PDO data used are part of the NCDC ERSST.v3 sets.
ftp://eclipse.ncdc.noaa.gov/pub/ersst/pdo/
ftp://eclipse.ncdc.noaa.gov/pub/ersst/pdo/el_nino_v3.dat
ftp://eclipse.ncdc.noaa.gov/pub/ersst/pdo/pdo.1854.latest.ts

Figure 1 illustrates monthly NINO3.4 and PDO data from January 1900 to February 2008. Also shown with a higher weighting are the NINO3.4 and PDO signals smoothed with a 36-month filter. You’ll definitely need to use the TinyPic links to view these graphs. Note that I used the filtering available with EXCEL, which aligns it with the last data point.

http://i25.tinypic.com/14dj904.jpg
Figure 1

Keep in mind that the PDO data is not a true SST anomaly. It has been standardized, which can amplify the signal minimally or greatly dependent on the data.

In Figures 2 through 5, I’ve broken the data into shorter periods for easier viewing.
Figure 2 covers January, 1900 through December, 1925. Figure 3 includes January, 1925 through December, 1950. Figure 4 covers January, 1950 through December, 1975. Figure 5 illustrates January, 1975 through February, 2008.

http://i30.tinypic.com/30adrph.jpg
Figure 2

http://i32.tinypic.com/2dt2hbq.jpg
Figure 3

http://i25.tinypic.com/dyx7bl.jpg
Figure 4

http://i27.tinypic.com/2zf4taw.jpg
Figure 5

A PAPER THAT ANSWERS THE QUESTION

In “ENSO-Forced Variability of the Pacific Decadal Oscillation”, Newman et al state in the conclusions, "The PDO is dependent upon ENSO on all timescales." Refer to:
http://www.cdc.noaa.gov/people/gilbert.p.compo/Newmanetal2003.pdf