http://www.lavoisier.com.au/articles/greenhouse-science/solar-cycles/IanwilsonForum2008.pdf Many people argue that the PDO (Pacific Decadal Oscillation) 
Index is largely a reddened response to El-Nino-ENSO forcing 
from the tropical Pacific ocean (see * below). However,
the graph above shows that this view is not compatible
with the observations.
The lower of the two graphs in this figure shows the phase of the 
PDO from 1660 to 2000 A.D., as indicated by proxy tree-ring 
data of Vernon and Franks (2006). Above it is a graph of the
(running) mean intensity of El Nino events over the same time 
period, as published from proxy tree-ring data by Gergis and 
Fowler (2006) [Advances in GeoScience, 6, pp. 173 - 179, 2006]. 
It doesn't take a rocket scientist to realize that every time the 
PDO switches positive, there is a progressive increase in the 
mean intensity of El Nino events, and every time the PDO 
switches negative, there is a progressive decrease in the mean
intensity of El Nino events.
Simple causational logic tells you that it is the El Nino that is 
reponding to the long term changes in the PDO and not the 
other way around. This evidence alone, should be enough to 
completly invalidate the models of Newman et al. [2003] and 
Shakun and Sharman (2009) [Geophysical Research Letters]. 
http://wattsupwiththat.com/2009/10/16/connecting-enso-pdv-and-the-north-and-south-pacific/#comments A much more likley explanation for the results that these 
authors are getting is that the underlying causal mechanism 
for both the El-Nino-ENSO phenomenon and the PDO are linked.
In this blog, I propose to show that the common underlying 
mechanism responsible for the changes seen in the ENSO 
and the PDO are the lunar tides and their effect upon the 
up-welling cool deep ocean water in the Pacific ocean.