‘That said there is a LOT of nonsense about the PDO. People like CPC are tracking PDO on a monthly basis but it is highly correlated with ENSO. Most of what they are seeing is the change in ENSO not real PDO. It surely isn’t decadal. The PDO is already reversing with the switch to El Nino.’ K Trenberth – hacked email
Trenberth is certainly wrong. The biology is definitive – as in the fantastic Russian fisheries publication someone linked to earlier. I can’t recommend it enough. The biological changes occur as a result of changes in upwelling of nutrient rich water. But the evidence of decadal changes in hydrology, cloud and sea surface temperature is equally compelling. As sure as sh.. – as we say in Australia (I have censored myself for sake of Bart’s delicate sensibilities – and the fact that I keep getting sent, albeit temporarily, to the sin bin) – it is decadal.
The changes have been attributed to solar UV in the 11 years Schwabe, the 22 year Hale and other diverse solar cycles – which drifts much more over longer timeframes than does solar irradiance. – See for instance – http://iopscience.iop.org/1748-9326/5/3/034008/fulltext – See also – http://iopscience.iop.org/1748-9326/5/2/024001/fulltext
The mechanism involves warming and cooling of ozone in the middle atmosphere – which influences sea level pressure at the poles and thus the path of storms spinning off the polar vortices.
The correlation between ENSO, the Southern Annular Mode (SAM) and solar cycles has been shown in many studies. In the Southern Hemisphere – increased SLP in higher latitudes pushes storms and cold Southern Ocean water towards the western coast of South America in the region of the Humboldt Current. The Humboldt Current is the thermal origin of ENSO and ENSO is of course is the dominant source of global climate variability.
I usually add the postscript that dynamical systems theory implies extreme climate sensitivity near points of chaotic bifurcation – suggesting that changing the composition of the atmosphere may not be entirely risk free.