By Joseph D’Aleo, Intellicast.com, April 28, 2008

In this version of the weekly Dr. Dewpoint series on Intellicast, the El Nino dominance since the late 1970s is explained.

John McLean in a paper here reported Australia’s CSIRO and Bureau of Meteorology (Power and Smith 2007) wrote about a period of unprecedented El Nino dominance the last 30 years, which they blamed on human activity. Vecchi (2006, 2007) speculated there was a just 1% probability that this was due to natural events.

McLean’s paper and the following discussion will show how the change had precious little to do with anthropogenic factors but was the result of a natural large scale cyclical climate flip-flop known now as the Pacific Decadal Oscillation.

The first hint of a basin wide cycle was a recognition of a major regime change in the Pacific in 1977 among climatologists that became known as the Great Pacific Climate Shift. Later on this shift was shown to be part of a cyclical regime change given the name Pacific Decadal Oscillation by Mantua (1997). This followed research first showing decadal like ENSO variability by Zhang in 1993.

Mantua found the Pacific Ocean temperature regime and overlying pressure patterns tended to persist in one mode (in terms of ocean temperature anomalies and overlying pressure and wind patterns) for two or three decades and then flip to very nearly the opposite mode for a similar period. The IPCC in their latest report (2007) also showed how during the positive phase of the PDO, the sea surface temperature pattern suggested more warmth in the eastern tropical Pacific (thus more El Ninos).

HAVE WE EXPERIENCED GREAT PACIFIC CLIMATE SHIFT II - A REGIME CHANGE IN PDO?

The PDO appears to have changed back to the cold mode in 1998 following the Super El Nino of 1997/98. Three straight years of La Nina followed. PDO bounced some during the early 2000s but this year dropped off dramatically again this past year as a strong La Nina developed. Since it is 30 years since the last climate shift, it appears likely this negative mode should continue.

See the full size graph here. See the full image filled blog describing how the PDO influences ENSO frequency and global temperatures here.