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PDO-ENSO Interactions Over the Past Four Centuries
Verdon, D.C. and Franks, S.W. 2006. Long-term behaviour of ENSO: Interactions with the PDO over the past 400 years inferred from paleoclimate records. Geophysical Research Letters 33: 10.1029/2005GL025052.

What was done
The authors used "proxy climate records derived from paleoclimate data to investigate the long-term behaviour of the Pacific Decadal Oscillation (PDO) and the El Niņo Southern Oscillation (ENSO)."

What was learned
Over the past 400 years, Verdon and Franks report that climate shifts associated with changes in the PDO "occurred with a similar frequency to those documented in the 20th century." In addition, and more importantly, they find that "phase changes in the PDO have a propensity to coincide with changes in the relative frequency of ENSO events, where the positive phase of the PDO is associated with an enhanced frequency of El Niņo events, while the negative phase is shown to be more favourable for the development of La Niņa events."

What it means
The two Australian scientists note that the numerous El Niņo events of the recent past "have been reported as unusual, and have even been suggested to be possible evidence of anthropogenic climate change [e.g., Trenberth and Hoar, 1996]." However, as they continue, "the paleo records suggest that the apparent lack of La Niņa events and high frequency of El Niņo events over the past two decades may not be abnormal and could be attributed to the fact that during this time the PDO has been in a positive phase," such that "when the PDO switches back to a negatively dominated phase, it is quite likely that the frequency of La Niņa events will increase once again." Consequently, there is no compelling reason to believe that the recent preponderance of El Niņo events over La Niņa events is a "fingerprint" of CO2-induced global warming.

Trenberth, K.E. and Hoar, T.J. 1996. The 1990-1995 El Niņo-Southern Oscillation event: Longest on record. Geophysical Research Letters 23: 57-60.

Reviewed 5 July 2006