February 13, 2007

More Bad News about El Nino

World Climate Report has brought you many essays regarding articles in the scientific literature with results that challenge popular perceptions about global warming and El Niño. Don’t look now, but, predictably, this year’s El Niño has brought with it its share of climate change/impact associations. (We wonder what happened to that warm winter that El Niño was thought responsible for?).

You’ve probably read this a dozen times already, but El Niño is related to a warm pool of water forming off the Pacific coast of equatorial South America, and during El Niño periods, Australia suffers from drought and fire, the southern and southwestern United States receive significantly above average winter precipitation, and the United States as a whole has warmer than average winters. The empirical linkage underlying these connections is significant in a statistical sense, but often surprisingly weak as covered many times in World Climate Report.

The science literature contains articles from modeling studies showing that higher concentrations of greenhouse gases will result in more El Niño years, and not surprisingly, the climate alarmists have made the most of that potential link. Of course, there are many other articles with results from models that show no link whatsoever between greenhouse gas buildup and the occurrence of El Niño conditions. Hard to make much out of those, so the alarmists don’t really bring them up.

Two articles have appeared recently in major journals that caught our eye but surely have been dismissed by those who embrace and promote the El Niño – global warming linkage. The first article is by Danielle Verdon and Stewart Franks of the University of Newcastle in New South Wales, and it appears in Geophysical Research Letters. They note that the state of El Niño Southern Oscillation (ENSO), the swing back and forth between the El Niño warm pools and La Niña cool pools, is strongly related to a much larger Pacific Decadal Oscillation (PDO) covering much of the North Pacific Ocean and impacting the entire Pacific Ocean. The phase of the PDO can not only change the probability of having El Niño or La Niña, but the phase of PDO also determines how strongly El Niño or La Niña will impact weather conditions from Australia to Florida. Once again, we learn that things are more complicated than they seem at first – any discussion about global warming, El Niño, and effects on regional climates must account for potential changes in PDO.

Verdon and Franks assemble proxy records of the PDO and reconstruct the positive and negative phases of PDO back to A.D. 1662 (Figure 1). The graph is based on tree ring chronologies from Alaska, the Pacific Northwest, and subtropical North America as well as coral fossil from Rarotonga located in the South Pacific. Given the step-like shifts back and forth in the PDO, the odds of El Niño or La Niña change over time and the impact of El Niño or La Niña on regional climates would thereby change as well. The Welsh pair concludes that “the paleo records suggest that the apparent lack of La Niña events and high frequency of El Niño 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. When the PDO switches back to a negatively dominated phase, it is quite likely that the frequency of La Niña events will increase once again.” We see this over and over – put some recent trend in a longer term perspective, and it would seem nothing unusual is happening.


Figure 1. Duration and timing of step changes in the Composite PDO Index from 1662–1998 (from Verdon and Franks, 2006).

Our second feature article appears in the Journal of Climate by a pair of scientists from the University of Maryland. These two questioned whether numerical climate models relied upon by the Intergovernmental Panel on Climate Change (IPCC) can realistically simulate ENSO and the impacts of ENSO on climate throughout various parts of the world.

Joseph and Nigam did a fine job evaluating many models used by the IPCC, and after their comprehensive work, they found that “Generating realistic ENSO variability remains challenging for climate models as is evident from the preceding analysis of ENSO amplitude, duration, recurrence, and seasonal timing.” They basically found that in terms of ENSO characteristics, the models have a long way to go in simulating what was observed from 1950 to 1999. We completely agree with their statement that “The study suggests that climate system models are not quite ready for making projections of regional-to-continental scale hydroclimate variability and change, even though they have begun to make inroads in simulating key features of ENSO variability.”

We agree that the models will get better, but we give Joseph and Nigam credit for daring to suggest that not only do the models struggle with ENSO, but they have a long way to go with other oscillations, such as the PDO. Getting the PDO and ENSO interactions correct would seem a long way down the highway. Joseph and Nigam write in the final sentence of their abstract “Predicting regional climate variability/change remains an onerous burden on models.”

World Climate Report could not say it any better!

References

Joseph, R. and S. Nigam, 2006. ENSO evolution and teleconnections in IPCC’s twentieth-century climate simulations: Realistic representation? Journal of Climate, 19, 4360-4377.

Verdon, D. C., and S. W. Franks, 2006. Long-term behaviour of ENSO: Interactions with the PDO over the past 400 years inferred from paleoclimate records. Geophysical Research Letters, 33, L06712, doi:10.1029/2005GL025052.




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