October 29, 2013
Recent El Nino Patterns More Active Than In Past 600 Years
April Flowers for redOrbit.com - Your Universe Online
Scientists from the University of New South Wales, the University of Hawaii International Pacific Research Center and the NOAA Geophysical Fluid Dynamics Laboratory have developed a new approach to analyzing paleo-climate reconstructions of the El Niño Southern Oscillation (ENSO) phenomenon. Their findings, published in Climate of the Past, resolve disagreements and reveals that ENSO activity during the 20th century has been unusually high compared to the past 600 years.
El Niño can spawn floods or cause droughts in many regions of the world, wreaking havoc across the globe. While scientists are still uncertain how a warming planet influences ENSO behavior, one way to determine the sensitivity of ENSO to climate change has been to look into the past. The instrumental record is too short to get a reliable picture of natural variations in ENSO magnitude and frequency, so climate scientists rely on geological and biological clues - lake sediment cores, corals, or tree rings as proxies for past ENSO behavior, for example. Reconstructions of ENSO from such paleo-proxies, however, have not been telling the same story.
The research team says that some of these discrepancies arise because the methods typically applied during ENSO reconstructions to combine individual paleo-proxy records do not handle small dating uncertainties among the proxies well. The most used approach combines the individual ENSO proxies and then calculates the activity of this combined ENSO signal. By turning this analysis around, by first calculating the activity of ENSO in each of the individual paleo-climate reconstructions and then combining the activity time series, Shayne McGregor, post-doctoral fellow at the University of New South Wales, and his team have gotten a much more consistent and robust view of ENSO's past activity. The team used virtual ENSO data obtained from two multi-century-long climate model simulations to confirm their new approach.
The scientists found that ENSO was more active during 1979-2009 than during any 30-year period between 1590 and 1880 when applying this improved method of reconstructing ENSO activity by synthesizing many different existing proxies and comparing these time series with instrumental data.
"Our results represent a significant step towards understanding where current ENSO activity sits in the context of the past." says Axel Timmermann, professor at the University of Hawaii and co-author of the study.
"Climate models provide no clear indication of how ENSO activity will change in the future in response to greenhouse warming, so all we have to go on is past records," explains McGregor. "We can improve the projections of climate models, however, by selecting those that produce past changes in ENSO activity consistent with the past instrumental records."
"Our new estimates of ENSO activity of the past 600 years appear to roughly track global mean temperature," says McGregor, "but we still don't know why."