El Nino More Active In 20th Century Than In Previous 700 Years
July 1, 2013

El Nino More Active In 20th Century Than In Previous 700 Years

Lawrence LeBlond for redOrbit.com - Your Universe Online

The El Nino Southern Oscillation (ENSO) is a tropical Pacific Ocean phenomenon that has important consequences for weather around the globe. During years with ENSOs, increased rainfall is seen across the southern US, which has the potential for causing destructive floods. Meanwhile, drought conditions embrace the Western Pacific, increasing the chance of devastating fires throughout Australia. It is also during El Nino years that hurricane activity in the Atlantic wanes.

Because of the dramatic consequences ENSO has on global weather patterns, forecasting it accurately matters greatly. However, providing accurate predictions is challenging due to the fact the ENSO varies naturally over decades and centuries. There are currently not enough instrumental records to accurately calculate if changes seen in recent weather patterns are directly attributed to natural or man made greenhouse gases. As well, reconstructions of ENSO behavior are usually missing adequate records for areas where ENSO develops.

But to now help scientists gain a better understanding of past ENSO patterns, a unique tree-ring record has been utilized. This record has been shown to reflect ENSO activity over the past several centuries and has been shown to be very good proxies in measuring temperature and rainfall patterns.

The project has been spearheaded by Jinbao Li and Shang-Ping Xie, two international scientists working at the International Pacific Research Center, University of Hawaii at Manoa. Li and Xie have compiled more than 2,200 tree-ring chronologies representing 700 years of data from the tropics and mid-latitudes in both the Northern and Southern hemispheres.

The study has been published in the June 30, 2013 online edition of Nature Climate Change.

By studying tropical tree-ring records, the team was able to generate an archive of ENSO activity with unprecedented accuracy. The tree-ring data was closely associated with other data from equatorial Pacific corals and temperature reconstructions of the Northern Hemisphere, meant to capture well-known teleconnection climate patterns.

By combining the data, Li and Xie found ENSO was unusually active in the late 20th century compared to all other times over the past 700 years, implying that the Pacific Ocean phenomenon is responding to the ongoing increase in global warming.

"In the year after a large tropical volcanic eruption, our record shows that the east-central tropical Pacific is unusually cool, followed by unusual warming one year later. Like greenhouse gases, volcanic aerosols perturb the Earth's radiation balance. This supports the idea that the unusually high ENSO activity in the late 20th century is a footprint of global warming" explained Li, lead author of the study.

"Many climate models do not reflect the strong ENSO response to global warming that we found," added coauthor Xie, who is also a Roger Revelle Professor at Scripps Institution of Oceanography, University of California at San Diego.

"This suggests that many models underestimate the sensitivity to radiative perturbations in greenhouse gases. Our results now provide a guide to improve the accuracy of climate models and their projections of future ENSO activity. If this trend of increasing ENSO activity continues, we expect to see more weather extremes such as floods and droughts."

A separate team of scientists from the University of Hawaii at Manoa last month reported in the journal Nature Geoscience that they discovered why the El Nino pattern falls into a particular cycle, peaking in late December and waning between February and April.

"An unusual wind pattern that straddles the equatorial Pacific during strong El Nino events and swings back and forth with a period of 15 months explains El Nino's close ties to the annual cycle," according to the researchers.

By studying the wind pattern's evolution, the scientists were able to discover a 15-month rhythm accompanying strong El Nino events, meaning it is "considerably faster than the three- to five-year timetable for El Nino events, but slower than the annual cycle."

That team believes using an improved representation of this 15-month tropical Pacific wind pattern in existing climate models will lead to improved El Nino forecasts. In addition, they say that recent climate model projections suggest future El Nino events will be accompanied by this combination tone wind pattern on a more frequent basis -- and that this discovery will likely alter the characteristics of future rainfall patterns caused by the weather event.