The temperature of the eastern tropical Pacific fluctuates between relative warm (El Niño) and relatively cold (La Niña) every 2 to 8 years, based on well-known instrumental records from the recent past. This is coupled with shifts in air surface pressure in the western Pacific – the Southern Oscillation. The patterns have a strong impact on the climate in many regions of the world, including the American Southwest: Warmer sea surface waters lead to relatively wetter winters there, while the cooler La Niña leads to drier ones.
Until now, scientists have not had a detailed enough record to see the longer term fluctuations in the El Niño Southern Oscillation, or ENSO, spanning the past millennium. But tree ring samples taken from the U.S. Southwest provided a year-to-year account dating back 1,100 years. In those narrow cores, wider rings reflect wetter seasons and thinner rings the drier years.
The tree ring analysis corresponded several other records, including instrument readings for Pacific sea surface temperatures, isotope analysis of modern and relic corals from around the Pacific, and other climate reconstructions.
|Porites corals, such as these from Ofu Island, contributed to the Pacific coral isotope record of El Niño. Image courtesy of Peter Craig, NPS
The record reveals an intriguing pattern: The variance between El Niño and La Niña becomes more pronounced in periods when the background temperature is warmer, and less so in cooler periods. The researchers found this cycle runs 50-90 years.
This connection between the overall temperature trend and the amplitude of the ENSO cycle could be interactive, with one enhancing the other. And that supports the idea that the continued warming of the climate may lead to enhanced ENSO variability and more extreme climate conditions around the globe.
Li earned his PhD working in the Tree Ring Lab at Lamont-Doherty Earth Observatory and is now a postdoctoral fellow at the International Pacific Research Center
of the University of Hawaii at Manoa. Lamont scientists Edward R. Cook and Rosanne D’Arrigo are co-authors of the paper.