Southwestern Trees Hold the History—and Future?—of El Niño

The impact of tropical Pacific ocean temperatures on the hydroclimate of the Southwest is profound: on timescales of 2-8 years, El Niño and La Niña events influence the amount of winter precipitation we receive. El Niños—defined by warmer-than-average ocean temperatures in the eastern tropical Pacific—bring more rain, and La Niñas—defined by cooler-than-average ocean temperatures in the eastern tropical Pacific—bring drought. Trees living across the Southwest region record these changes in moisture in the width of their rings, providing us with a history of precipitation going back thousands of years or longer in some places.

By counting and measuring tree-rings, scientists can reconstruct the history of precipitation and drought in the Southwest.

In a recent publication in Nature Climate Change (if you can’t access the article read more on ScienceDaily), a group of scientists lead by Jinbao Li have capitalized on the strong relationship between the tropical Pacific and southwestern precipitation to create a 1,100-year record of the history of the El Niño/Southern Oscillation (ENSO). The main idea is that the history of moisture recorded in the tree-ring-based North American Drought Atlas is also a record of El Niño and La Niña events. Going a step further, the authors converted their time series to a variance time series that shows the strength of El Niños and La Niñas, and they found that the strength of these events varies through time. I was impressed by this reconstruction for two main reasons: 1) During the period of instrumental observations (1880 AD-present), their tree-ring-based time series of ENSO variance looks very much like the real time series of ENSO variance from ocean temperature measurements. 2) Back through the last millennium, their tree-ring record of ENSO matches up well with coral records of ENSO from fossil corals from the tropical Pacific.

This is all pretty cool, but what can we do with this new information on the history of ENSO? The authors’ main point is that during past time periods when the mean state of the eastern tropical Pacific was warmer, ENSO variance was stronger—meaning stronger El Niños and La Niñas. Thus, since we expect the eastern tropical Pacific to warm in coming decades due to increasing greenhouse gas concentrations in the atmosphere, ENSO variance could also increase. This is critical information, since ENSO has such big climatic impacts around the world--it would be good to predict future ENSO behavior so societies can adapt accordingly. Should we expect stronger droughts, more floods and mudslides? What can we do to prepare for an increase in the frequency of these events? The results of this paper fill an important knowledge gap since climate models can’t yet give us a good idea if ENSO will change in the future: some project no change, others project an increase in variance, and still others project a decrease in variance.

However, it’s also clear that other factors can affect wintertime precipitation in the Southwest, like North Atlantic and North Pacific ocean temperatures. That’s why ideally, reconstructions of ENSO should come from the tropical Pacific, where the phenomenon takes place, so we don’t have to worry about these confounding factors. Nevertheless, this new record of ENSO variance looks pretty good—it suggests a relatively stable relationship between tropical Pacific ocean temperatures and Southwest precipitation over the last millennium—and is moving our understanding of ENSO behavior forward.