Scientists are getting closer to learning what makes El Niño squall. Over the last ten thousand years, the ocean-atmosphere phenomenon known as the El Niño-Southern Oscillation (ENSO) has varied in its intensity, and therefore its influence on climate. Now, a new study examining evidence of this long-term variability at El Niño’s source — the eastern equatorial Pacific Ocean — suggests the strength of the phenomenon is highly sensitive to even small changes in climate. That sensitivity, the authors suggest, could have implications for how it plays into future climate change.
El Niño, or more precisely, ENSO, is born again every two to seven years or so, when the trade winds that normally push the warm waters of the equatorial Pacific westward weaken, or even reverse direction. The warm water sloshes back to the east, raising the average temperatures of the eastern equatorial Pacific. That change in ocean temperatures also affects the atmosphere, further altering the trade winds and disrupting rainfall patterns and climate worldwide.
Thanks to modern satellite data, scientists now better understand the year-to-year changes that produce ENSO, says Athanasios Koutavas, a paleoclimatologist at the City University of New York. For example, those data reveal a short-term correlation between ENSO-related changes in sea-surface temperatures and the ever-shifting location where the equatorial trade winds meet, called the Intertropical Convergence Zone (ITCZ). During El Niño, the ITCZ is farther south and closer to the equator, while during La Niña, it is farther north and away from the equator, Koutavas says. “We don’t know, however, what the cause and effect relationship is,” he says.
The two may, in fact, be feeding back on each other: Warm seas draw the ITCZ closer to the equator, while the location of the convergence zone determines the total strength of the winds on the equator, which also controls the sea-surface temperatures. Satellite data have only been collected for a few decades, however, and how the two effects have interacted over much longer time scales — such as hundreds, or thousands, of years — is still not well-known, he says.
How sensitive ENSO and the ITCZ are to present-day climate change is not clear, because scientists have no direct measurements for how they are responding. Climate models, however, do show that the ITCZ can shift rapidly in response to changes in climate at high latitudes, such as the amount of sea ice in the Arctic and the circulation of the deep ocean. Such changes are likely to be among the first signals as the planet warms — and if the ITCZ shifts in response to them, changes in ENSO are likely to follow suit, Koutavas says.
Previous work has also suggested that ENSO was highly variable during the Holocene, the epoch that includes modern times and stretches back to about 9500 B.C. Models and oxygen isotopes in corals have suggested that ENSO was weaker during the mid-Holocene, from about 9,000 to 5,000 years ago, but returned to conditions similar to the present day about 4,500 years ago. Much of the earlier research, however, has relied on data from locations that are far from the “center of action,” at ENSO’s source in the tropical Pacific, Koutavas says.
To better understand how ENSO changed over the Holocene, and to better predict how it might change in the future, Koutavas and his team turned to the paleo-climate record. They studied the chemical compositions of foraminifera in both late-Holocene and mid-Holocene sediments from the eastern equatorial Pacific. The team not only assessed the average compositions of the creatures within a section of sediment, to estimate mean climate conditions on the scale of hundreds of years, but also measured the compositions of individual forams, which can act as high-resolution, “near-monthly recorders of sea-surface conditions,” Koutavas says.
The individual foram data confirmed that ENSO was significantly dampened during the mid-Holocene, which is likely related to the ITCZ shifting to the north, the team reported in the December Geology. Although those data appear to confirm the sensitivity of both ENSO and ITCZ to climate change, what that means for current climate remains unclear, he notes. In the mid-Holocene, changes in Earth’s orbit were responsible for altering the climate, by increasing the amount of solar radiation reaching the northern hemisphere in summertime. Those conditions do not exist today, he says, but anthropogenic effects on climate are altering sea ice extent.
“If the models are right and the ITCZ is sensitive” to changes in sea ice, the ITCZ is likely to move northward, Koutavas says. That could reduce the frequency or strength of El Niños in the Pacific, as in the Holocene. Models also suggest that global warming could weaken thermohaline circulation, however, which could push the ITCZ southward. “We are looking at two competing effects,” he says. “We don’t know the final answer but the point is, we have good reason to believe [sea ice and circulation] will change with changing climate. And models tell us that when these two change, they affect the ITCZ, the eastern Pacific and ENSO.”
“It’s an interesting study,” says George Philander, a climate scientist at Princeton University. “We understand El Niño fairly well now, and understanding it in the past can help provide confidence in our theories.”