Solving the mystery of weak El Niños
A new study says El Niño events may have recently diminished due to a chain of climate trends starting with the accelerated warming of subtropical waters in the North Atlantic and continuing with stronger southerly winds in the tropical Pacific. A likely reversal of these trends in the future will mean much stronger El Niño activity, the researchers said.
An El Niño event is the warm phase of the El Niño Southern Oscillation, characterized by unusually warm water in the Pacific Ocean. The 1980s and 1990s saw strong El Niño activity, when the two strongest El Niño events in the observational record occurred in 1982 and 1997. These events triggered vast climate impacts, causing billions of dollars in damages worldwide.
Although scientists had predicted more extreme El Niño events, or the so-called super El Niños, in response to global warming, the next 20 years brought relatively weak El Niño activity. Even the strong El Niño of 2015 did not reach the impacts of the 1982 and 1997 events.
Yale professor Alexey Fedorov and former Yale Ph.D. student Shineng Hu, now with the Scripps Institution of Oceanography at the University of California-San Diego, may have found an answer.
Their data analysis and modeling suggest that the strengthening of cross-equatorial (north-south) winds in the tropical Pacific over the past 30 years is most likely to blame for weak El Niño events. “If we want to predict how El Niño will change in the future, we need to predict how cross-equatorial winds will change,” said Fedorov. “This remains a difficult problem since a vast majority of climate models have cross-equatorial winds much weaker than what is being observed.”
In addition, the researchers found that the stronger winds in the tropical Pacific are the result of the anomalous warming of waters in the subtropical North Atlantic over this period. “Other mechanisms do exist, as well, such as the Atlantic Meridional Overturning Circulation and the asymmetrical distribution of aerosol emissions in two hemispheres,” Hu said. “Our study potentially has broad implications for past and future climate changes.”
Grants from the National Science Foundation and NASA supported the study. The researchers also acknowledged the NASA Earth and Space Science Graduate Fellowship, the Scripps Postdoctoral Fellowship, and computational support from the Yale University Faculty of Arts and Sciences High Performance Computing facility and the NSF/NCAR Yellowstone Supercomputing Center.