Wildfire risk growing in tropics
Along with being one of the strongest El Niños on record, this year’s edition of the cyclical weather event in the Pacific will be one of the most studied.
NASA, for example, has been tracking the effects of El Niño via satellite data, which shows global impacts, from increasing fire danger in some tropical regions to a reduction of certain types of pollution in other areas.
Some of the findings were presented this week at the American Geophysical Union meeting in San Francisco, where researchers said that atmospheric rivers, significant sources of rainfall, tend to intensify during El Niño events, and that California may see some relief from an extreme multiyear drought.
An El Niño, happens when sea surface temperatures in the equatorial Pacific Ocean warm up. The increased ocean surface temperatures influence air and moisture movement around the globe. Approximately 15 years of observations by NASA’s fleet of Earth-observing satellites show how El Niños affect multiple interconnected Earth systems.
Duane Waliser, chief scientist of the Earth Science and Technology Directorate at NASA’s Jet Propulsion Laboratory in Pasadena, California, and his colleagues analyzed the historical record of atmospheric rivers. These concentrated rain bands account for 40 percent of California’s water supply. Their results suggest the number of atmospheric rivers California receives will remain the same, at an average 10 per year, but they will be stronger, warmer and wetter.
“Overall we’ll likely get more precipitation, but maybe less in terms of snowfall,” Waliser said, adding that they may contribute to more flooding.
One statistical study looked at the relationship between the strength of El Niño and precipitation.
“What we learned is weak El Niños don’t necessarily change the odds of precipitation being much different from normal,” said Martin Hoerling, a research meteorologist with the Earth Systems Research Laboratory at the National Oceanic and Atmospheric Administration in Boulder, Colorado. “The rare occurrence of a strong El Niño, like what we’re currently experiencing, however, greatly increases the odds of a wet California winter,” Hoerling said.
El Niño-driven shifts in global wind patterns also change the distribution of tropospheric ozone. Tropospheric ozone exists in the atmospheric layer closest to the surface and comprises ozone produced naturally and from human pollution.
Previous studies showed that El Niño has a strong effect on ozone distribution in the tropics, and new research suggests that there are also impacts in the mid latitudes.
“El Niño is just one factor in the variability,” said Mark Olsen, an atmospheric research scientist at Morgan State University in Baltimore and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “But you do see regions like the central United States where El Niño explains 20 to 25 percent of the variability.”
Another team of scientists, including Jim Randerson, an earth system scientist, at the University of California, Irvine, said El Niño can drive changes in the number and size of fires increases in tropical forests across Asia and South America.
“The change in atmospheric dynamics shifts the rainfall,” Randerson said. “So El Niño causes less rain to fall in many areas of the tropics, making forests more vulnerable to human-ignited fires.”
Fires in tropical forests also accelerate carbon dioxide buildup in the atmosphere and reduce air quality. Indonesia, for example, has carbon-rich peatlands that ignite as soon as the rain stops, which is what happened this fall, Randerson said. Meanwhile, Southeast Asia, Central America, and the southern Amazon have very high fire risk for 2016. El Niño tends to reduce rainfall in their wet seasons, and less rain means drier vegetation and drier air, which make forests vulnerable to dry season burning.
More on NASA’s El Niño research at this web page.