Certain disease outbreaks, including some of the worst pandemics of the 20th century, are linked to weather patterns in the Pacific Ocean, according to new research. Scientists said tracking these climate changes can help officials anticipate and plan for surges in illnesses.
The El Niño-Southern Oscillation (ENSO) cycle causes ripples through the global climate, changing rainfall and air currents. These shifts, in turn, can cause disease carriers to interact in new ways, creating novel pathogens. Weather changes can also increase the number of people exposed to a disease, increasing the likelihood of an outbreak.
El Niño is the warm phase of the ENSO, characterized by unusually high sea surface temperatures along the equator in the Pacific, lasting between nine months and two years, according to the National Oceanic and Atmospheric Administration.
The temperature changes seem small -- usually 0.5 to 1 degree Celsius -- but they can alter rainfall patterns all over the world. "It's a shift in the large-scale dynamics of the tropical atmosphere," said Jon Gottschalck, head of forecast operations at NOAA's Climate Prediction Center. "It's not regular, and it's not predictable. They call it an oscillation, but it's an aperiodic oscillation."
After three to five years, the ENSO usually cycles to La Niña, when sea surface temperatures reach a low point. Gottschalck said this leads to more rainfall over Indonesia while weakening jet streams -- fast-moving, high-altitude air currents -- and pushing them further north. This leads to drier conditions in the southern United States.
Currently, the planet is in the middle of a La Niña phase, and parts of the American South, like Texas, experienced record drought last year.
Shifts in bird migrations create flu strains
These regular precipitation patterns change how migratory birds interact. The birds are vectors for the influenza virus. As their populations split off and reunite every few years due to ENSO, new flu strains emerge. "Flu is interesting because it mutates all the time," explained Jeffrey Shaman, an assistant professor in the Environmental Health Sciences department at the Mailman School of Public Health at Columbia University. The illness is transmitted through feces, fluids and aerosols.
As the flu virus reproduces in its host, it changes over time. When carriers are separated, the virus mutates differently in the two groups. Once La Niña comes around, disparate bird populations start to mingle again, increasing the likelihood that a carrier is infected with more than one variety of the virus.
These two versions can then form hybrid viruses in a process known as reassortment. "This reassortment happens more often than there are pandemics. In order for there to be a pandemic, there needs to be a radical change," said Shaman.