Study authors Jeffrey Shaman of Columbia University's Mailman School of Public Health and Marc Lipsitch of the Harvard School of Public Health said worldwide pandemics of influenza that caused widespread death and illness in 1918, 1957, 1968 and 2009, were preceded by La Nina conditions in the Pacific.
The researchers studied records of ocean temperatures in the equatorial Pacific in the fall and winter before the four most recent flu pandemics emerged and found all four pandemics were preceded by below-normal sea surface temperatures -- consistent with the La Nina phase of the El Nino-Southern Oscillation.
Other research has shown the La Nina pattern alters the migration, stopover time, fitness and interspecies mixing of migratory birds -- and these conditions could favor a kind of gene swapping that creates novel, and therefore potentially more variations, of the influenza virus.
"We know that pandemics arise from dramatic changes in the influenza genome," Shaman said in a statement. "Our hypothesis is that La Nina sets the stage for these changes by reshuffling the mixing patterns of migratory birds, which are a major reservoir for influenza."
During La Nina the sea surface temperature across the equatorial Eastern Central Pacific Ocean drops several degrees.
The findings were published online in Proceedings of the National Academy of Sciences.