The La Niña weather pattern could be triggering flu pandemics such as 2009's swine flu, scientists suggest.
Worldwide pandemics caused widespread death and illness in 1918, 1957, 1968 and 2009 - all years preceded by La Niña conditions in the equatorial Pacific.
The reason, say Jeffrey Shaman of Columbia University and Marc Lipsitch of the Harvard School of Public Health that the La Niña pattern is known to alter the migratory patterns of birds - believed to be a primary reservoir of human influenza.
They suggest that alterations in migration patterns promote the development of dangerous new strains of influenza.
"We know that pandemics arise from dramatic changes in the influenza genome," says Shaman.
"Our hypothesis is that La Niña sets the stage for these changes by reshuffling the mixing patterns of migratory birds, which are a major reservoir for influenza."
The researchers studied ocean temperatures records for the equatorial Pacific covering the fall and winter before the four most recent flu pandemics emerged.
They found that all four were preceded by the below-normal sea surface temperatures that are associated with the La Niña phase of the El Niño-Southern Oscillation.
Research shows that the La Niña pattern alters the migration, stopover time, fitness and interspecies mixing of migratory birds. These conditions could favor the kind of gene swapping that creates new variations of the influenza virus, say the scientists.
More worryingly, changes in migration don't just alter the pattern of contact among bird species. They can also change the ways that birds come into contact with domestic animals such as pigs - and it was gene-swapping between avian and pig flu viruses that's believed to have triggered the 2009 swine flu pandemic.