Weather experts are not sure exactly what causes La Nina, but there are many theories.
However, many other La Nina events have not seen novel flu strains spread around the world, they caution.
La Nina is the cold cousin of El Nino - the two collectively making up the El Nino Southern Oscillation (ENSO).
Even with the aid of these devices, NOAA still has not been able to calculate the exact behavior of La Nina.
In Proceedings of the National Academy of Sciences (PNAS), they say that flu-carrying birds may change migratory patterns during La Nina conditions.
Yet the fact that many other La Nina periods have not been followed by a pandemic indicate that other factors must also be involved.
Phenomena like La Nina have always been around, but large areas of extreme warming have only come about with climate change, the study said.
La Nina may be the underlying cause of the drought in the South, but rising temperatures from global warming make that drought more severe.
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This has been caused by cold La Nina temperatures in the Eastern Tropical Pacific that pushed the jet stream northward, shifting moisture and rainfall eastward.
US-based scientists found that the last four pandemics all occurred after La Nina events, which bring cool waters to the surface of the eastern Pacific.
Temperatures have been essentially flat since 1998, with the exception of another brief 2010 El Nino spike immediately followed by a dramatic 2011 La Nina decline.
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Both teams said that temperatures would have been higher if it had not been for the La Nina weather pattern that brought cooling to some regions.
First, the Pacific is going through a cooler period, called La Nina (with this one being particularly strong), and the Atlantic is going through a warmer period.
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But recent research has shown that some wild birds' patterns of flights and stopovers during migrations, or moulting times, differ between El Nino and La Nina years.
The pattern is common when La Nina -- the cooler than normal waters in the Eastern Equatorial Pacific -- influences upper air patterns across the southern United States.
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The La Nina pattern, which generally cools the ocean globally, happens every few years and was in play for much of 2011 and the early part of 2012.
Temperatures in both previous years were dragged down by the Pacific La Nina effect which makes it four times more likely that we will suffer a bad summer in Europe.
Most of the country will experience hotter than normal temperatures this summer because of "La Nina, " which will remain in force through August, according to the National Oceanic and Atmospheric Administration.
Yet both years were the hottest La Nina years on record and that shows the underlying global warming at work, said climate scientist Stefan Rahmstorf of the Max Planck Institute in Germany.
Davies and Molloy are therefore properly cautious with their conclusions, but nonetheless note that a comparison of the beginning and endpoints for their study, which minimizes the La Nina contribution, still showed a decline in cloud height.
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The best hope lies in trying to identify the broader changes under way - for example with the retreat of the Arctic sea-ice or the cycle of warming in the Pacific Ocean with El Nino and La Nina.
"All of the computer weather models agree that most of the U.S. will be warmer than usual, but at least we can see the end coming for La Nina, " said Ants Leetmaa, director of NOAA's Climate Prediction Center.
Nevertheless, the last four pandemics - the Spanish Flu that began in 1918, the Asian Flu of 1957, the Hong Kong Flu of 1958 and the swine flu of 2009 - were all preceded by periods of La Nina conditions.
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