La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño-Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3–5 °C.
During La Niña winters...the polar jet stream is strong and the subtropical jet stream weaker in the vicinity of North America. Stronger than normal high pressure in the central Pacific deflects the jet stream further north, thus concentrating temperature contrast and accelerating jet stream winds.
During La Niña, stronger trade winds increase the upwelling of cold water along the South American coast and then transport the cold water westward along the equator, where trade winds from both hemispheres converge. In the oceans, water temperatures generally decrease with increasing depth much in the same way that temperatures decrease with height in the lower atmosphere.
May and June are typically the wettest months in Texas, so La Niña’s exit may provide more opportunities for the state to recover from the drought. “After such a dry April, it will be interesting to see if May 2012 will live up to being a wet and stormy month or will the recent trend of dry weather continue,” Bob Rose, a meteorologist for the Lower Colorado River Authority, wrote this week on the LCRA’s website. “Today’s long-range forecast data seems to suggest there will be some opportunities for rain over the next couple of weeks and the dry trend of April may not necessarily continue this month or next.”
The La Niña weather pattern, one of the big factors behind the Texas drought, has finally left, according to a report out this week from the National Weather Service’s Climate Prediction Center.
But will she come back? It’s a vital question, because the last two years we’ve seen back-to-back La Niñas. That resulted in the worst single-year drought on record in Texas.
La Ninas often feature a strong northern jet stream in the U.S., which drives cold, stormy weather into the northern tier while dry, warm conditions bubble up in the South Central and Southwest. 2011’s La Nina pattern was tied to the historic drought conditions in Texas and flooding from the northern Plains through the Mississippi Valley. Not to mention, the pattern promoted highly volatile conditions at the intersection of the cold/wet and warm/dry extremes, leading to the historic tornado season from the Midwest to Southeast.
The La Nina pattern, linked to one the most extreme weather years on record in 2011 in the U.S., is forecast to fade away this spring according to the National Oceanic and Atmospheric Administration (NOAA). La Nina is characterized by cooler than average waters in the equatorial Pacific, and NOAA finds the seas are warming in the eastern Pacific.
A repeat of La Niña ocean conditions from one year to the next is not uncommon: repeating La Niñas occurred most recently in 1973-74-75, 1998-99-2000 and in 2007-08-09. Repeating La Niñas most often follow an El Niño episode and are essentially the opposite of El Niño conditions. During a La Niña episode, trade winds are stronger than normal, and the cold water that normally exists along the coast of South America extends to the central equatorial Pacific.
La Niña episodes change global weather patterns and are associated with less moisture in the air over cooler ocean waters. This results in less rain along the coasts of North and South America and along the equator, and more rain in the far Western Pacific.
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. What pandemics have in common is that they all feature novel strains of the virus to which people have not developed immunity.
US-based scientists found that the last four pandemics all occurred after La Niña events, which bring cool waters to the surface of the eastern Pacific. In Proceedings of the National Academy of Sciences (PNAS), they say that flu-carrying birds may change migratory patterns during La Nina conditions.