Thiruvananthapuram, Jan 25
Last year's transition from El Nino to La Nina was about the most sudden ever, according to Dr Tony Barnston, Chief Forecaster, International Research Institute (IRI) for Climate and Society at Columbia University.
Rapid flips like this in the past have ended up precipitating a two-year La Nina, such as right after the El Nino episodes of 1972-73 and 1997-98.
The likelihood of this happening with the current La Nina is unknown, Dr Barnston says.
The term La Nina refers to a period of cooler-than-average sea-surface temperatures in the eastern and central equatorial Pacific Ocean that occurs as part of natural climate variability.
La Nina has been found to correlate well with a reasonably good concurrent Indian monsoon, though without any direct cause-effect relationship.
This situation is roughly the opposite of what happens during El Nino events, when waters in above-mentioned Pacific region are warmer-than-normal.
A strong El Nino, as witnessed during 2009, has correspondingly been associated with dry or drought conditions in India, but with honourable exceptions as in 1997.
Both are part of a larger climate cycle known as the El Nino-Southern Oscillation, or ENSO.
“Even if we do have a second year of La Nina developing in northern hemisphere summer 2011, we expect at least a brief return to neutral conditions from May to July of 2011,” he added.
Based on current observations and on predictions from models, the IRI sees at least a 90 per cent chance that La Nina conditions will continue through March 2011.
Because the Pacific is the largest ocean on the planet, any significant changes in average conditions there, such as those that occur during La Nina or El Nino, can have consequences for temperature, rainfall and vegetation in faraway places.
Once developed, La Nina conditions typically persist for 9-12 months, peaking sometime during November, December, or January.
But 2010 was an interesting and lively year for climate scientists, the IRI recalls.
For the first four months of this year, El Nino conditions prevailed in the tropical Pacific, but that quickly changed, and by June, a La Nina pattern had emerged.
Since 1950, the world experienced six major La Niña events, which were linked to widespread flooding in some areas.
What La Niña does is increase the likelihood that certain areas will get above-normal or below-normal rainfall. Hence, it can be associated with droughts as well.
It keeps east Africa drier-than-usual, sparking food-security concerns in areas lacking irrigation, including parts of Somalia, Kenya, Ethiopia and Tanzania.
Areas in southeastern South America, central southwest Asia, and the southern US may also see lower-than-normal rainfall for the first quarter of 2011.
But La Nina probably isn't to blame for the recent flooding in southeastern Brazil, says Dr Barnston.
The more likely culprit there was a pocket of above-average sea-surface temperatures in the southwest Atlantic that promoted low atmospheric pressure and an increased tendency for heavy rainfall, he says.