Falling surface temperatures in the tropical Pacific Ocean are leading climate scientists to predict that La Ni帽a is set to replace El Ni帽o. But the rapid switch has highlighted the difficulty of modelling such changes in climate.
Both La Ni帽a and El Ni帽o affect weather conditions across the globe. The mild El Ni帽o seen in 2002 caused, for example, severe droughts and wildfires in Australia. La Ni帽a will bring more rain here, and in Indonesia and South-East Asia. But it will also result in more active hurricanes in the Atlantic.
During a La Ni帽a event, the surface water in the Pacific is one to three degrees colder than average. 鈥淲e first spotted signs in March to April 鈥� now it is clear we are definitely heading for a La Ni帽a,鈥� says Vernon Kousky, a meteorologist at the US National Weather Service鈥檚 Climate Prediction Center (CPC) in Maryland.
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The model at the CPC now predicts a La Ni帽a with a 70 per cent likelihood. But this was only one of 15 models that were assessed by the International Research Institute for Climate Prediction (IRICP) when they issued a combined forecast on Friday.
Of these, only three predicted that La Ni帽a conditions would prevail at the end of the summer. One predicted that El Ni帽o would continue and the remaining 11 suggested that ocean temperatures would be normal.
Transition state
Despite the majority of models predicting normal ocean temperatures, Kousky is certain La Ni帽a is on the way because 鈥渢he prediction techniques we have available do not deal with transitions very well鈥�. About 70 per cent of El Ni帽os will evolve into a La Ni帽a, he estimates.
The statistical comparison of current ocean conditions with historical records is the basis of one type of predictive model, and the IRICP prediction gave more weight to these.
The second class are dynamic models, which simulate the global ocean-atmosphere interaction to make predictions. But these struggle with the flip from El Ni帽o to La Ni帽a because it happens within a month, while they need three or four months of input data. 鈥淲e see a transition before the model picks it up鈥� explains Kousky.
The World Meteorological Organisation also issues El Ni帽o forecasts, to which Mike Davey, a scientist at the Meteorological Office in Bracknell, UK, contributes. The process, he explains, involves contacting all the centres to get both objective and subjective forecast information. Then a draft text is drawn up to try to reach a consensus.
鈥淭his will then be circulated and discussed at all the centres 鈥� it is very personal at that stage,鈥� he says.
