Falling surface temperatures in the tropical Pacific Ocean are leading climate scientists to predict that La Ni単a is set to replace El Ni単o. But the rapid switch has highlighted the difficulty of modelling such changes in climate.
Both La Ni単a and El Ni単o affect weather conditions across the globe. The mild El Ni単o seen in 2002 caused, for example, severe droughts and wildfires in Australia. La Ni単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単a event, the surface water in the Pacific is one to three degrees colder than average. “We first spotted signs in March to April now it is clear we are definitely heading for a La Ni単a,” says Vernon Kousky, a meteorologist at the US National Weather Service’s Climate Prediction Center (CPC) in Maryland.
The model at the CPC now predicts a La Ni単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.
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Of these, only three predicted that La Ni単a conditions would prevail at the end of the summer. One predicted that El Ni単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単a is on the way because “the prediction techniques we have available do not deal with transitions very well”. About 70 per cent of El Ni単os will evolve into a La Ni単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単o to La Ni単a because it happens within a month, while they need three or four months of input data. “We see a transition before the model picks it up” explains Kousky.
The World Meteorological Organisation also issues El Ni単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.
“This will then be circulated and discussed at all the centres it is very personal at that stage,” he says.
