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The current La Nina is associated with record atmospheric pressure conditions and has ocean temperatures in line with the significant 1988/89 event, the Bureau says.
In its latest assessment issued on Wednesday 5 January 2011, the Bureau states that a major La Nina event continues to affect the Pacific Basin and that long-range forecast models suggest that the current La Nina is likely to persist into the southern hemisphere autumn.
Measurements of the difference in atmospheric pressure between Tahiti and Darwin in Australia, the so called Southern Oscillation Index (SOI), are at record levels with a value for December 2010 of +27. This is the highest December SOI value ever recorded and the highest value for any single month since November 1973. Positive values of the SOI are associated with strong La Nina events.
Also, ocean cooling is comparable with the most significant recent La Nina event in 1988/89. The tropical Pacific Ocean remains much cooler than average for this time of year, with temperatures below the surface up to 4C below normal in central and eastern parts which is comparable to the La Nina event of 1988 – the most significant recent La Nina event.
“All climate indicators of ENSO remain beyond La Nina thresholds,” the Australian BOM statement says.
The La Nina cooling events alternate with the better known El Nino warming events and the cycle is known as the ENSO. During La Nina waters in the central and eastern tropical Pacific Ocean become cooler while Pacific trade winds grow stronger and warmer sea temperatures develop to the north of Australia. Together these give an increased probability that eastern and northern Australia will be wetter than normal – which is what is currently happening. The current event has contributed to 2010 being Australia's the third wettest year on record, and Queensland having its wettest December on record.
Models predict La Nina event will persist through southern hemisphere summer. Although most dynamical models surveyed by the Australian BOM indicate a gradual rise in Pacific Ocean temperatures during the coming months, they also show that sea surface temperatures are likely to remain at levels typical of a La Niña event during the first quarter of 2011.
The models surveyed, with one exception (NCEP), predict the central Pacific will warm during the first quarter of 2011, with the NASA model predicting a faster decay of La Niña conditions relative to the other models.
The Bureau's own model, called POAMA, suggests that central Pacific Ocean temperatures will start to warm in coming months. All thirty POAMA forecasts favour a gradual weakening of La Niña conditions. However, despite a gradual weakening, POAMA predicts that the central Pacific Ocean will remain at temperatures typical of a La Niña event until at least the end of the southern hemisphere summer. A gradual weakening of the event, perhaps accelerating during the southern autumn, would be consistent with previous La Niña episodes, reports the Australian BOM.
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