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The Australian Bureau of Meteorology suspects that ‘unusual conditions’ in tropical Pacific during the past fortnight may increase the chance of El Nino this year.
But it also observed that model outlooks spanning the traditional transition period of February to May generally have lower accuracy than outlooks made at other times of year.
El Nino is a condition in which tropical and adjoining east Pacific warms up relative to the west, taking away storm-building, clouding and rains from Asia to the South America coast.
El Niño watchThis is normally expected to leave an impact on the Asian summer, including the monsoon, though there is no direct cause-effect relationship.
The bureau’s tracker is now locked at an ‘El Nino watch.’
This indicates that there is about a 50 per cent chance of El Nino forming in 2015 – double the normal likelihood of an event. This is due to the warmer-than-average sub-surface temperatures in the tropical Pacific Ocean, the bureau said in an update.
Pacific cyclonesInternational models surveyed by it indicate that central and eastern tropical Pacific will warm to El Nino thresholds by mid-year.
The bureau recalled that in the western Pacific, super cyclone Pam and tropical storm Bavi had straddled the equator, producing one of the strongest reversals in the trade winds in recent years.
El Nino happens when weakening trade winds (east to west, which sometimes even reverse direction) allow the warmer water from the western Pacific to flow toward the east.
A reversal of winds has taken place in the wake of the cyclones.
This flattens out the sea level, builds up warm surface water off the coast of South America, and increases temperatures in east Pacific.
Warm anomalyThis change is expected to increase the already warm sub-surface temperatures currently observed in the tropical Pacific Ocean.
It may in turn raise tropical Pacific Ocean surface temperatures in the coming months.
However, the bureau said that it is too early yet to say whether the reversal in the trade winds is a short term fluctuation or the beginning of a sustained trend.
International models surveyed have strengthened their outlooks for the likelihood of El Nino, it said.
All eight models are suggesting ocean temperatures will exceed El Nino thresholds by mid-year.
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