Unusual warming of waters in the central equatorial Pacific has prompted the US government to declare an El Nino event and predict a better-than-even chance that it will linger through the middle of the year.
The US National Oceanic and Atmospheric Administration said the above-average sea-surface temperatures had exceeded key thresholds, triggering the declaration of the "long-anticipated" El Nino.
However, the location of the main warming – about 10 degrees west of the International Dateline rather than to the east - and its timing early in the year are puzzling climate experts looking for similar events.
"Climate scientists are monitoring this with amazement," said Cai Wenju, a principal CSIRO research scientist who has published widely on the El Nino Southern Oscillation (ENSO) climate pattern. "We only understand what we have seen."
El Ninos involve the relative warming of sea-surface temperatures in the eastern equatorial Pacific, compared with western regions. In such events, the typical east-to-west trade winds abate or reverse, and large areas of the western Pacific including eastern Australia receive reduced rainfall.
Currently, the area of most anomalous warmth is located about 7000 kilometres west of the area where El Ninos are typically centred. They also tend to appear in the late autumn to winter period for the southern hemisphere.
"All these are very different from a classic El Nino," Dr Cai said.
While Japanese researchers have identified similar central Pacific warming events - dubbing them El Nino Modoki, or "same but different" in Japanese - the current pattern is about 1500 kilometres further to the west than previous ones, Dr Cai said.
Upgraded reading
Earlier this week, the Bureau of Meteorology upgraded its ENSO Tracker to "watch" level, reflecting the recent warming. "Weakened trade winds are forecast to continue, and this may induce further warming," the bureau said.
Andrew Watkins, the bureau's supervisor of its Climate Prediction Services unit, said Australia's definition differs from the NOAA's so it is yet to declare an El Nino event.
"Even by their definition, it's very weak," Dr Watkins said. "It just scrapes over the line."
For the bureau to declare an El Nino event, greater warmth needs to persist in the monitored areas - 0.8 degrees above average compared with 0.5 degrees - and the atmosphere must begin to "couple" with the changed ocean conditions, reinforcing them.
El Ninos can trigger droughts in Australia and south-east Asia while bringing floods to Peru and Chile in the east. For now, though, the effects are not expected to be serious, the NOAA said
"Due to the expected weak strength, widespread or significant global impacts are not anticipated," the NOAA said in a statement.
Since El Ninos also mean the Pacific slows its uptake of heat from the atmosphere, such events typically nudge global surface temperatures higher.
The main meteorological agencies have declared 2014 to have been the hottest year globally on record, and the extension of at least El Nino-like conditions well into this year may result in 2015 setting new highs.
Dr Cai said this year was beginning to resemble 1988, a warm year that followed a big El Nino event, and so 2015 could be "another warmest year".
The bureau is already predicting a warmer than usual autumn for most of Australia.
