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.

Heating up: 2015 could be another very warm year, climate scientists believe. Credit:Reuters

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."