ReliefWeb

What is El Niño?

Peruvian fishermen identified El Niño centuries ago when every three to seven years, during the months of December and January, fish in their coastal waters virtually vanished. During normal conditions, surface temperatures are warm in the western Pacific Ocean and trade winds blow towards the west. During El Niño, warm water appears in the central and eastern Pacific Ocean and trade winds weaken, or even reverse. Once sea surface temperatures in the equatorial Pacific Ocean rise 0.5°C above their historical average for three months in a row, and when atmospheric conditions shift accordingly, scientists typically declare an El Niño event. During El Niño, the South Pacific Convergence Zone (SPCZ), a band of low-level convergence, cloudiness and precipitation, moves further away from the Pacific Island countries located west of the dateline. Rainfall patterns tend to follow this warmer water eastwards, reducing the amount of rainfall in the western Pacific and across large parts of South and South East Asia. This also leads to tropical cyclones forming over a more expansive area of ocean which increases their intensity before they reach populated western Pacific Islands and countries in East Asia.

Current conditions and forecast

Despite the tropical Pacific Ocean being primed for an El Niño during much of the first half of 2014, the atmosphere above has largely failed to respond, and hence the ocean and atmosphere have not reinforced each other. As a result, some cooling has now taken place in the central and eastern tropical Pacific Ocean, with most of the key NINO regions returning to neutral values. Over July 2014, model forecasts have slightly delayed the El Niño onset. At this time, the consensus of forecasters expect El Niño to emerge during August-October 2014 and to peak at weak strength during the first half of 2015. A strong El Niño is not favored in any of the scenarios, and slightly more models call for a weak event rather than a moderate event. While the chance of an El Niño in 2014 has clearly eased, warmer-than-average waters persist in parts of the tropical Pacific and therefore the establishment of El Niño before the year’s end cannot be ruled out.