El Niño is the negative phase of the El Niño-Southern Oscillation. El Niño events are associated with the appearance of a warm ocean current off the South American coast and sustained negative Southern Oscillation Index (SOI) values.

El Niño refers to the extensive warming of the central and eastern tropical Pacific that leads to a major shift in weather patterns across the Pacific. El Niño events are often accompanied by cooler than normal sea surface temperatures (SSTs) in the western Pacific, and to the north of Australia. Over much of Australia, but particularly eastern Australia, El Niño events are associated with an increased probability of drier conditions.

Read more about historical El Niño events and their effect on Australia in the detailed analysis of past El Niño events, or about typical El Niño rainfall patterns here. The ENSO Wrap-Up discusses current conditions and indicators of El Niño and La Niña events.

Changes to the atmosphere and ocean circulation during El Niño events include:

• Sustained warmer-than-usual SSTs across the central and eastern tropical Pacific Ocean.
• Decreased convection or cloudiness over tropical Australia, Papua New-Guinea, and Indonesia - the focus of convection migrates from the Australian/Indonesian region eastward towards the central tropical Pacific Ocean.
• A decrease in strength of the trade winds (easterlies) across the tropical Pacific Ocean (but not necessarily in the Australian region).
• Sustained negative values of the SOI, typically below −8.

The map below shows a typical El Niño pattern of sea surface temperature anomalies in the tropical Pacific. This image is for the week ending 26 November 2006.