La Niña is the positive phase of the El Niño-Southern Oscillation, sometimes thought of as the "opposite of El Niño". A La Niña event is indicated by sustained positive Southern Oscillation Index (SOI) values.
La Niña refers to the extensive cooling of the central and eastern tropical Pacific Ocean, often accompanied by warmer than normal sea surface temperatures (SSTs) in the western Pacific, and to the north of Australia. La Niña events are associated with increased probability of wetter conditions over much of Australia, particularly over eastern and northern areas. La Niña events have been correlated with higher numbers of tropical cyclones during the cyclone season (November to April).
Read more about historical La Niña events and their effect on Australia in the detailed analysis of past La Niña events, or about typical La Niña 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 La Niña events include:
- Sustained cooler-than-usual SSTs across the central and eastern tropical Pacific Ocean.
- Increased convection or cloudiness over tropical Australia, Papua New-Guinea, and Indonesia.
- An increase in strength of the trade winds (easterlies) across the tropical Pacific Ocean (but not necessarily in the Australian region).
- Sustained positive values of the SOI, typically above +8.
The map below shows a typical pattern of sea surface temperature anomalies in the tropical Pacific during a La Niña event. This image is for the week ending 16 January 2011.