The 2009–10 El Niño commenced in May 2009, reaching its peak in late December 2009 before breaking down in the first quarter of 2010. The Pacific Ocean returned to neutral by late April 2010, but continued to cool rapidly during autumn.
As early as April 2010, a number of climate models from meteorological agencies around the world suggested a La Niña event could commence later in 2010. Subsequently, the first observed signals of a potential La Niña became apparent in the tropical Pacific Ocean during the following month.
As sea surface temperatures approached values (or 'thresholds') associated with a La Niña in July 2010, and as long-range outlooks became more consistent, the Bureau of Meteorology announced that a La Niña event was more likely than not to persist for the rest of the year.
By October 2010, and with the event showing parallels to the La Niña events of the early 1970s, seasonal outlooks were increasingly suggesting wet conditions for northern and eastern Australia. As a result, the Bureau began briefing key federal and state government agencies of increased flood and tropical cyclone risk, and decreased bushfire potential, over the summer period.
The La Niña strengthened further during spring and into summer, peaking around January 2011, before weakening during autumn 2011. The 2010–11 La Niña drew to a close in May 2011, with both Pacific Ocean and atmospheric indicators returning to neutral levels by mid-year.
Monthly sea surface temperature anomalies (differences from normal) in the Pacific Ocean indicate where the ocean is warmer than usual (red) and cooler than usual (blue).
December 2009 – peak of the 2009–10 El Niño; warmer than normal sea surface temperatures in the central and eastern Pacific
May 2010 – between the end of El Niño and the start of the 2010–11 La Niña; relatively neutral state of the ocean
January 2011 – peak of the 2010–11 La Niña; cooler than normal sea surface temperatures in the central and eastern Pacific