During the 2010–12 La Niña events, record and near-record high Southern Oscillation Index (SOI) values occurred in many individual months and multi-month periods.
The surface air pressures near Tahiti were consistently high throughout both the 2010–11 and 2011–12 La Niña events, and they were particularly high during the first event. These high SOI values reflect large changes in the typical weather patterns over the South Pacific, which relate to the strong trade winds experienced over the same period and an enhanced Walker Circulation.
The El Niño–Southern Oscillation (ENSO) is described in more detail in The three phases of ENSO.
Monthly SOI values for the 2010–11 and 2011–12 La Niña events – numerous record and near-record values for individual months are indicated
Comparison to past La Niña events
The 2010–11 La Niña was one of the strongest on record, comparable in strength to the events of 1917–18, 1955–56 and 1975–76. Widespread impacts were experienced across a large part of Australia, including record rainfall and severe flooding.
While the intensity of atmospheric indicators during the event was exceptional, tropical Pacific Ocean indicators did not reach record cool levels. This may have been partly due to the general warming trend in the Pacific Ocean, which has warmed around 0.5 °C since 1950.
The 2011–12 La Niña was a weaker event, but still of moderate strength by both atmospheric and oceanic measures. Despite flooding in a number of areas during summer, the impact of this second event upon Australia's climate was generally less significant than during the previous event.
The previous two La Niña episodes were also multi-year events, lasting from September 2007 to March 2009 and from May 1998 to March 2001, respectively.
The 2007–08 and 2008–09 La Niña events were weak to moderate, with relatively minor impacts across Australia. Although the 2007–08 event brought the typical heavy rainfall to most of northern Australia and the eastern tropics, the southern half of the Murray–Darling Basin did not receive the above-average rainfall and cool temperatures typical of past La Niña events. The 2008–09 La Niña was a short event, bringing significantly above-average rainfall across the north of Australia, although most parts of southeastern Australia received below-average rainfall, consistent with persistent drought conditions in place for the region since at least 2000.
The 1998–2001 La Niña persisted for three years with three distinct peaks over the summers of 1998–99, 1999–2000, and 2000–01. The event was generally moderate in strength, with widespread above-average rainfall and flooding, particularly in New South Wales and Queensland. While much of the north received record high falls, parts of the southeast and Tasmania missed out on the heavy rainfall, and hence did not get the relief from the dry conditions that had started around late 1996.