Major climate drivers such as the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) have a well-established influence on mean rainfall and temperature in many parts of Australia. This prompts the question of whether there is evidence of differential influence on different parts of the frequency distribution.
Whilst, in many parts of eastern Australia, the ENSO influence on extreme temperatures manifests as an overall shift of the frequency distribution, more complex relationships exist in parts of southern Australia, particularly Victoria and southern South Australia. In this region, on the one hand, El Niño is associated with an increased frequency of individual extreme hot days from late winter to mid-summer, with an especially strong signal for the most extreme extremes in spring. In contrast, El Niño is associated with a reduced frequency of prolonged heatwaves; for example, all of the 10 summers in which Melbourne has had five or more consecutive days above 33 °C have occurred following springs in which the value of the Oceanic Niño Index (ONI) is +0.2 °C or lower. Most of southern Australia's most significant extended heatwaves, such as those of 1939, 2009 and 2014, have occurred under cool neutral or weak La Niña conditions. A detailed explanation for the shift in ENSO-extreme heat relationship with increased length of heatwave is still unclear, but it opens for further investigation the question of whether there is an ENSO signal in the speed of movement of synoptic-scale systems in the region.