Product Code: IDCKGEERE0
The purpose of this article is to describe the average impact of El Niño events on Australian rainfall patterns. To do this, the twelve strongest El Niño years which had the "classic" autumn to autumn pattern of evolution and decay were used. Three-month composites of Australian rainfall data were calculated for each and every period from autumn (March to May) of the onset year, to autumn of the decay year for these particular events. A discussion of the winter-spring (June to November) and summer (December to February) periods, precedes the display of the overlapping three-month rainfall patterns.
The onset years for the 12 strongest "classic" or "canonical" El Niño events are 1905, 1914, 1940, 1941, 1946, 1965, 1972, 1977, 1982, 1991, 1994 and 1997. Maps of rainfall amounts and rainfall deciles for these and any other year are freely available from our archives.
Figure 1 shows the mean rainfall deciles for total winter/spring (June to November) rainfall for the twelve El Niño years listed above. The average Southern Oscillation Index (SOI) values across the six month period for each of these El Niño years are shown in Table 1. For each of the twelve years, the rainfall percentiles for the winter/spring period were calculated against all years between 1900 and 2004. These percentiles were then averaged for each point in Australia, and the result mapped.
The map above shows that the average impact is for the six-month total rainfall to be "below average" (that is, in deciles 2 or 3 and indicated by red shades on the map) across much of eastern Australia. Regions specifically affected include the northeastern half of Tasmania, almost all of Victoria, almost all of New South Wales (excluding coastal districts), eastern and coastal parts of South Australia, and much of Queensland. Small coastal in southern Western Australia are affected, as is the north of the Northern Territory.
|Year||Average SOI||Year||Average SOI|
|Table 1: Average (June to November) SOI values for the twelve El Niño years. Strongly negative SOI values sustained across many months form one of the indicators of an El Niño event.|
It should be remembered that the winter/spring period covers a lot of the dry season for northern Australia. In the dry season, zero monthly rainfall totals are quite common in some northern and central parts even in ordinary years, so it is not surprising that there is little or no consistent El Niño effect for this time of year across central and southern parts of the Northern Territory and adjacent parts of Western Australia. In no parts of the country is there a consistent tendency towards "above average" (decile 8 or higher) rainfall in El Niño years.
It should not be expected that winter/spring rainfall in any given
El Niño year will follow the pattern of Figure 1,
nor should it be expected that "above average" rainfalls will not
occur during an El Niño year.
To see what happened as regards total winter/spring rainfall in
each of these El Niño years, click on the
The composite picture for summer (December to February) is shown below in Figure 2. The average Southern Oscillation Index (SOI) values across the three month period for each of these El Niño years are shown in Table 2. For each of the twelve years, the rainfall percentiles for the summer period were calculated against all years between 1900 and 2004.
Generally speaking, El Niño's impact on Australian rainfall diminishes from November onwards, so that by summer the El Niño-induced tendency towards drier than average conditions has almost entirely broken down across the east and south of the country. The most significant exceptions are the east of Cape York and northwest Tasmania, both of which have a moderate response. Scattered areas along the Queensland/New South Wales border show a slight response in the opposite sense, that is, a weak tendency for wetter conditions. There is a somewhat stronger and more widespread tendency for wetter than average conditions over the southeast of Western Australia.
|Year||Average SOI||Year||Average SOI|
|Table 2: Average (December to February) SOI values for the twelve El Niño years. Even though the average summer SOI usually remains negative at the end of an El Niño event, it is quite common for the index to rise during this 3-month period.|
It should not be expected that summer rainfall in any given
El Niño event will follow the pattern of Figure 2.
Furthermore, as the influence of El Niño often
decreases during the season, it is possible for there
to be a marked contrast between the rainfall patterns of
early and late summer.
That is, hot and dry conditions in the first part of
the season can be replaced by milder, wetter weather in
the second part.
To see what happened as regards total summer rainfall in
each of these El Niño events, click on the
The following table shows the evolution of 3-month rainfall deciles averaged over the 12 El Niño events. Each overlapping 3-month period is shown from autumn (March to May) of the onset year, to autumn of the decay year.
Click on an image to display higher resolution versions.
|MarchMay (year 0)||AprilJune (year 0)||MayJuly (year 0)|
|JuneAugust (year 0)||JulySeptember (year 0)||AugustOctober (year 0)|
|SeptemberNovember (year 0)||OctoberDecember (year 0)||NovemberJanuary (year 0)|
|DecemberFebruary (year 0)||JanuaryMarch (year 1)||FebruaryApril (year 1)|
|MarchMay (year 1)|
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