Product Code: IDCKGEERL0
The purpose of this article is to describe the average impact of La Niña events on Australian rainfall patterns. To do this, the twelve strongest La Niña 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" La Niña events are 1910, 1916, 1917, 1938, 1950, 1955, 1956, 1971, 1973, 1975, 1988 and 1998. 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 La Niña 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 shows that the average La Niña impact for winter/spring rainfall is not the exact opposite of the corresponding El Niño impact. The blue shadings in Figure 3 indicate "above average" rainfall (deciles 8 and 9) for the six month period. Most notable is the much broader impact across northern and central Australia. Some of this is undoubtedly due to heavier rainfalls in October and November associated with an early start to the northern wet season in La Niña years, but this is not the whole story.
|Year||Average SOI||Year||Average SOI|
|Table 1: Average (June to November) SOI values for the twelve La Niña years. Strongly positive SOI values sustained across many months form one of the indicators of a La Niña event.|
Northeast Tasmania, northern Victoria, and eastern parts of South Australia show a La Niña response (that is, a tendency towards wet conditions in La Niña years) similar in strength to the El Niño response (a tendency towards dry conditions in El Niño years). For northern South Australia and northern Queensland, the La Niña response is quite a bit stronger than the El Niño response. In no part of the country is there a consistent tendency towards "below average" rainfall in La Niña years.
The effect of La Niña events on winter/spring rainfall is limited for coastal areas of Victoria and New South Wales, extending almost to Fraser Island in southern Queensland. The reasons for the lack of a consistent response of winter/spring rainfall to El Niño and La Niña events for the coastal areas of New South Wales and southern Queensland are the subject of current research, but it should be noted that rainfall in these parts often arises from the lifting of on-shore air streams as they flow over the Great Dividing Range. A lot of the month to month and year to year rainfall variability from these airstreams seems to be due to the chaotic nature of the mid-latitude weather systems, which form a major feature of the weather and climate patterns of southern and central Australia.
It should not be expected that winter/spring rainfall in
any given La Niña year will follow the pattern
of Figure 1, nor should it be expected that "below average"
rainfall will not occur in a La Niña year.
To see what happened as regards total winter/spring rainfall in
each of these La Niña years, click on the appropriate year
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, in relative terms La Niña's impact on Australian summer rainfall is less than the winter-spring impact. This is especially the case over Tasmania, SA, the NT and WA which have large areas of deciles 5 and 6 in the image above. As in the June-November period, the impact from La Niña is more widespread and intense during summer than the corresponding impact from El Niño.
There is a contraction and weakening of the effect in western Queensland, but in the east of that state, the La Niña-induced tendency towards wetter than average conditions continues to be moderate to strong. This is significant as December to February are, on average, three of the four wettest months in this part of the country. In eastern NSW, there is an increased tendency for wet conditions in comparison with the impact during the winter-spring period. As with eastern Queensland, this is significant in northern NSW because the region experiences a natural summer rainfall maximum.
|Year||Average SOI||Year||Average SOI|
|Table 2: Average (December to February) SOI values for the twelve La Niña years. There is usually a slight fall in the SOI during summer, but with significant positive values still being recorded in many events. In a few cases, the SOI rose during summer.|
It should not be expected that summer rainfall in any given
La Niña event will follow the pattern of Figure 2.
To see what happened as regards total summer rainfall in
each of these La Niña events, click on the
The following table shows the evolution of 3-month rainfall deciles averaged over the 12 La Niña events. Each overlapping 3-month period is shown from autumn (March to May) of the onset year, to autumn of the decay year. In general terms, La Niña's impact on Australian rainfall for overlapping three-month periods, is more widespread, intense and long-lasting than its El Niño counterparts.
Click on an image to display a high resolution version.
|March - May (year 0)||April - June (year 0)||May - July (year 0)|
|June - August (year 0)||July - September (year 0)||August - October (year 0)|
|September - November (year 0)||October - December (year 0)||November - January (year 0)|
|December - February (year 0)||January - March (year 1)||February - April (year 1)|
|March - May (year 1)|
Further sources of information: