Australian rainfall patterns during El Niño and La Niña events
The information below has been updated. The relevant link is located here.
In the article Rainfall in El Niño years, the effects of changes in the Southern Oscillation on Australian Winter/Spring (June to November) rainfall were described. Following on from that article, this article presents a composite of the El Niño effect for the same period, taken across the twelve strongest El Niño years, to show the average effect on Winter/Spring rainfall. This has recently been extended to show the average Summer impact across Australia. A composite of the La Niña effect, taken across the twelve strongest La Niña years, is also given for comparison. The maps for these composites give a good idea of the areas of Australia whose rainfall is affected by the El Niño-Southern Oscillation (ENSO) phenomenon.
Figure 1 shows the mean rainfall deciles for total Winter/Spring (June to November) rainfall for the twelve El Niño years 1905, 1914, 1940, 1941, 1965, 1972, 1977, 1982, 1987, 1991, 1994 and 1997. 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 1998. These percentiles were then averaged for each point in Australia, and the result mapped.
The map 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 the southern half of Queensland. Relatively small coastal areas of southern Western Australia, the Northern Territory and northern Queensland are also affected.
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 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 appropriate year ( 1905, 1914, 1940, 1941, 1965, 1972, 1977, 1982, 1987, 1991, 1994, 1997 ).
A composite picture for Summer (December to February) rainfall has also been calculated (Figure 2). 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.
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 appropriate year ( 1905/06, 1914/15, 1940/41, 1941/42, 1965/66, 1972/73, 1977/78, 1982/83, 1987/88, 1991/92, 1994/95, 1997/98 ).
A map of Winter/Spring rainfall patterns has also been calculated for twelve La Niña events 1910, 1916, 1917, 1938, 1950, 1955, 1956, 1971, 1973, 1974, 1975 and 1988 (see Figure 3). Table 3 shows the average SOI values for June to November for these twelve La Niña years.
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 (October to April) in La Niña years, but this is not the whole story.
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 air streams 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 3, 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 ( 1910, 1916, 1917, 1938, 1950, 1955, 1956, 1971, 1973, 1974, 1975, 1988 ).
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