Australian rainfall patterns during positive Indian Ocean Dipole

years


Introduction

This page describes the average impact of positive Indian Ocean Dipole (IOD) events on Australian rainfall patterns. A positive IOD year is characterised by cooler than normal water in the tropical eastern Indian Ocean, near Indonesia, and warmer than normal water in the tropical western Indian Ocean, near Africa. A positive IOD sea surface temperature pattern often results in a decrease of rainfall over parts of Australia.

Eleven years since 1958 have been identified that have general acceptance as positive IOD years. The eleven IOD years selected are 1961, 1963, 1967, 1972, 1977, 1982, 1983, 1994, 1997, 2006 and 2007. Maps of rainfall amounts and rainfall deciles for these and other years are freely available from our archives.

An IOD event usually starts around May or June, peaks between August and October and then rapidly decays. Hence the focus of this article is on the austral winter and spring (June to November). In the last section, a display of overlapping three-month rainfall patterns is used to show the evolution of typical positive IOD events.

Winter and spring - Positive IOD years

Figure 1 shows the mean rainfall deciles for total winter-spring (June to November) rainfall for the eleven positive IOD years listed above. For each of the eleven years, the rainfall deciles for the winter-spring period were calculated against all years between 1900 and 2007. These deciles were then averaged for each point in Australia, and the result mapped.

Winter/Spring mean rainfall deciles across
Australia for eleven positive IOD years.
Click for a high resolution image. Figure 1: Australian winter-spring mean rainfall deciles for eleven positive IOD events.

Alternative image formats: hi-res colour gif | low-res B&W gif | high-res B&W gif | colour pdf | B&W pdf

The map above shows that during positive IOD years, the mean winter-spring rainfall is below average (that is, in deciles 2 or 3 and indicated by red shades on the map) across much of central and southern Australia. It should be noted that in no part of the country is there a consistent tendency towards above average (decile 8 or higher) rainfall in positive IOD years.

It should not be expected that winter-spring rainfall in any given positive IOD year will follow the pattern of Figure 1, nor should it be expected that "above average" rainfalls will not occur during an IOD year. To see what happened as regards to total winter-spring rainfall in each of these positive IOD years, click on the appropriate year (1961, 1963, 1967, 1972, 1977, 1982, 1983, 1994, 1997, 2006, 2007).
 

Winter and spring - Positive IOD and El Niño

Some positive IOD events, but not all, occur during the same year as an El Niño. In 2007 a positive IOD occurred in the same year as a La Niña. This is the only clear occurance of a positive IOD event coinciding with a La Niña; 1967 showed a weak La Niña like pattern in the Pacific Ocean after August, but was clearly neutral in the atmosphere. The relationship between El Niño and the IOD is complicated, with the level of dependence of the two phenomenon an area of active research.

Figure 2 shows the mean rainfall deciles for total winter-spring rainfall for seven of the recent positive IOD years that also occurred during an El Niño event. The seven positive IOD/El Niño years used are 1963, 1972, 1977, 1982, 1994, 1997 and 2006.

Winter/Spring mean rainfall deciles across
Australia for seven positive IOD years and El Niño years.
Click for a high resolution image. Figure 2: Australian winter-spring mean rainfall deciles for seven positive IOD coinciding with El Niño events.

Alternative image formats: hi-res colour gif | low-res B&W gif | high-res B&W gif | colour pdf | B&W pdf

The map above shows that during positive IOD/El Niño years, the mean winter-spring rainfall is below average across nearly all of Australia. There is a significant shift in the pattern of Figure 2 towards dry conditions in the east of Australia. The shift towards the east of the country is in line with the impact expected in an El Niño year (see map of Australian rainfall patterns during El Niño events), however, there is still a signal of reduced rainfall across central Australia, which is not usual when we look at El Niño years on their own. Once again, in no part of the country is there a consistent tendency towards above average (decile 8 or higher) rainfall.

Winter and spring - Positive IOD and neutral ENSO

Since 1958 only three positive IOD events have occurred during ENSO neutral years. The three positive IOD/neutral ENSO years are 1961, 1967 and 1983. As three years is a small sample size, mean rainfall deciles for total winter-spring rainfall has not been plotted. However, the maps below are included so that rainfall patterns from each individual year can be viewed seperately.

Click on an image to display higher resolution versions.
 

1961 1967 1983

Figure 3: Australian winter-spring mean rainfall deciles for 1961, 1967 and 1983.

The maps show that during the neutral ENSO/positive IOD years analysed here, winter-spring rainfall patterns have similarities to that in Figure 1, with below average rainfall across central and parts of southern Australia. Both 1961 and 1967 show a particularly strong trend of below average rainfall over central and southern Australia. In contrast to 1961 and 1967, 1983 was wetter than average over much of southeast Australia as well as over southwest Western Australia. All three years share in common above average rainfall along the east coast of Australia.

Evolution of positive IOD events

Figure 4 below shows the evolution of three-month rainfall deciles averaged over the eleven positive IOD events. Each overlapping 3-month period is shown from April (April to June) to December, to encompass a typical event from start to finish.

Click on an image to display higher resolution versions.
 


April–June May–July June–August
July–September August–October September–November
October–December

Figure 4: Three month rainfall deciles for Australia averaged over all eleven positive IOD events.

An animation of the above composites is available here.

Further sources of information:

References

Meyers G., McIntosh P., Pigot L., Pook M., 2007: The Years of El Niño, La Niña, and Interactions with the Tropical Indian Ocean, Journal of Climate, 20, 2872-2880.

Saji N.H., Goswami B.N., Vinayachandran P.N., Yamagata T., 1999: A dipole mode in the tropical Indian Ocean, Nature, 401, 360-363.

Saji N.H., T. Yamagata, 2003: Structure of SST and Surface Wind Variablity during Indian Ocean Dipole Mode Events: COADS Observations, American Meteorological Society, 16, 2735-2751.

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