Product Code: IDCKGEIRN0

Introduction

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

Ten years since 1958 have been identified that have general acceptance as negative IOD years. The ten negative IOD years selected are 1958, 1960, 1964, 1971, 1974, 1975, 1989, 1992, 1993 and 1996. 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 page 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 negative IOD events.

Winter and spring - Negative IOD years

Figure 1 shows the mean rainfall deciles for total winter-spring (June to November) rainfall for the ten negative IOD years listed above. For each of the ten 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.

Figure 1: Australian winter-spring mean rainfall deciles for ten negative 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 negative IOD years, the mean winter-spring rainfall is above average (that is, in deciles 8 or 9 and indicated by blue shades on the map) across most of southern Australia. It should be noted that in no part of the country is there a consistent tendency towards below average (decile 3 or lower) rainfall.

It should not be expected that winter-spring rainfall in any given negative IOD year will follow the pattern of Figure 1, nor should it be expected that below average rainfall will not occur during a negative IOD year. To see what happened as regards to total winter-spring rainfall in each of these negative IOD years, please click on the appropriate year (1958, 1960, 1964, 1971, 1974, 1975, 1989, 1992, 1993, 1996).
 

Winter and spring - Negative IOD and La Niña

Some negative IOD events, but not all, occur during the same year as a La Niña. In contrast, negative IOD events rarely occur in the same year as an El Niño with 1993 the only recorded case of this happening since 1958. The relationship between La Niña and the IOD is complicated, with the level of dependence of the two phenomena an area of active research.

Since 1958, four negative IOD events have occured during the same year as a La Niña. The four negative IOD/La Niña years are 1964, 1971, 1974 and 1975. As four 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 separately.

Click on an image to display higher resolution versions.
 

1964	1971	1974
1975

Figure 2: Australian winter-spring mean rainfall deciles for 1964, 1971, 1974 and 1975.

The maps above show that during the negative IOD/La Niña years analysed here, winter-spring rainfall has been above average over large parts of Australia in each of the four years. This shift in pattern towards above average rainfall is in line with the impact expected in a La Niña year (see map of Australian rainfall patterns during La Niña events). However, it should be noted that in each of the four years, above average rainfall was also observed over most of, or parts of, the Gascoyne district, interior and southwest of WA, areas where La Niña generally has little impact.

Winter and spring - Negative IOD and neutral ENSO

Figure 3 shows the mean rainfall deciles for total winter-spring rainfall for five of the recent negative IOD years that occurred during ENSO neutral periods. The five negative IOD/neutral ENSO years are 1958, 1960, 1989, 1992 and 1996.

Figure 3: Australian winter-spring mean rainfall deciles for six negative IOD events occurring in neutral ENSO years.

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

The map shows that for neutral ENSO/negative IOD years, the mean winter-spring rainfall is similar in pattern to Figure 1, with above average rainfall across most of southern Australia. However, there is a significant shift in the pattern towards below average rainfall in an area covering northern WA and the western NT as well as along the central east coast.

Evolution of negative IOD events

Figure 4 below shows the evolution of 3-month rainfall deciles averaged over the ten negative IOD events. Each overlapping 3-month period is shown from April (April to June) to December.

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 rainfal deciles for Australia averaged over all ten negative IOD events.

An animation of the above composites is available here.

Further sources of information:

• Australian rainfall patterns during positive Indian Ocean Dipole years
• Current weekly IOD index data
• POAMA IOD forecast
• About POAMA IOD forecasts
• Australian rainfall patterns during La Niña events
• The Indian Ocean and its influence on Australian climate
• Australian Climate Influences
• Seasonal Climate Outlooks
• ENSO Wrap-Up

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.