THE CLIMATE KELPIE BLOG: Indian Ocean driving wetter than average winter–spring outlook

Posted by BCG on 20th September 2021

Many of Australia’s main agricultural producing regions this year have experienced an excellent start to the season with above average rainfall. The main driving force behind this has been activity in the Indian Ocean region.

So what’s been happening? ….

The Indian Ocean Dipole explained:

Indian Ocean sea surface temperatures impact rainfall and temperature patterns over Australia. Warmer than average sea surface temperatures can provide more moisture for frontal systems and lows crossing Australia.

Sustained changes in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean are known as the Indian Ocean Dipole or IOD.

Dr Andrew Watkins, Head of Operations Climate services said: “The IOD is one of the key drivers of Australia’s climate and can have a significant impact on agriculture. This is because events generally coincide with the winter crop growing season. The IOD has three phases: neutral, positive and negative. A negative IOD often results in more rainfall than average over south-eastern Australia.

“Events usually start around May or June, peak between August and October and then rapidly decay when the monsoon arrives in the southern hemisphere around the end of spring, Dr Watkins said.

“This is good news for Australian farmers, particularly in the south east of the country where crops are likely to benefit from the rain during the growing season which will clear before harvest when growers want drier conditions in order to get crops off and protect grain quality. That is unless the El Niño–Southern Oscillation moves out of neutral and enters an active La Niña during spring. A La Niña would increase the chances of above average spring/summer rainfall for much of eastern and northern Australia which will be welcomed for many industries but could cause headaches for others for example Australia’s forecast bumper grain harvest.”

Figure 1: Westerly winds intensify along the equator, allowing warmer waters to concentrate near Australia. This sets up a temperature difference across the tropical Indian Ocean, with warmer than normal water in the east and cooler than normal water in the west. A negative IOD typically results in above-average winter–spring rainfall over parts of southern Australia as the warmer waters off northwest Australia provide more available moisture to weather systems crossing the country.

Dr Watkins said the IOD is a strong climate driver: “Farmers can gain even more confidence in the seasonal outlook predicted due to the negative IOD by looking back at previous years which had a negative IOD event.”

Negative IOD over the years

The map below shows rainfall during negative IOD years is generally above average (decile 8 or higher, indicated by the blue shading) across the mainland’s southeast. In no part of the country is there a tendency towards below-average rainfall (decile 3 or lower, indicated by the red shading).

*Figure 2: Winter-spring mean rainfall deciles for 11 negative IOD years.*

Winter–spring warm in the far north, cool in the southeast

The temperature deciles maps show the typical maximum (left) and minimum (right) temperature impacts during a negative IOD. A negative IOD often results in cooler than average maximum temperatures (decile 3 or lower, indicated by the blue shading) over south-eastern mainland Australia. Maximum and minimum temperatures in the far north are typically warmer than average.

*Figure 3: Winter-spring mean maximum temperature deciles for 11 negative IOD years.*

*Figure 4: Winter-spring mean minimum temperature deciles for 11 negative IOD years.*

About the maps

Since 1960, when reliable records of the IOD began, to 2016 there have been 11 negative IOD and 10 positive IOD events.

The decile maps above are composites of 11 negative IOD years since 1960. The maps provide guidance on the typical conditions to expect during a negative IOD. For each of the 11 years, the deciles for the winter – spring period are calculated against all years between 1900 and 2019 for the rainfall map, and 1910 and 2019 for the temperature maps. These deciles are then averaged for each point in Australia, and the result mapped. Rainfall and temperatures for all negative IOD events may not follow an identical pattern, and areas of below-average rainfall and above-average temperature can still occur during a negative IOD.

In summary Dr Watkins said: “Eastern Australia is likely to receive above median winter-spring rainfall due to the large parts of the eastern Indian Ocean being warmer than average. Western WA is in for a drier spell with below median rainfall likely during September to November. The northern tropics and south-eastern parts of Australia are likely to be above median for daytime temperatures during August to October, but parts of central NSW are more likely to experience below median daytime temperatures. All of Australia except for the south-west is very likely to have above median temperatures overnight during August to October.”

Contact

Dr Andrew Watkins, 03 9669 4000, andrew.watkins@bom.gov.au

More Information

Bureau of Meteorology’s Climate Driver Update.