A positive Indian Ocean Dipole has been forecast by the Australian Bureau of Meteorology, and it is likely to be a key climate driver for the coming Spring. The IOD is the Indian Ocean’s version of El Nino and typically brings reduced rainfall and an increase in temperatures to significant parts of Australia through Winter and Spring.
But, with most of the media hype about Australian weather focused on the coming El Nino in the Pacific Ocean, a positive Indian Ocean Dipole (IOD) doesn’t get as much airtime or public discussion. So, what is it, and how is it likely to impact Australia this Spring?
From deserts to temperate rainforests and everything in between, climate drivers and extremes of weather have a significant impact on the daily lives of Australians.
Talk about El Nino is certainly gaining momentum in Australia, with many recent online articles and media reports focusing on the likelihood of a developing El Nino. This is certainly a valid topic of conversation, as an El Nino event will have a significant effect on the weather in Australia, particularly for the more populated eastern half of the continent.
The IOD is another major climate driver that has just as much impact on the Australian weather, particularly across Southern and Central Australia. In this article, we will explain what the Indian Ocean Dipole is, how it is measured, and how it may affect Australian weather in the coming months.
NEGATIVE IMPACTS OF A POSITIVE INDIAN OCEAN DIPOLE IN AUSTRALIA
Most people will be concerned with knowing exactly how the Indian Ocean Dipole affects their weather in Australia, so let’s discuss that first.
The IOD has a significant impact on seasonal variations in rainfall, the distribution of that rainfall, and temperatures. It typically has the largest effect on the weather between May and December, with peak impact most commonly occurring from August to November.
This period coincides with the most productive growing season for many farmers and general agriculture practices across Southern Australia. Thus, the importance of reliable rainfall becomes critical for ensuring crops are strong and healthy with good-sized yields when it comes time to harvest.
If the rains don’t fall, the result can be catastrophic for farmers. This, in turn, causes significant ongoing effects on supply chains and then consumers.
REDUCED WINTER AND SPRING RAINFALL FOR SOUTHERN AND CENTRAL AUSTRALIA
With the recent lack of rainfall in Australia, research and historical weather data during a positive IOD is painting a grim picture of what may lie ahead. Positive IOD events have impacted Australia significantly in past years, and there is every likelihood that this 2023 event will result in similar impacts.
One of the main effects of a positive IOD is a reduction in Winter and Spring rainfall across Southern and Central Australia, as shown in the image below. With less moisture feeding in from the tropics, frontal systems and any synoptic scale triggers for rain or storms have less moisture to work with. In addition, high-pressure systems are typically stronger due to the downwelling or sinking air from aloft.
These factors also result in higher-than-normal daytime maximum temperatures due to fewer clouds and more sunny days, as shown in the image below.
However, the aforementioned lack of clouds causes colder-than-normal temperatures during the night, with much of the daytime solar radiation escaping. This means a much greater risk of frost, which can severely hamper crops by freezing the tender, soft growth.
Now, if this is not bad enough, on some occasions we also see an El Nino develop at the same time as a positive IOD. This leads to even more sinking air over Australia, associated with the downwelling arm of the Walker Circulation, which makes up the El Nino Southern Oscillation (ENSO) over the Pacific Ocean.
The double impact from two significant global climate drivers compounds the drying effect, causing below-average to well below-average rainfall as can be seen in the image below.
It’s entirely possible for record-low rainfall to be experienced over parts of Australia during a year with both a positive IOD and an El Nino. This is particularly likely in the extended areas around Eastern Australia, where El Nino has a greater impact.
With an outlook for widespread below-average rainfall over the coming months, let’s have a look at what late Winter and then Spring in Australia might look like with a positive IOD in effect.
POSITIVE IOD AUSTRALIAN WEATHER PATTERNS TRANSITIONING FROM WINTER INTO SPRING
During the latter stages of Winter, reliable cold fronts begin to become weaker and also shift further south. The Southern Ocean is where the westerly belt of frontal systems combines with moisture from the Indian Ocean to make up the majority of typical late Winter rainfall.
This is when farmers look to the skies, hoping to receive the significant rains that will carry their crops forward into Spring, where rainfall eventually tends to diminish month by month.
With the approach of Spring, we see the inland of Western Australia heat up. This is the main heat engine of Australia, and it begins to bake the dry interior. With already below-average rainfall it begins to form a heat dome, which pools up over the interior, ready to be pushed ahead of any frontal systems.
This sort of setup is very common in Spring over Southern Australia, where very changeable weather is commonplace.
WHAT SPRING WEATHER PHENOMENA DOES A POSITIVE IOD CREATE?
A positive IOD enhances the aforementioned weather patterns and brings a greater potential for hotter-than-average temperatures. This, in turn, creates a marked increase in fire danger, with hot and often strong N to NW winds to fan any fires that do start.
Dry Microbursts from Thunderstorms
To further compound the fire danger risk, any thunderstorm activity that is starved of moisture is likely to be quite high based on lower rainfall totals.
These higher-based storms commonly create damaging winds from dry microbursts caused by the evaporative cooling effect. This is where a descending parcel of air accelerates downwards as the rain evaporates into the dry air below the thunderstorm.
High-based thunderstorms in warmer air tend to produce very frequent lightning strikes. Whilst fires started by lighting are a common occurrence, the heavy rain usually associated with thunderstorms is often enough to quickly extinguish them.
Thunderstorms with little to no rain produce ‘dry lightning’, which can easily ignite numerous fires on already-dry crop areas, scrubland, and forests. The high and gusty winds produced by the storm then fan the flames, without the higher water loading available to extinguish them.
By November, the heat dome over inland Western Australia is often strong enough to bring heatwave conditions over Southern Australia. This is especially prevalent further inland, where any shallow coastal weather changes fail to reach.
This prolonged heat from positive Indian Ocean Dipole conditions turns inland areas into dustbowls, where strong westerly winds behind cold fronts or troughs can whip up dust and carry it vast distances. In 2009, dust storms generated in Australia carried dust as far as New Zealand. Despite this not being a positive IOD year, it did occur at the end of a 4-year severe drought across many parts of Australia.
WHAT IS THE INDIAN OCEAN DIPOLE (IOD)?
Now that we know how a positive Indian Ocean Dipole event is likely to affect the weather in Australia, let’s take a brief look at what the IOD is.
The Indian Ocean Dipole is a climate driver that measures the difference in sea surface temperature anomalies between the Western Tropical Indian Ocean and the Eastern Tropical Indian Ocean.
The IOD can be categorized into three phases; Positive, Negative, and Neutral. Each phase has a different impact on Australian weather, with a specific focus on Southern and Central Australia. Since 1950, when reliable record keeping on the IOD commenced, there have been 14 positive phases and 14 negative phases.
The official measurement term for the IOD is the Dipole Mode Index (DMI), where positive events are characterised by anomalies greater than +0.4C and negative events by anomalies less than -0.4C.
Neutral events are not counted; however, these make-up years whereby the anomaly value is within these bounds with no significant impacts on either side of the Indian Basin. Records thus far tend to indicate that events do balance out over time with no long-term bias towards any particular side of the Indian Ocean.
WHEN WILL A POSITIVE IOD EVENT DECLARED FOR AUSTRALIA?
As of the time of writing this article, the current IOD value is +1.05C and rising rapidly. This is the second consecutive week it has been above the positive IOD threshold of +0.40C. The Bureau of Meteorology generally requires 5 consecutive weeks of the IOD index above that threshold before they will officially declare a positive IOD event.
So, whilst a positive IOD event has not yet officially been declared, the Bureau expects the current trend to intensify and become a moderate to strong event at its peak by October. Confirmation of this event could well happen within weeks. The other event yet to be declared officially is of course El Nino. This is still in the ‘alert’ stage but is highly likely to eventuate in the coming weeks.
With a positive IOD event and El Nino potentially joining forces, there are obvious concerns about how this will impact Australia as the weather continues to warm. Stay tuned for further updates on the status of both the Indian Ocean Dipole and El Nino in the coming weeks.
The latest Bureau of Meteorology climate driver update news can be found at http://www.bom.gov.au/climate/enso/