Take home messages

• El Niño does not equal drought, and La Niña does not equal flood.
• Six key Australian climate drivers have no, or variable, rainfall outcomes over the summer and autumn periods in south-west Victoria.
• The key climate drivers have their greatest influence over winter and spring in the south-west.
• If you’re interested in understanding the historic climate driver effects in your area, the Local Climate Tool is a good test.
• Monitor the climate situation from a trusted source that can tell you whether oceanic climate phenomena are coupling with the atmosphere.

The 2024 start to the season was challenging for growers in south-west Victoria, not helped by consistent predictions of a wetter spring that failed to occur.

Key climate drivers

The El Niño Southern Oscillation (ENSO) phenomena is the major climate driver in the Pacific Ocean and has three states: El Niño, neutral and La Niña. El Niño is a warming of the central Pacific and an associated cooling of the ocean north of Australia. La Niña is a cooling of the central Pacific and a warming of the ocean north of Australia. Temperature changes suppress or enhance the moisture sources for rainfall. The tropical pressure changes in ENSO events can also make it harder or easier to draw tropical moisture down from the tropics. Historically, over a four-year period we average one El Niño, one La Niña and two neutral phases, but the actual distribution of events is more random, with occasional larger periods between events, or in the case of La Niña, multiple (two or three) events in a row.

The Indian Ocean Dipole (IOD) is the key climate driver in the Indian Ocean and has three states: positive, neutral and negative. The positive phase is cooler water off Indonesia and warmer water off Africa, and the negative phase is warmer water off Indonesia and cooler water off Africa. Like ENSO, this phenomenon has the ability to suppress or enhance moisture supply from the tropics. The events also have the ability to change pressure over Victoria which can make it harder or easier to draw moisture down in northwest cloud bands. The historic distribution of IOD events is less than for ENSO, with the incidence of positive or negative events about once every six years. Sometimes the length of time between events can be as long as 10 years in the case of the negative phase, while positive IODs have a much greater likelihood of two or three in a row.

The Southern Annual Mode (SAM) is the key climate driver around Antarctica and consists of positive, neutral and negative phases. Positive SAM is when the winds blow stronger around Antarctica dragging weather systems south. Negative SAM is when the winds slow down and allow weather systems to drift north to mainland Australia. SAM is different to other climate drivers in that its rainfall effects in winter are reversed when the events occur in summer. The chance of a negative or positive event in winter and a negative event in summer is around once every six years, but the chance of a positive event in summer has been getting more frequent at once every four years.

The Subtropical Ridge of high pressure (STR) is not so much a climate driver but a blocker or assister of weather systems, and is often influenced by the major climate drivers. The STR is the positioning of the high pressure systems around the mid latitudes of the world, usually sitting somewhere close to Victoria. The STR has a seasonal pattern, where it is positioned over Melbourne in summer and moves north to the top of the Great Australian Bight in winter. Sometimes the STR positioning is further north or south of normal, and this can block weather systems, or allow the introduction of tropical moisture from the north, or the passing of plentiful cold fronts from the south. Another feature of the STR is the physical pressure of the systems. Higher pressure systems move slower than lower pressures, and can spend many days or up to a week blocking weather systems from passing across the state. If the high system is positioned off the east coast in the Tasman Sea, it can lead to many days of a rain pattern from the north. Pressures are highest in winter and the STR is capable of its greatest misdemeanours during this season.

The Madden-Julian Oscillation (MJO) is a cloud wave that travels just below the equator and takes around 40 days to make a complete circuit of the Earth. It often kicks off the burst of the northern wet season, monsoonal rains. Scientists have labelled one to eight positions of the MJO that start at one when the MJO is off Africa, and end at eight when the MJO is in the middle of the Pacific Ocean. Positions four and five are when the cloud build up is to the northwest or northeast of Australia. The MJO is a cloud source that can deliver moisture when the right triggers pass the state to draw the moisture down to Victoria. Consequently, the existence of the MJO in the right position increases the likelihood of rainfall but doesn’t guarantee it. While positions four and five increase the chances of rainfall, positions two and three in the central Indian Ocean can suppress rainfall over Victoria.

Table 1 shows months of the year where these six climate drivers have affected rainfall across Victoria. The peak months are in winter and spring, whereas in summer and particularly autumn, the correlations are poorer.

Table 1: Key climate drivers and the months of the year that they have statistically affected rainfall somewhere in Victoria.

Table 2 shows the same data but specifically for south-west Victoria. This analysis shows that winter and spring are still the most reliable for the influence of the climate drivers’ on rainfall, while summer and autumn are particularly poor. Almost no climate driver has had a strong effect in the critical early autumn months of March and April.

Table 2: Key climate drivers for south-west Victoria and the months of the year that they have statistically affected rainfall.

It is worth noting that autumn is always a poor time from climate predictability in southern Australia because the tropical oceans are usually in reset mode during autumn. At this time the tropical climate drivers are yet to indicate what they might do in the coming winter and spring. The undersea temperatures sometimes show signs of possible ENSO or IOD formation in the coming year, but it’s always uncertain as to whether those phenomena actually form.

The Forecasts4Profit and Local Climate Tool websites (originally funded by GRDC) are a great place to look at the historical effects of ENSO and IOD across south-east Australia. Figures 1–3 are derived from this tool and show tercile distributions of the rainfall effects for varying seasons and the ENSO and IOD phase. A tercile distribution is divided into thirds. The red colour shows the percentage of time rainfall was in the lowest third of records; the yellow colour shows the percentage of time rainfall was in the middle or average third of records; and the blue colour shows the percentage of time rainfall was in the highest third. So by definition, the historic record including all years, shows a 33% chance of low, average and high rainfall.

In autumn, the climate driver response is varied, some close to the historic chance, and perhaps some swayed a bit wetter in the case of La Niña. The winter and spring responses for the drier climate phases, El Niño and positive IOD are quite similar, with the odds of drier conditions moving up to 50–75%. The wetter climate drivers of La Niña and negative IOD show a opposite effect, where in spring the odds of wetter conditions increase to 50–65%. Winter behaviour is slightly different, where the negative IOD can kick in earlier for wetter conditions, while a La Niña in winter tends to show behaviour closer to the long-term average.

All of these pie charts show that the existence of a particular climate driver doesn’t guarantee a certain outcome, just an increased likelihood. So El Niño doesn’t ‘equal’ drought and La Niña doesn’t ‘equal’ floods, and occasionally, the exact opposite has occurred. What has historically happened is the odds substantially increase, but don’t guarantee anything. The Local Climate Tool is a good way of testing the historic outcomes for various climate drivers in your own area. In your mind, you might be thinking a drought is strongly on the cards, but historically, droughts in El Niño or positive IOD years are only around 10% of all events rather than a ‘lay down misère’.

Figure 1. Autumn rainfall tercile graphs for the phases of ENSO and IOD.

Figure 2. Winter rainfall tercile graphs for the phases of ENSO and IOD.

Figure 3. Spring rainfall tercile graphs for the phases of ENSO and IOD.

ENSO and IOD are called ‘coupled ocean’ and ‘atmospheric’ phenomena. These events are fully functioning when the ocean surface and depth combined with atmospheric wind, pressure and cloud patterns are all ‘singing from the same hymn sheet’. All phenomena have these five key features that make up whether they are coupled or not.

In an El Niño, we would expect to see a warm equatorial ocean surface from South America across to the International Date Line. We also expect to see warm water to depth under this warm surface. Atmospherically, we would expect to see increased cloud at the Date Line and less to the north of Australia, westerly trade winds in the western Pacific, and much higher air pressure at Darwin.

In a La Niña, we would expect to see a cool equatorial ocean surface from South America across to the International Date Line, and cool water to depth. Atmospherically, we expect to see decreased cloud at the Date Line and increased cloud north of Australia, strong easterly trade winds in the western half of the Pacific and much lower air pressure at Darwin.

The IOD behaves similarly but with different geographic positioning. A positive IOD is a large patch of cool water off the island of Sumatra in Indonesia and warm water off the coast of Africa. There is cool water to depth in the eastern Indian Ocean and warm water to depth in the west. Atmospherically, there is less cloud off Sumatra and more off Africa, higher air pressure off Sumatra and lower off Africa, and strong easterly trade winds blowing away from Indonesia.

A negative IOD is a large patch of warmer water off the island of Sumatra in Indonesia and cool water off the coast of Africa. There is warm water to depth in the eastern Indian Ocean and cool water to depth in the west. Atmospherically, there is more cloud off Sumatra and less off Africa, lower air pressure off Sumatra and higher off Africa, and strong westerly trade winds blowing into Indonesia.

What happened in 2024

The year 2024 was characterised by a very dry autumn and a very late autumn break, in some regions, the latest on record. No climate phenomenon was in existence to cause this, except very unfavourable high pressure positioning that started over South Australia and moved slowly over to western Victoria as autumn progressed. It’s interesting to look back in hindsight at a number of models that started to pick up this drying trend as early as February right through to May. It wasn’t until late autumn that there was strong model consensus for drier conditions. This large, slow-moving, high-dominated weather pattern probably had some warm water in the central Indian Ocean as its origin. Autumn is a time to be concerned about the climate, but not a time to put much faith in climate forecasts. So, spend more time making decisions with known information: your soil moisture profile, has the break occurred yet, has optimal seeding time changed, and the weed profiles of paddocks.

As early as February, there was talk from climatologists of a potential La Niña in 2024. This prediction was based on observations that the ocean was cooler at depth in the eastern Pacific and if this upwelled to cool the surface, a La Niña could occur. The problem with predictions in autumn is that they are notoriously inaccurate, as too much random and unpredictable weather needs to occur in the western Pacific, and this year was no different. Strong trade winds in the central Pacific failed to fire up all year to lift the cool undersea water to the surface. Pressure at Darwin also remained high for much of the season, making it more difficult to move tropical moisture down south. What became frustrating was that the majority of world models were fixated on the formation of a La Niña and increased chance of wetter conditions. The weather features needed to kick start the climate of the Pacific didn’t occur until January 2025, and even then, only for two weeks it would seem. With so much talk about La Niña about, it was important to concentrate on the five features that indicate an event is more likely, or whether it remains just speculation. The Fast Break newsletter and Very Fast Break YouTube clips provide monthly climate analysis during the growing season and hope to cut through the hype that can abound in the media.

I have developed a list of things that give me confidence in a forecast, as opposed to ‘that’s interesting, but…’. In increasing confidence level:

• there is a consensus between models
• it’s the time of year where there is model skill (accuracy)
• you can see from ocean and atmosphere observations where the forecast is derived from
• you can see ocean or atmosphere observations moving towards a climate driver formation
• you actually have a climate driver formed and coupled between the ocean and the atmosphere
• the models have good form between predictions and actual.

In 2024, these conditions were not met in autumn and failed to be met as the months ensued throughout the growing season.

Conclusions

It’s important to make farm decisions in autumn by placing little emphasis on unreliable model forecasts at that time. The existence of any climate driver changes the odds of winter and spring rainfall, but doesn’t guarantee any outcome. During the season, don’t get caught up in the climate hype and try to find information on whether any climate event is coupling the ocean and atmosphere. If you’re ever wondering what the various climate drivers actually meant for your location historically, the Local Climate Tool is a useful source of information.

Useful resources

GRDC South rainfall history and climate driver tool (https://climatetool.forecasts4profit.com.au/)

The Break newsletters (https://agriculture.vic.gov.au/support-and-resources/newsletters/the-break)

Contact details

Dale Grey
Agriculture Victoria
195-229 Lyttleton Tce, Bendigo VIC 3550
0409 213 335
dale.grey@agriculture.vic.gov.au
@eladyerg