Transcript for Beyond ENSO: understanding wet and dry regimes in southeast Australia

[Speaker: Jacqui Lloyd (DEECA)]

Hi everybody. Thank you for joining us. We're just waiting a few minutes until a few more people trickle in through the door.
All right. I think we'll maybe give it 30 more seconds. And we can get started.

Alright, that'll do. Let's go! Thank you, everybody. Welcome to this webinar being hosted by the Hydrology and Climate Science team at the Department of Energy, Environment and Climate Action.

This webinar is showcasing some of the research from the Victorian Water and Climate initiative. As with previous webinars, this one will be recorded with the intention of making it available on our website after the event.

I'd like to acknowledge the Traditional Owners of the lands on which we are meeting today.

In my case, it's the land of the Wurundjeri people of the Kulin Nation. I'd like to pay my respects to elders, past and present, and extend that acknowledgement to those across other parts of Victoria or Australia where people may be joining us today.

My name's Jacqui Lloyd. I am in the Hydrology and Climate Science team where we manage the Victorian Water and Climate initiative. Other team members have been working very hard to coordinate this and make sure the tech runs smoothly. So a big thanks to Geoff Steendam, Rachel Brown, Sandra Dharmadi, and Gil Aitken.

The second phase of the Victorian Water and Climate Initiative, or VicWaCI for short, has been underway for a few years now, and we are approaching the end of the current phase which will be marked by the release of our VicWaCI synthesis report later in the year, and supported by a webinar series - including today.

So for today's webinar we have Doctor Irina Rudeva from the Bureau of Meteorology, who will be talking about understanding wet and dry regimes in Southeast Australia. Irina will be discussing the key processes driving persistent wet and dry seasons, emphasising seasonal differences, and helping to understand Victoria's rainfall variability and its influences.

By default we run this webinar with cameras and microphones switched off. However, I encourage you to use the Q&A function in Teams if you have any comments or questions at any time and we'll put those to Irina at the end of the presentation. And just before I begin formally introducing Irina, I'd like to thank everybody for joining us today. And I hope you enjoy.

So Doctor Irena Rudeva is a climate scientist at the Bureau of Meteorology, specialising in the mechanisms of climate variability. Her research focuses on understanding individual climate extremes and long term trends, contributing valuable insights into climate dynamics.

Dr Rudeva led the Bureau's contribution to the second phase of the VicWaCI, which furthers our understanding of the interplay between climate and water resources. So please, Irina, thank you very much for presenting today.

[Speaker: Dr. Irina Rudeva (Bureau of Meteorology)]

Thank you. Jacqui can you hear me OK?

[Speaker: Jacqui Lloyd (DEECA)]

Yes.

[Speaker: Dr. Irina Rudeva (Bureau of Meteorology)]

OK, great. Thank you for the introduction and thank you everyone for joining this webinar today. As Jacqui said, I'll be talking about rainfall variability in Victoria in and in Southeast Australia in general, focusing on wet and dry regimes, as has been told already and the title says beyond ENSO.

This is to highlight that there are, even though ENSO, or variability or oscillation in the tropical Pacific is an important driver of global climate variability. When we think about a particular region, there are other processes that might influence regional climates and also may have a strong impact on it, and that depends.

So the drivers are of a particular region or rainfall in the particular region, depends on the location of that region and also on the time of the year. So seasonality will be also a big aspect of this presentation.
Sorry, I'm just trying to. Yes, I can change slides now. Yes.

So this is a slide with a few topics that will be presented as a part of VicWaCI webinars by the Bureau of Meteorology. There will be other presentations by other partners or link projects, but there are main three main topics that we cover at the Bureau of Meteorology as a part of this initiative.

And the first talk was already presented by Ulrike Bende-Michl months ago and that was on catchment response to drought conditions. And one of the main outcomes of that work was that recovery of high intensity rainfall after the drought plays a critical role in the recovery of run off in the post drought environments, and today we'll be talking.

We'll continue talking about the rainfall variability and how high intensity rainfall changes throughout the time. There will be one more webinar on the changes in rainfall patterns in the past and also in the future that will be presented by Surendra Ruaniyar in the in the next few months. But as we started to prepare for this presentation today on rainfall variability, we quickly realised that we can possibly run a whole series of webinars just on this topic.

Therefore, we decided to simplify our task and split this topic into two. So today I'll be talking mainly about them or about rainfall variability in general and then Eun-pa Lim will in the next few weeks will present on climate drivers to the South of Australia. Because usually when we talk about rainfall variability, we talk about ENSO and other tropical processes but Eun-pa will focus on how the climate to the South of Australia and also in the upper levels of the atmosphere, impact Victorian climate.

So today we have a general presentation on processes that may impact rainfall in Victoria. And when we started this project, or this second phase of VicWaCI three years ago, we realised that there had been many studies that had already looked at the rainfall variability in this area of in our region.

And we decided to summarise what we could do in literature review and we wrote a paper, a review paper, that was published in 2023, led by Rosanna McKay. So you can find this paper online. All papers or the majority of papers published under this initiative are Open Access papers so you can find all of them and read if you want to know more details. But this slide just quickly summarises what we found.

And the schematic on the top left shows the key processes that we think are important for Southeast Australian rainfall, and you can see ENSO there. So that is in the top right of that plot. So that is an oscillation in the tropical Pacific.

There is a similar process in the Indian Ocean which is called the Indian Ocean Dipole or the IOD for short. That is also very important. Then I'll be talking about the SAM which is the Southern Angular Mode which represent the shift in the jet vacation jet is the area of high wind speeds to the south of Victoria.

There are also other processes that impact rainfall variability. For example weather systems there are lows and fronts and highs, thunderstorms and there is one more small symbol there which says MJO - this is the Madden-Julian Oscillation. This is a pulse of tropical convection that propagates across tropical oceans from the west to the east.

And there is a lot of information that we need to put together in that review paper and we try to summarise that with this diagram on the right hand side. So each circle - red and blue circles - they represent trends in the key drivers that were identified during that review process.

For example ENSO is represented by the biggest circle. I'm not sure if you can see my yeah, so if you can see my cursor, so the blue circle represents. The impact of these drivers on Victorian or SE Australian rainfall is shown by season. And each quadrant shows specific season, for example, summer is shown in the top right.

And then the colour represents the trend in this in this driver, for example, blue colour for ENSO means that the in historical period, the trend in ENSO was negative or La Nina, like other drivers, had a positive trend, but it doesn't mean that they impact on Southeast Australian rainfall was also positive or increase in rainfall. It can be either decreasing or
increasing.

And that is represented by the letter. So W means wet conditions and D means dry conditions. There is a lot of information in this plot, but what I want you to get from this plot is that most of the drivers in the cool season in winter, which is in the bottom left, contribute to a dry and cold season trend in Southeast Australia.

Another important message that I want you to take from this plot is that there is a significant seasonal variation in drivers that affect rainfall variability and trend.

Let's have a closer look to a few key drivers that are, ENSO, IOD and SAM. And so a small schematic of what these drivers mean is shown on the right hand side. So ENSO as I said is an oscillation in the tropical Pacific. IOD is a similar oscillation in the Indian Ocean. And then SAM represents the north-south displacements, or intensification of the jet to the south.

And what this plot shows is the correlation between these drivers and rainfall in individual grid points. Stippling shows that the relationship is not statistically significant.

So what you can clearly see in this plot is that the best of the season where the relationship are the strongest, is spring. So in spring both positive ENSO and positive IOD have a drying impact on Victorian rainfall.

The SAM or positive phase of SAM, suggests weather conditions in Southeast Australia. In other seasons though the relationship is much weaker. For example, in summer there is a very weak relationship with ENSO in general, with a few exceptions in northern Victoria. Also SAM has - wetting positive SAM - has a wetting effect on rainfall in the far west. But otherwise the relationship is not significant.

What is also important is that in spring, when all drivers have strong impact on Victorian rainfall, or SE Australian rainfall, they are often acting in phase. So if there is a positive or strong positive phase ENSO there is a higher likelihood that IOD will be also in a positive phase and SAM will be in a negative phase. So they kind of all amplify each other.

But this is not the case in winter for example. And in winter IOD is the main driver.
But it often it is uncorrelated with ENSO, it has a weak relationship with ENSO and SAM. So that season is particularly influenced by strong phases of IOD, but other drivers have a relatively minor effect on rainfall.

Let's move on and this slide shows a similar information, but for the southern Murray Darling Basin, and it represents how the mean rainfall for this region correlates with the three key drivers - again ENSO, IOD, and SAM. And what you can see that the correlation now is shown throughout the year from the start of or from mid-summer using sliding seasons until the end of the year.

And at the start of the year or the first half of the year, there is no strong relationship with any of these drivers. And then in winter IOD is again the main layer that really defines the changes in rainfall in the southeast. And then towards the end of spring, you can see that ENSO and SAM become more significantly related to the rainfall than the IOD.

How can we use this information to understand trends? The plot at the bottom shows again a few seasons from late autumn to mid spring.
Now the bars show the trends. And grey bars show the observed rainfall trends. And coloured bars show trends that are induced, if you like, or are related to, the trend in those drivers. And you can see that ENSO, even though it plays some role in rainfall variability, it doesn't really contribute much to rainfall trends. Maybe with some exception in spring.

The main driver of rainfall cool season trends in the southeast of Australia is the IOD, which is shown by the pink bars. And it almost entirely explains the rainfall in some seasons, but not at the end of this plot in mid-spring where it significantly overestimates the trend, even though it is balanced with the opposite tendencies from the two other drivers, it is still the way too much. But in winter and late autumn the IOD provides good information on rainfall long term changes.

But this is all good and nice, but we all know that rainfall is brought about by weather systems. So even though we may feel the impacts of large scale drivers it's actually cyclones, fronts, thunderstorms that bring rainfall. So how we can reconcile this information? And there was a relatively, it's not very old but a few year old, study led by Acacia Pepler, who looked at the rainfall changes between the first two a - little bit less than two - decades of the 21st century compared to the last two decades of the 20th century.

And cool season declines can be well seen in this plot in the southern parts of Australia, and that study looked at different regions, not just at Victoria. But will be only talking about the southeast, which is shown in the bottom plot on the left hand side. The warm season trend is actually in the opposite direction, it has slightly increased.

But these increases or the trends in the warm season are still not statistically significant. The trends in the cool season are statistically significant. Therefore, we'll be focusing on them. And when there's this trend or decline in the cool season, rainfall was separated by contributions from different weather systems.

They showed that the rainfall declined from all weather systems. And what was interesting was the decline in rainfall intensity per weather system was even more significant, especially for fronts, for example, than the decline in the frequency of those weather systems. How we can understand this decline in rainfall intensity, we'll get back to it later.

But here is another study that we did as a part of this phase of the project already. So we decided, OK, so the previous study here, it compared 2 long periods. What if we compare wet and dry seasons? Or relatively short periods? And the idea was that if we do so, we will probably find a weather regime that brings a lot of rainfall during the wet seasons, but is possibly absent during the dry seasons.

And what we found (so we looked at the mainly at the lows and highs and the combinations) when we did so, we realised that the frequency of weather regimes between wet and dry periods doesn't change significantly. There is a small reduction in the amount of weather regimes that bring the highest amount of rainfall, but the main change comes from the rainfall intensity.

There is a plot on the left hand side but I don't expect you to understand all of this. Again, this work is published and it was led by Chiara Holgate, who is now working at ANU. And you can find this study online.

But the key message here is that: during dry periods rainfall is - or during wet periods, sorry - rainfall is increased from all weather combinations or weather regimes and in dry periods it's the opposite. So all right.

Again, how we can reconcile this information? Because, oh, sorry, I didn't mention here that at the bottom right I show moisture sources for rainfall in Southeast Australia and you can see that the main moisture source is the Coral Sea. It contributes to rainfall during high intensity rainfall regimes. But interestingly, or possibly that's what to be expected during dry regimes, the same weather combination that is a high pressure system of the Tasman Sea and a low pressure system to the east.

There is the biggest decline in moisture that is brought into this region happens over the coral Sea. The Southern Ocean contributes to the rainfall in both wet and dry periods and relatively equally.

So, if you remember at the start of this presentation I said that positive phase of IOD and positive phase of ENSO both contribute to dry conditions. And conversely the opposite is true. The negative phases of these indices provide wetter conditions, or support weather conditions, in the southeast.And if you look at the schematics at the top of the slide, they both show the negative phases.

The IOD is shown on the left hand side and the ENSO is shown on the right hand side. They both have one common feature that is increased rainfall to the north of Australia. So this information tells us that when the moisture or rainfall to the north of Australia is increased then we are more likely to have wetter conditions in the Southeast.

If you go back to the moisture sources, you can easily see that this is not the source - the region to the north of Australia - is not the moisture source region for our, for where we are. But still the rainfall there helps possibly helps intensify the weather systems that later affects moisture from the nearby oceans.

And also if you remember, there was a contribution from different drivers to the trend in the Southeast and we showed that the IOD plays a critical role in the cool season decline. So what we did next, we compared the moisture, we correlated the moisture to the north of Australia with the IOD from the start of autumn until the end of spring. And this correlation is shown in the plot on the right hand side.

And all the time through the 20th century, or at least the second part of the 20th century, when we have good observational records, the moisture to the north of Australia is well correlated with the IOD. Even in autumn, when typically IOD is believed not to be developed yet, so it is believed that it is developing at the start of winter.

So there is there was a very good relation between the moisture to the north with the IOD until at the end of the 20th century and then this relationship dropped. But it dropped not in all seasons, but more strongly in autumn. And there was literally no change in spring and then it recovered after 2010.

But the period when this relationship dropped this aligns with the Millennium Drought, that we are all familiar who live in Vic. And the Millennium Drought is known for two key anomalies: so one is that the rainfall, the mean rainfall decline wasn't that dramatic, but there was no very wet periods in the southeast.

And the 2nd important signature of the Millennium Drought was that the main rainfall declined occurred in autumn. And these findings here support the role of moisture to the north of Australia and rainfall in Victoria, especially in autumn. So that period of reduced relationship coincides with the with lack of high intensity rainfall in events in the South East.

This is this can be applied to circulation regimes from autumn to spring. What about summer? Summer is quite different and this relationships with the large scale drivers are not as well established in summer because the atmosphere operates differently. But what I'm showing here is another study that is still in progress, but hopefully will be published soon.

And there is a busy plot on the left hand side. What I want you to focus on is the black line. We chose the number of hot and dry compound events in in Vic. And black dashed line shows that trend is this number in this number of days and you can see that it increased from around 5 days in 1980 to over 15 days per season and season is from November to March. So that's extended summer.

What is important here is that we found that termination of this compiled hot and dry events is associated with increased rainfall to the north of Australia again. But this time it's driven not by ENSO. And I'm just to remind you the relationship with the ENSO and rainfall in the southeast is quite weak. So increased rainfall variability or rainfall amounts to the north of Australia in summer is driven by the Madden-Julian Oscillation when it's in the specific phase in in that location.

So. It's really important. You know, if we want to understand this, how this compound hot and dry events are going to change in the future to be able to simulate MJO. The problem is that it is not currently well simulated by the models, but that's small caveat that we need to keep in mind. And that brings me to my last slide with the summary.

So here we reviewed the role of different climate drivers in rainfall variability. I didn't show that plot, but individual climate drivers explain 30 maybe 40% of rainfall probability is most at most, and that's for spring. In other seasons, their impact on rainfall are much weaker. In some seasons, other drivers, not just ENSO, but drivers like IOD and SAM have a better skill in predicting or explaining rainfall variants.

But models tend to predict ENSO more accurately compared to other climate drivers. And therefore it is often relied upon even in seasons where there is actually no correlation with Victorian rainfall. So just keep that in mind. And if you hear that there is a strong ENSO in February, for example, it doesn't mean that there will be less rainfall in that month because the relationship is really weak.

There is something interesting that we've found about the predictability of SAM, and I think Eun-pa will talk more about it in the future webinar. So don't miss it if you are interested.

And finally, moisture content to the north of Australia is an important predictor of rainfall in Victoria and this rainfall in that area, it can be modulated by a range of drivers such as ENSO, IOD and MJO, as well as Australian Monsoon and other processes.

And that brings me to the last point of this presentation is that I think I've persuaded you that there are many processes in the atmosphere that it may impact regional climate, and seasonal outlooks incorporate all this information in their systems and provide a reliable way of predicting rainfall.

It's better than just relying on one or two key drivers that we think are important. So please look at the seasonal outlooks and they are quite skilful. I can give you an example, for example, December 20 November, December 2023 last year when there was strong El Nino.

But also we had quite wet conditions and actually the seasonal outlook was neutral so neither wetter nor drier conditions, but a lot of people assumed that it would be dry because it was El Nino. Yeah, that wasn't the case.

And yeah, just view the seasonal outlooks. Thank you for your attention. And I'm happy to answer questions if there are any.

[Speaker: Jacqui Lloyd (DEECA)]

Thank you.

Page last updated: 05/02/25