Transcript for The Antarctic stratospheric vortex, ozone, and their impact on southeast Australian rainfall

[Speaker: Geoff Steendam, DEECA]

Thanks everyone for joining us today. We'll just wait a minute or two just for others to join. We’ll just wait another 20 seconds or so then we'll kick off.

We might, might make a start.

So thanks everyone for joining us for this webinar today. We really hope that you enjoy the event.

This event today is hosted by the Hydrology and Climate Science team at the Department of Energy, Environment and Climate Action, or DEECA, and these webinars showcase the research from the Victorian Water and Climate Initiative.

I'd like to start today by acknowledging the traditional owners of the lands on which we're meeting. In my case, this is the land of the Wurundjeri people of the Kulin nation, and I'd like to pay my respects to the elders, past and present, and extend that acknowledgement to those across other parts of Victoria and Australia where people might be joining us today.

As with previous webinars in this series, we'll be recording the presentation and we're hoping to have those recordings of the series available on a website. We're still working on that, but will advise everyone through our e-mail list when we've got those recordings available.

So my name's Geoff Steendam and I work in the Hydrology and Climate Science team in DEECA where we manage the Victorian Water and Climate Initiative. And also in the team we have Sandra Dharmadi, Rachel Brown and Jacqui Lloyd.

The second phase of the Victorian Water and Climate Initiative, or VicWaCI for short, is currently wrapping up and will soon be releasing a report that summarises the findings from this phase of the research, including the types of findings that you'll hear about from Eun-Pa today.

And before I introduce today's presenter, I'll just let you know that earlier this week DEECA released a report titled Victoria's Climate Science Report 2024. The report provides information on the State's changing and future climate for a broad audience to support climate related decisions through all sectors of Victoria's economy, interested stakeholders, and the public.

The report, or the content of the report, is not a VicWaCI report, but it does incorporate some of the findings of VicWaCI research, along with a lot of other information. And so one of the team I think will post in the Q&A function, just a link to that report - if you'd like to view that.
So now on to today's presentation.

I'm very excited today to have our guest presenter, Dr Eun-Pa Lim from the Bureau of Meteorology. She'll be talking about the Antarctic stratospheric vortex and ozone and their impact on southeast Australian rainfall. Dr Eun-Pa Lim is a senior research scientist at the Bureau of Meteorology specialising in climate dynamics and seasonal prediction.

Eun-Pa began her career at the Bureau in 2006 and since then has been involved in the SEACI phase two program, the VicCI program and now VicWaCI, contributing to the improvements of understanding of the variabilities and predictabilities of the large scale climate drivers such as ENSO, IOD, SAM and the Antarctic stratospheric Polar vortex and ozone, which you'll hear about today.

So today's webinar follows on nicely from the presentation - our last presentation - from Doctor Irina Rudeva in the last webinar, where Irina presented on some of the more general large scale climate drivers that influence rainfall variability in Victoria. Whereas Eun-Pa today will focus on the impacts of climate drivers to the South of Australia.

So by default the webinar will run with microphones of audience and cameras also off, but we invite you to use the Q&A function to post any questions you have during the presentation or at the end and we'll go to those questions at the end of the presentation today. So thanks very much Eun-pa and over to you.

[Speaker: Dr. Eun-Pa Lim (Bureau of Meteorology)]

Thank you so much Geoff for the introduction. And I'd like to thank also Jacqui and Rachel for organising this webinar. So as Geoff mentioned I have been interested in the various large scale climate drivers that are related to Australian rainfall to better predict our seasonal mean rainfall at long lead times.

But in VicWaCI two, my research has been very focused on the Antarctic stratospheric vortex, ozone, and their impact on southeast Australian rainfall, with significant contributions from my colleagues at the Bureau.

So in this presentation, I'd like to provide you with a brief overview of some main characteristics of the Antarctic stratospheric vortex and its impact on the Antarctic ozone and the Southern Hemisphere surface climate. And then I will show you some recent examples of the stratospheric impact on southeastern Australia and the warm season rainfall.

The Antarctic stratospheric polar vortex refers to this band of strong westerly winds blowing from the west to the east around Antarctica in the stratosphere, which is the layer at 10 to 50 kilometre altitude and where ozone is rich, especially in the low level.

And these stratospheric zonal winds blow almost parallel to latitudinal circles. Therefore, we often take the zonal-means of these zonal winds and look at their north-south meridional changes, and vertical changes.

So these plots show the climatological mean of the zonal-means on a wind, displayed as a function of vertical pressure levels from 1000 to one hectopascal level and latitude from the South Pole to the equator from April to December.

The Southern Hemisphere stratospheric westerly jet, which is commonly called the polar vortex, generally starts developing in autumn and peaks in winter in the latitude band of 40 to 50 degrees south at the top of the stratosphere.

The average winter jet speed is about 300 kilometres per hour, which is like four to five times faster than our metro trains. So you can sense how fast this stratospheric winter jet is.

Then, when the season progresses to spring, the Antarctic region becomes warmer and the stratospheric jet becomes weaker and therefore it becomes more easily disturbed by large scale atmospheric waves propagating vertically from the lower stratosphere and upper troposphere into the higher levels of the stratosphere, which further weakens and warms the vortex, causing it to contract towards the pole and move downward.

Then the polar vortex completely breaks down in the upper to mid stratosphere by late November to early December. So this is the general life cycle of the Southern Hemisphere polar vortex.

This seasonal evolution of the polar vortex sometimes happens faster than normal, leading to anomalous spring polar vortex warming and weakening, and earlier vortex breakdown. And at other times it happens slower than normal, leading to anomalous spring polar vortex cooling with persistently strong vortex winds and delayed vortex breakdown.

So here is an example of a typical slower than normal seasonal evolution of the polar vortex.

Here the contours show the climatological zonal-mean zonal winds as before. And the colour shadings show the zonal-mean zonal wind anomalies associated with the spring polar vortex variations measured at 60° south at 10 hectopascal level.

The slower than normal vortex evolution often starts with an equator, where shift of the winter stratospheric jet with increased westerlies on the equatorward side and decreased westerlies on the polarward side of the jet. Therefore the total vortex area gets larger.

These meridional dipole anomalies of the jet move poleward and downward with time, leading to a stronger and cooler polar vortex in spring. In the spring, vortex anomalies move further downward and promote a poleward shift of the tropospheric jet, which is expressed as a positive phase of the Southern Annular Mode (SAM). With this nearly zonally symmetric high pressure anomalies in the middle latitude and low pressure anomalies in the high latitudes in our mid-spring and summer seasons.

A stronger and cooler springtime polar vortex can result in significantly lower Antarctic ozone concentrations from spring to early summer seasons. And a stronger vortex and associated lower Antarctic ozone together can promote the positive SAM in late spring to early summer.

And this positive SAM is well known to promote higher than normal rainfall over southeastern Australia from spring to summer seasons. Therefore, the dynamical evolution of the polar vortex from winter to early summer, from the top of the stratosphere to the surface, is an important source of long lead predictabilities of the SAM and associated Southern hemisphere regional climate anomalies, including southeastern Australia rainfall anomalies.

Interestingly, the Antarctic stratospheric vortex variability has been exceptionally large over the last five years, swinging from this near-record weak polar vortex in spring 2019 to the strongest polar vortex takes on record in late spring 2020, followed by the vortex being significantly stronger than normal in late spring 2021 and 2022.

Even in 2023, when the vortex just resisted breaking down.

And these four consecutive strong polar vortices significantly contributed to this extraordinary persistent positive SAM in spring and summer seasons of the last four years. In particular, the polar vortex in late spring 2020, was the strongest on record in the last 45 years, so we closely examine this super-vortex event for its cause, predictability and impact on the SAM and associated southeastern Australian summer season rainfall, particularly focusing on the role of very low Antarctic ozone during that time.

And we found that this super-vortex event in late spring 2020 actually didn't follow the canonical dynamical evolution that starts with an equatorward shift of the winter stratospheric jet, which means lack of long lead predictability.

Instead, it was caused by an anomalous low atmospheric circulation pattern characterised by the significant low pressure anomalies in the South Pacific that strongly persisted in September 2020.

This circulation pattern reduced the upward propagating wave activity, therefore leaving the vortex undisturbed and staying very strong. This extraordinary circulation pattern of September 2020 was not well predicted by the Bureau's seasonal forecast system of that time, and so was the polar vortex. Even at one month lead time.

In addition to this intrinsic unpredictability of this super-vortex event, another detrimental factor to the predictions of the vortex and its impacts could be that in most of the seasonal forecast systems, including ours, ozone is prescribed with its monthly long term average rather than time-varying realistic information.

However, when we used in our model experiment, this observed 2020 spring ozone that had this big hole over Antarctica, we could obtain about 10 to 20% improvement in predicting the strength of the polar vortex at different vertical levels at one month lead time - as indicated by this darker orange-colour shading in the experimental forecast with the observed ozone compared to the control forecast with the climatological mean ozone.

And significant improvements were found in predicting the subsequent positive SAM and increase summertime rainfall over the eastern part of southeastern Australia at three months lead time. Which highlights a scope for further improvements in the forecast skill for southeastern Australia warm season rainfall, by improving the representation of more realistic ozone feedback in a forecast system.

So this study was recently published in the Journal of Geophysical Research, so please check it out if you are interested in more details of this work.

The second example I'd like to share with you today is the spring climate of 2022- which was the wettest spring on record over southeastern Australia, with its rainfall being double the spring average rainfall and 60% more than the previous record set in spring 1916. As you would remember, this extreme rainfall caused devastating floods across southeast Australia, resulting in human casualties and huge economic losses.

The anomalous large scale climate context for this extreme rainfall was characterised by a moderate La Nina, whose maximum cooling was found over the Date Line in the NINO4 region, and a moderate negative phase of the Indian Ocean Dipole (IOD) with higher SST anomalies in the Eastern Pole and slightly lower SST anomalies in the Western Pole.

And the SAM was in its positive phase with the nearly zonal symmetric higher pressure anomalies in the middle attitude and low pressure anomalies in the high latitudes, which are strongly forced by the stronger-than-normal Antarctic stratospheric polar vortex in spring 2022, as I showed you earlier.

All of these are very well known large scale conditions for a higher than normal rainfall over southeastern Australia in spring. Therefore, some kind of wetter spring over southeastern Australia in 2022 was highly foreseeable.

So when the 9-member ensemble forecast were initialised on the 1st of September 2022, and their ensemble mean forecast was verified for the following September to November mean, the Bureau's dynamic decisional forecast system “XSS2” skilfully predicted the anomalous tropical SST conditions of La Nina and the negative IOD. Although, the Central Pacific cooling of the La Nina was somewhat unpredicted and the negative IOD was quite overpredicted.

XXS2 also skilfully predicted the anomalous extratropical atmospheric circulations with the positive SAM and the stronger than normal polar vortex. Consequently, significantly higher than normal rainfall over Australia was very well predicted by XXS2 at the shortest lead time. However, the ensemble mean forecast for southeastern Australian rainfall anomaly was only 2.5 standard deviations, compared to the observed rainfall anomaly being nearly 4 standard deviations.

Also, the forecast spring rainfall for 2022 was only the second highest following the forecast spring rainfall in 2010 that had much stronger La Nina and stronger negative IOD. So although this extreme rainfall over southeastern Australia in spring 2022 was very well captured by our ensemble mean forecast, it's record-breaking extremity was not.

Interestingly, out of the 9 forecast members, there was one member that predicted the rainfall anomaly being even greater than 4 standard deviations. So we compared this wettest member with the driest member, as shown here with the green and brown bars.

This comparison revealed that the La Nina forecast weren’t very different between them and the negative IOD was even stronger in the driest member than the wettest member. However, the positive SAM was significantly stronger in the wettest member, largely because of this pronounced high pressure anomolies south of Australia.

And the stratospheric polar vortex was much stronger in the wettest member than the driest member. This vortex in the wettest member was significantly stronger than the the observed vortex as well. So these results seem to suggest that the local atmospheric circulation within the positive SAM was important for the rainfall extremity, which could be partly associated with the stronger polar vortex in spring 2022.

So here are some take home messages.

The Antarctic stratospheric polar vortex variability is an important source of long lead predictability for the SAM and associated southeastern Australian rainfall in our warm seasons, which can assist better management of water resources in our region.

And the stratospheric polar vortex variability has been exceptionally large over the past five years, with four consecutive strong vortices in late spring seasons that significantly influenced the surface SAM to be positive, that contributed to wetter-than-normal conditions over southeastern Australia during the recent warm seasons, including the wettest spring on record in 2022 and the wet summer under El Nino in 2023 last year.

And our case study confirmed that the Antarctic ozone offers additional predictability to the same and associated southeastern Australian warm season rainfall by amplifying the vortex and the SAM anomalies.

Currently this ozone and circulation feedback is missing in seasonal prediction systems. So improving the representation of the realistic ozone information and its interaction with atmospheric temperatures and circulations in seasonal forecast systems can improve the forecast skill for southeastern Australian warm season rainfall. Which is not an easy task, but researchers and forecasters together are working on it. So that's all. And thank you so much for your attention.

Page last updated: 05/02/25