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Is Australia’s climate confusing you? Here’s why rainfall and temperatures don’t always behave as expected

<div class="theconversation-article-body"><a href="https://theconversation.com/profiles/carly-tozer-1404662">Carly Tozer</a>, <a href="https://theconversation.com/institutions/csiro-1035">CSIRO</a> and <a href="https://theconversation.com/profiles/james-risbey-2011">James Risbey</a>, <a href="https://theconversation.com/institutions/csiro-1035">CSIRO</a> In the past few years in Australia, seasonal rainfall and temperatures have left a lot of people confused. Sometimes, the hot, dry conditions usually associated with an El Niño have not eventuated. Similarly, there have been years where a La Niña did not lead to the cool, wet conditions expected. It’s important for scientists to better understand all weather processes at play, so we can manage expectations around what Australia might experience when climate drivers such as El Niño and La Niña are forecast in future. That’s where our <a href="https://journals.ametsoc.org/view/journals/mwre/aop/MWR-D-23-0112.1/MWR-D-23-0112.1.xml">new research</a> comes in. We examined the state of play in November 2020 and November 2021. La Niña conditions occurred in both years. November 2021 followed the La Niña script and was wet and cool, but November 2020 departed from expectations and was dry and warm. We set out to determine why. We found the differences could be explained by fluctuations in the path of storms over the Australian continent. These fluctuations can be hard to predict well in advance, which makes it difficult to say for certain how a particular La Niña or El Niño event will affect Australia. <h2>A tale of two Novembers</h2> For large parts of Australia, the presence of a La Niña or El Niño <a href="https://theconversation.com/what-to-expect-when-youre-expecting-an-el-nino-the-answer-might-surprise-you-198510">shifts the odds</a> of experiencing wet or dry conditions. Our analysis of Novembers 2020 and 2021 shows how actual outcomes can differ from, or align with, expectations. The first step in our analysis was to examine <a href="https://theconversation.com/what-does-el-nino-do-to-the-weather-in-your-state-218257">other climate drivers</a>, including the Indian Ocean Dipole and Southern Annular Mode. We wanted to know if these drivers were in the same phase – negative, neutral or positive – during those two Novembers a year apart. So what did we find? In addition to La Nina, both Novembers occurred during positive Southern Annular Mode phases and very weak negative Indian Ocean Dipole phases. These phases are typically associated with more rainfall in Australia. So this didn’t explain why November 2020 was hot and dry. Next, we looked to the <a href="http://www.bom.gov.au/climate/about/?bookmark=mjo">Madden Julian Oscillation</a>. When this driver is located in the Australian region, it has been linked to more rainfall in Australia. Although the oscillation was in different phases during November 2020 and 2021, we found in general, this driver does not strongly influence rainfall across all of Australia in November. It was time to look for answers elsewhere. <h2>Jet streams: a key piece in the puzzle</h2> Next, we examined weather systems – in other words, the movement of high and low pressure systems across the globe. These systems are partly controlled by jet streams, which are bands of wind in the upper atmosphere. The effect of jet streams on weather systems, including storms, means they influence rainfall in the regions they pass over. We found there was a strong jet stream over Australia in November 2021. This would have assisted the development of any rain-bearing low-pressure systems moving in from the west, allowing these systems to travel across the Australian continent. These systems brought rain and contributed to the very wet conditions. In November 2020, the jet stream was largely absent over Australia. Instead, it was pushed south of the continent, which means rainfall systems received little help and were also largely steered south. That contributed to the dry month. But why did the jet streams develop in the first place? They form in part due to temperature differences, and are found in the zones where the temperature contrast between warm and cool air is strongest. In November 2021, Australia experienced cooler temperatures over land, but above-average sea surface temperatures in the waters off northern Australia. This pattern set up the zone of strongest temperature contrasts over the continent, which led to a persistent jet stream there. In November 2020, Australia was relatively warm both over land and on the sea surface to the north. This meant that the strongest temperature contrasts (and the jet stream) now sat at the junction between the warm continent and cooler Southern Ocean. <h2>But wait, there’s more</h2> So why were temperatures over Australia so different? To help answer this question we shifted our analysis from the monthly timescale to the daily timescale. That’s because atmospheric features such as jet streams vary strongly from day to day. We found for about the first three weeks of November 2021, a large low-pressure system – also known as a trough – was sitting south of Australia. It pumped cold air onto the continent, cooling it down. This maximised the north-south temperature difference between the warm sea surface temperatures to the north of Australia and the cool of the continent. And as we know, this aided the development of the jet stream over Australia. In November 2020, the continent started off relatively warm. And for a large portion of the month, there was a large high-pressure system over Australia, pulling warmer air from the tropics over the continent. This system would have also promoted clear skies over Australia and enhanced heating coming from the sun, contributing to the warm Australian continent in November 2020. <h2>More puzzle pieces to fit</h2> November 2020’s hot, dry conditions were not the only time a climate driver has failed to bring the conditions some had anticipated. Just last year El Niño <a href="https://www.theguardian.com/australia-news/2024/jan/03/experts-warned-el-nino-was-likely-to-bring-australia-a-hot-dry-summer-what-happened">did not deliver</a> expected dry conditions, leaving many people scratching their heads. Climate drivers play an important role in shaping rainfall. But they’re not the whole story. As our research shows, sometimes they are confounded by changes in weather patterns, which might mean that our expectations of a wet month or season don’t always play out. When it comes to Australia’s climate puzzle, these findings show there’s more to understand about the role of weather.<img style="border: none !important; box-shadow: none !important; margin: 0 !important; max-height: 1px !important; max-width: 1px !important; min-height: 1px !important; min-width: 1px !important; opacity: 0 !important; outline: none !important; padding: 0 !important;" src="https://counter.theconversation.com/content/233345/count.gif?distributor=republish-lightbox-basic" alt="The Conversation" width="1" height="1" /> <a href="https://theconversation.com/profiles/carly-tozer-1404662">Carly Tozer</a>, Senior Research Scientist, <a href="https://theconversation.com/institutions/csiro-1035">CSIRO</a> and <a href="https://theconversation.com/profiles/james-risbey-2011">James Risbey</a>, Researcher, Oceans and Atmosphere, <a href="https://theconversation.com/institutions/csiro-1035">CSIRO</a> Image credits: Shutterstock This article is republished from <a href="https://theconversation.com">The Conversation</a> under a Creative Commons license. Read the <a href="https://theconversation.com/is-australias-climate-confusing-you-heres-why-rainfall-and-temperatures-dont-always-behave-as-expected-233345">original article</a>. </div>

Domestic Travel

Floods in Victoria are uncommon. Here’s why they’re happening now – and how they compare to the past

Think Victoria and disasters and you’ll think bushfires. But floods can hit – just not as often. Today is one of those days, with much of the state under a flood watch. Premier Dan Andrews says the floods are likely to be the <a href="https://www.abc.net.au/news/2022-10-13/nsw-victoria-tasmania-wet-weather-flooding-rainfall-live-updates/101530310?utm_campaign=abc_news_web&utm_content=twitter&utm_medium=content_shared&utm_source=abc_news_web" target="_blank" rel="noopener">most significant</a> in years. Evacuations are likely. Floodwaters are pushing down the Goulburn to the Murray. Major flooding in the Maribyrnong, which runs through towns and Melbourne’s west. Emergency services say evacuations may be necessary. Towns are sandbagging flood-prone areas. Some have been cut off by rising waters. The state’s largest dam, Dartmouth, is spilling over. So is Lake Eildon’s dam. And the Thomson dam may well spill this weekend, for the first time in decades. This isn’t the last of it – Victoria’s emergency management commissioner Andrew Crisp has warned intense rains and floods could last up to <a href="https://www.3aw.com.au/in-this-for-a-while-emergency-management-commissioners-warning-to-victorians/" target="_blank" rel="noopener">six to eight weeks</a>. Even as the rest of the eastern seaboard has faced the brunt of three consecutive La Niña years, Victoria has had little flooding until now. Tasmania, too, is facing rare flooding, while flood-weary New South Wales is bracing for more. These heavy rains are unusual. Dense cloud bands have crossed the desert, carrying moisture evaporating from seas off north-west Australia. Rain has fallen across almost the entire continent in the last two weeks. Our rain events are usually regional – <a href="https://theconversation.com/on-our-wettest-days-stormclouds-can-dump-30-trillion-litres-of-water-across-australia-191949" target="_blank" rel="noopener">not national</a>. <h2>Why doesn’t Victoria have as many floods?</h2> Victoria’s claim to fame in disasters is that it’s the most <a href="https://insidestory.org.au/we-have-still-not-lived-long-enough/" target="_blank" rel="noopener">bushfire-prone region</a> in the world (followed by California and Greece). Fire risk also comes from climate. Victoria’s temperate climate means dry summers and less rain than its northern counterparts – around 520 millimetres of rain a year falls on average in Melbourne, compared to 1175mm a year in Sydney and 1149mm in Brisbane. Up north, rain tends to fall intensely, whereas Victoria’s rain tends to fall more as drizzle. What’s different this time? September was wetter and colder <a href="http://www.bom.gov.au/climate/current/month/vic/summary.shtml" target="_blank" rel="noopener">than usual</a> in Victoria, which meant the ground was already saturated in many areas. Colder weather means less water evaporates. Together, that made the state primed for floods. For a flood to happen, you need a high rate of run-off, where rain hits saturated soils and flows overland rather than sinking in, as well as intense rains in a short period. Victoria is more familiar with flash floods. That’s because the stormwater drains in cities and towns can be overwhelmed by sudden dumps of rain, flooding streets. The good news is this flooding is usually over quickly, in contrast to the flooded rivers we see up north. This situation may be different. With the state’s major dams beyond capacity or very close to it, water is already spilling over. Dams in Australia are often dual-purpose, storing drinking water and allowing us some control over floods. While Brisbane’s dams are designed with gates to permit floodwater release, Victoria’s dams tend to just have dam walls. When dams overflow, they can add to floods in low-lying areas downstream. There’s also usually a lag time in riverine floods, as it takes hours or sometimes days for rain falling in the headwaters to end up as floodwater downstream. <h2>What floods has Victoria seen before?</h2> The largest was in 1934. More than 140mm of rain fell over two days in Melbourne, and more than double that in Gippsland. The <a href="https://trove.nla.gov.au/newspaper/article/10998182" target="_blank" rel="noopener">enormous flood</a> that followed was most devastating in Melbourne, where the Yarra <a href="https://knowledge.aidr.org.au/resources/flood-melbourne-and-victoria-1934/" target="_blank" rel="noopener">broke its banks</a> and formed a lake from the city out to the outer suburbs. Thirty-six people died, and thousands of people were left homeless. Floods in the capital and in the regions are <a href="https://www.theage.com.au/national/victoria/a-brief-history-of-victorian-floods-20171202-gzxcem.html" target="_blank" rel="noopener">rare but not unknown</a>. In 1891, floods forced more than 3,000 people from their homes in Richmond, Collingwood and Prahran. In 1909, western Victorian rivers broke their banks, flooding many towns and causing four deaths. The <a href="https://www.floodvictoria.vic.gov.au/learn-about-flooding/flood-history/post-2000-floods" target="_blank" rel="noopener">most recent</a> big floods took place during the previous La Niña cycle from 2010-2012, with western Victoria taking the brunt of the damage. Flooding in Victoria has also reduced because people have shifted the course of rivers – particularly the Yarra. In 1879, 2,000 workers began a monster task: removing an entire loop of the Yarra west of the Docklands. One reason? Straight rivers flow faster, meaning floodwaters can discharge more quickly. Engineer John Coode was responsible for designing the new course for the Yarra, which also had the benefit of a wide new channel to improve access for ships. In the process, his workers created what’s now known as Coode Island. In 1896, Victoria’s Parliament passed the <a href="http://classic.austlii.edu.au/au/legis/vic/hist_act/yia1896218/" target="_blank" rel="noopener">Yarra Improvement Act</a> in a bid to reduce the damage caused by floods. Workers widened and deepened the river, and removed billabongs near the Botanic Gardens in the process. In the 1930s, engineers built another channel through an old quarry leading to the creation of Herring Island. These changes were mainly about improving navigation for ships – but they had the double benefit of reducing flooding in the lower reaches. In part, it was about British ideals of what rivers should look like, using highly modified rivers like London’s Thames as a guide. <h2>What’s next?</h2> Changing the course of rivers, raising dams and building levees can make us feel like we’re in control. Unfortunately, it’s not that simple, as <a href="https://theconversation.com/its-time-to-come-clean-on-lismores-future-people-and-businesses-have-to-relocate-away-from-the-floodplains-184636" target="_blank" rel="noopener">Lismore’s residents found</a>. Flood control measures can actually make the impact of <a href="https://theconversation.com/to-stop-risky-developments-in-floodplains-we-have-to-tackle-the-profit-motive-and-our-false-sense-of-security-184062" target="_blank" rel="noopener">large floods worse</a> by giving us a false sense of security about living on floodplains. This is unlikely to be the last flood before La Niña finally relents. It’s worth knowing your state’s history of disasters – so you can be better prepared. After all, we can’t control nature. This article originally appeared on <a href="https://theconversation.com/floods-in-victoria-are-uncommon-heres-why-theyre-happening-now-and-how-they-compare-to-the-past-192391" target="_blank" rel="noopener">The Conversation</a>. Image: Twitter

Travel Trouble

The wild weather of La Niña could wipe out vast stretches of Australia’s beaches and sand dunes

Australians along the east cost are bracing for yet another round of heavy rainfall this weekend, after a band of stormy weather soaked <a href="https://theconversation.com/on-our-wettest-days-stormclouds-can-dump-30-trillion-litres-of-water-across-australia-191949">most of the continent</a> this week. The Bureau of Meteorology has <a href="https://www.youtube.com/watch?v=QUsNQ_-fNbM&ab_channel=BureauofMeteorology">alerted</a> southern inland Queensland, eastern New South Wales, Victoria and northern Tasmania to ongoing flood risks, as the rain falls on already flooded or saturated catchments. This widespread wet weather heralds <a href="https://theconversation.com/la-nina-3-years-in-a-row-a-climate-scientist-on-what-flood-weary-australians-can-expect-this-summer-190542">Australia’s rare third</a> back-to-back La Niña, which goes hand-in-hand with heavy rain. There is, however, another pressing issue arising from La Niña events: coastal erosion. The wild weather associated with La Niña will drive more erosion along Australia’s east coast – enough to wipe out entire stretches of beaches and dunes, if all factors align. So, it’s important we heed lessons from past storms and plan ahead, as climate change <a href="https://theconversation.com/2022s-supercharged-summer-of-climate-extremes-how-global-warming-and-la-nina-fueled-disasters-on-top-of-disasters-190546">will only exacerbate</a> future coastal disasters. <figure><iframe src="https://www.youtube.com/embed/QUsNQ_-fNbM?wmode=transparent&start=0" width="440" height="260" frameborder="0" allowfullscreen="allowfullscreen"></iframe><figcaption>Ongoing flood risk for eastern Australia | Bureau of Meteorology.</figcaption></figure> <h2>How La Niña batters coastlines</h2> La Niña is associated with warmer waters in the western Pacific Ocean, which increase storminess off Australia’s east coast. Chances of a higher number of tropical cyclones increase, as do the chances of cyclones travelling further south and further inland, and of more frequent passages of east coast lows. Australians had a taste of this in 1967, when the Gold Coast was hit by the largest storm cluster on record, made up of four cyclones and three east coast lows within six months. 1967 wasn’t even an official La Niña year, with the index just below the La Niña threshold. Such frequency didn’t allow beaches to recover between storms, and the overall erosion was unprecedented. It <a href="https://impact.griffith.edu.au/seawall-engineering/">forced many</a> local residents to use anything on hand, even cars, to protect their properties and other infrastructure. Official La Niña events occurred soon after. This included a double-dip La Niña between 1970 and 1972, followed by a triple-dip La Niña between 1973 and 1976. These events fuelled two cyclones in 1972, two in 1974 and one in 1976, wreaking havoc along the entire east coast of Australia. Indeed, 1967 and 1974 are considered <a href="https://www.dailytelegraph.com.au/newslocal/northern-beaches/one-of-the-storms-that-hit-us-in-1974-was-among-the-three-worst-since-white-settlement/news-story/0cd5ca874d6b37206762d8485e4eb442">record years</a> for storm-induced coastal erosion. Studies show the extreme erosion of 1974 was caused by a combination of large waves coinciding with <a href="https://ro.uow.edu.au/cgi/viewcontent.cgi?article=1066&context=scipapers">above-average high tides</a>. It took over ten years for the sand to come back to the beach and for <a href="https://www.jstor.org/stable/4300263">dunes to recover</a>. However, <a href="https://www.nature.com/articles/s43247-022-00437-2">recent studies</a> also show single extreme storms can bring back considerable amounts of sand from deeper waters. La Niña also modifies the direction of waves along the east coast, resulting in waves approaching from a more easterly direction (<a href="https://www.surfline.com/surf-news/forecaster-blog-la-nina-conditions-mean-surf/97904">anticlockwise</a>). This subtle change has huge implications when it comes to erosion of otherwise more sheltered <a href="https://au.news.yahoo.com/how-la-nina-may-damage-queensland-tourist-hot-spots-041805874.html">north-facing beaches</a>. We saw this during the recent, and relatively weaker, double La Niña of 2016-18. In 2016, an east coast low of only moderate intensity produced extreme erosion, similar to that of 1974. Scenes of destruction along NSW – including a collapsed backyard pool on <a href="https://www.wrl.unsw.edu.au/news/wrl-coastal-engineers-document-the-worst-erosion-at-collaroy-since-1974">Collaroy Beach</a> – are now <a href="https://www.abc.net.au/news/2016-06-06/nsw-weather-large-waves-hit-collaroy-coast/7479846#:%7E:text=NSW%20weather%3A%20Collaroy%20swimming%20pool%20collapses%20as%20giant%20waves%20hit%20beachfront%20houses,-Posted%20Sun%205&text=Waves%20up%20to%208%20metres,as%20wild%20weather%20battered%20NSW.">iconic</a>. <a href="https://www.nature.com/articles/s41598-017-05792-1">This is largely</a> because wave direction deviated from the average by 45 degrees anticlockwise, during winter solstice spring tides when water levels are higher. <h2>All ducks aligned?</h2> The current triple-dip La Niña started in 2020. Based on Australia’s limited record since 1900, we know the final events in such sequences tend to be the weakest. However, when it comes to coastal hazards, history tells us smaller but more frequent storms can cause as much or more erosion than one large event. This is mostly about the combination of storm direction, sequencing and high water levels. For example, Bribie Island in Queensland was hit by relatively large easterly waves from ex-Tropical Cyclone Seth earlier this year, coinciding with above-average high tides. This caused the island to <a href="https://www.abc.net.au/news/2022-01-27/bribie-island-changes-could-create-new-caloundra-bar/100777038#:%7E:text=Ex%2DTropical%20Cyclone%20Seth%20has,splitting%20the%20island%20in%20two.">split in two</a> and form a 300-metre wide passage of seawater. Further, the prolonged period of easterly waves since 2020 has already taken a toll on beaches and dunes in Australia. Traditionally, spring is the season when sand is transported onshore under fair-weather waves, building back wide beaches and tall dunes nearest to the sea. However, beaches haven’t had time to fully recover from the previous two years, which makes them more vulnerable to future erosion. Repeated <a href="https://www.usc.edu.au/about/structure/schools/school-of-science-technology-and-engineering/coast4d">elevation measurements</a> by our team and citizen scientists along beaches in the Sunshine Coast and Noosa show shorelines have eroded more than 10m landwards since the beginning of this year. As the photo below shows, 2-3m high erosion scarps (which look like small cliffs) have formed along dunes due to frequent heavy rainfalls and waves. On the other hand, we can also see that the wet weather has led to greater growth of vegetation on dunes, such as native spinifex and dune bean. Experiments in laboratory settings show dune vegetation can dissipate up to 40-50% of the water level reached as a result of waves, and <a href="https://www.sciencedirect.com/science/article/abs/pii/S0272771418307583">reduce erosion</a>. But whether this increase in dune vegetation mitigates further erosion remains to be seen. <h2>A challenging future</h2> The chances of witnessing coastal hazards similar to those in 1967 or 1974 in the coming season are real and, in the unfortunate case they materialise, we should be ready to act. Councils and communities need to prepare ahead and work together towards recovery if disaster strikes using, for example, sand nourishment and sandbags. Looking ahead, it remains essential to further our understanding about coastal dynamics – especially in a <a href="https://www.abc.net.au/news/2022-09-03/gold-coast-no-stranger-to-beach-erosion/101381812#:%7E:text=a%20huge%20challenge-,Millions%20spent%20to%20protect%20Gold%20Coast%20beaches%2C%20but,change%20poses%20a%20huge%20challenge&text=In%201967%2C%20Gold%20Coast%20beaches,and%20ruined%20the%20tourist%20season.">changing climate</a> – so we can better manage densely populated coastal regions. After all, much of what we know about the dynamics of Australia’s east coast has been supported by coastal monitoring programs, which were implemented <a href="https://www.publications.qld.gov.au/dataset/coastal-observation-program-engineering">along Queensland</a> and NSW after the 1967 and 1974 storms. Scientists predict that La Niña conditions along the east coast of Australia – such as warmer waters, higher sea levels, stronger waves and more waves coming from the east – will become <a href="https://theconversation.com/climate-fuelled-wave-patterns-pose-an-erosion-risk-for-developing-countries-184064">the norm under climate change</a>. It’s crucial we start having a serious conversation about coastal adaptation strategies, including implementing a <a href="https://www.usc.edu.au/about/unisc-news/news-archive/2022/january/coastal-erosion-may-force-retreat-from-the-sea#:%7E:text=Giving%20up%20land%20to%20the,of%20the%20Sunshine%20Coast%20researcher.">managed retreat</a>. The longer we take, the higher the costs will be.<img style="border: none !important; box-shadow: none !important; margin: 0 !important; max-height: 1px !important; max-width: 1px !important; min-height: 1px !important; min-width: 1px !important; opacity: 0 !important; outline: none !important; padding: 0 !important;" src="https://counter.theconversation.com/content/191941/count.gif?distributor=republish-lightbox-basic" alt="The Conversation" width="1" height="1" /> <a href="https://theconversation.com/profiles/javier-leon-262182">Javier Leon</a>, Senior lecturer, <a href="https://theconversation.com/institutions/university-of-the-sunshine-coast-1068">University of the Sunshine Coast</a> This article is republished from <a href="https://theconversation.com">The Conversation</a>. Read the <a href="https://theconversation.com/the-wild-weather-of-la-nina-could-wipe-out-vast-stretches-of-australias-beaches-and-sand-dunes-191941">original article</a>.

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Is Australia’s climate confusing you? Here’s why rainfall and temperatures don’t always behave as expected

Floods in Victoria are uncommon. Here’s why they’re happening now – and how they compare to the past

The wild weather of La Niña could wipe out vast stretches of Australia’s beaches and sand dunes

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