THE CLIMATE KELPIE BLOG: What La Niña means for Australia this summer

Posted by BCG on 7th December 2020

After months of uncertainty, La Niña conditions have fully developed and settled across the Pacific Ocean. So, what exactly is La Niña, and what does it mean for farmers across Australia?

Technically, La Niña is the positive phase of the El Niño Southern Oscillation (ENSO). It is associated with cooler-than-average sea surface temperatures in the central and eastern tropical Pacific Ocean. La Niña events tend to begin in autumn, mature during winter, spring and early summer, then begin to decay in late summer. Events generally end in the autumn of the following year. The greatest impact normally occurs during the winter, spring and early summer period.

During La Niña, ENSO ‘chases’ greater amounts of moist tropical air across Australia, often bringing lower temperatures and higher winter and spring rainfall across much of the country. For example, this year’s La Niña brought welcome relief from a long and crippling drought that peaked with the devastating bushfires across much of the eastern seaboard.

But there can also be some potentially less welcome impacts of La Nina, particularly for Northern Australia. While first rains of the northern wet season tend to arriver earlier during La Niña years, they tend to also bring an increased chance of severe weather events including floods and tropical cyclones.

To help farmers across Australia understand and prepare for the potential impacts of La Nina, the Bureau of Meteorology’s Severe Weather Outlook evaluates the likelihood of weather extremes like flooding, heatwaves and dangerous bushfire weather over the months ahead.

An increased risk of cyclones for northern Australia

This year the Bureau’s Tropical Cyclone Season Outlook model predicts a 66 per cent chance of an above-average number of tropical cyclones in the Australian region.

Typically, between nine and 11 tropical cyclones form each season in the Australian region, with around four crossing Australia’s coastline. In fact, there has never been a season without at least one tropical cyclone crossing the coast. In recent decades we have seen fewer tropical cyclones in the Australian region, which is consistent with the expected impacts of climate change.

The number of tropical cyclones can vary a lot between years, and this can be caused by several factors. One of the main ones is the temperature of waters to the north of Australia, as warm ocean temperatures are the fuel for tropical cyclones.

One of the strongest tropical cyclones to make landfall on the Queensland coast was tropical cyclone Yasi. It coincided with the last significant La Niña in 2010-12.

Video: Severe weather outlook for the 2020–21 season.

More rain and greater risk of floods

La Niña can cause changes to much more than just the number of tropical cyclones. The biggest impact is to rainfall, with large parts of northern and eastern Australia typically having much more rain than normal (Figure 1). This outlook increases the risk of widespread and prolonged flooding over large parts of eastern and northern Australia especially in Queensland, New South Wales and Victoria.

Figure 1. For large parts of Australia, a La Niña means a high chance of exceeding normal rainfall from November 2020 to January 2021. Source: Bureau of Meteorology.

The weather patterns associated with La Niña can lead to repeated flooding in the same location or very long duration flooding. In Queensland in the 2010-11 season, some rivers and townships experienced several flood peaks that progressively increased in size over the course of the summer (Gympie, Roma, Dalby and Goondiwindi) and other locations remained at high flood levels for weeks (Rockhampton, St George and Theodore).

The potential for intense rainfall also increases risks of flash flooding in urban area and fast-responding streams.

Ex-tropical cyclones and tropical lows can also bring large amounts of rainfall inland. In 2010-11, moisture from ex-tropical cyclones Anthony and Yasi made it all the way to Victoria, contributing to flooding in the west of the state.

This periodic flooding can cause disruption to inland communities and agriculture. But it’s also an important recharge for the environment, such as watercourses through Queensland’s Channel Country, and Kati Thanda-Lake Eyre.

Normal potential for extreme heat and bushfire

As we saw over 2019-20 season, severe weather, such as bushfires and heatwaves can have devastating impacts on the Australian community. The climate influences during the last two summers favoured drier conditions over Australia. Long-term drought and warmer conditions increased fire risk.

This season the pendulum has swung toward wetter conditions. La Niña decreases the risk of large-scale and prolonged bushfires (Figure 2). Instead it favours above-average rainfall for much of Australia.

The climate outlooks show that with the notable exception of NSW, much of Australia has normal fire potential in the months ahead. This contrasts with the very elevated bushfire risk that we experienced in the preceding three summers.

Figure 2. The seasonal bushfire outlook for December 2020 to February 2021 shows a normal outlook for much of Australia, with the notable exception of NSW. Areas shown in red have above-normal fire potential. Source: Bushfire and Natural Hazards CRC/Australasian Fire and Emergency Service Authorities Council

When we talk about a ‘normal’ season, it’s important to remember that southern Australia is one of the most fire-prone places on earth. There has been a long-term increase in extreme fire weather and in the length of the fire season. This season, though there will be fires, the prevailing conditions are unlikely to support fires as severe and as extensive as those we saw last season.

However, a wetter spring can lead to greater grass and vegetation growth. It only takes a few weeks of hot, dry and windy weather to raise the fire risk, so keep an eye on your local fire forecasts and warnings.

Summer also brings heatwaves, which are Australia’s most deadly type of severe weather. Warm nights, after hot days, gives both human and livestock bodies little chance to cool down and recover, and heat stress becomes a significant health risk.

This summer, rain and increased humidity thanks to La Niña means we may face fewer extreme heat days across the country compared to recent years. However, in some parts of southern Australia, while heatwaves may not reach the extreme temperatures of the past few years, they are likely to last longer and be more humid. Heat load is cumulative, and both of these allow heat stress to build up in livestock.

More information

Severe weather presents a particular challenge for agriculture. It’s important to know your weather and know your risk – so you’re ready to act and stay safe. Always follow the advice from emergency services on what to do before, during, and after severe weather.

You can stay up to date with the latest forecasts and warnings on the Bureau website and subscribe to receive climate information emails.

More information

agriculture@bom.gov.au
Bureau of Meteorology climate outlook

This article was adapted from the Bureau of Meteorology Blog. Read more or subscribe at BOM Blog.

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