Dancing with La Niña

The Bureau of Meteorology (BOM) has declared Australia is experiencing ‘La Niña’. How will it affect Australian weather?

La Niña (La Neen-ya, meaning ‘little girl’ in Spanish) and her sibling, El Niño (El Neen-yo, ‘little boy’), are part of the naturally occurring El Niño-Southern Oscillation (ENSO) climate pattern. In a process described as a “dance between ocean and atmosphere”, the ENSO cycle drives movement of heat and moisture from the eastern Pacific Ocean towards Australia through a combination of sea surface temperature and air pressure changes.

La Niña is generated by a build-up of stronger than average easterly trade winds, together with a corresponding decrease in sea temperature off the tropical west coast of South America. As the strong winds push warm water to the west, cold water is drawn up from the deeper ocean towards the surface, cooling the atmosphere above and causing it to descend without forming clouds or rain. Conversely, the enhanced trade winds pile up warm surface waters in the western Pacific and off Australia’s east coast. Warming ocean temperatures heat the atmosphere, causing the air to rise and develop into moisten-laden clouds that deliver intense storms with above-average rainfall.

FREQUENCY AND STRENGTH

La Niña events occur at irregular intervals of between two and seven years. The last major La Niña event in Australia spanned the summers of 2010–12, resulting in the wettest two-year period on record, with widespread flooding in many parts of the country and devastating cyclones in the tropical north, including category five Yasi.

The current La Niña developed in the eastern Pacific during autumn–spring 2020 and peaked in December/January. Its effects are expected to continue into at least autumn 2021, although some La Niña events have been known to last more than two years. While not predicted to be as extreme as in 2010–12, BOM modelling suggests this year’s La Niña will be ‘moderate to strong’.

 LA NIÑA IN 2021 

How will La Niña affect Australia’s weather in 2021?

Typical of her ilk, La Niña is likely to bring above-average rainfall in central, northern and eastern Australia, with an increased risk of widespread flooding. This will be all the more significant for eastern Queensland and northern NSW because December–March are usually the wettest months for these regions. 

Increased rainfall is likely to be exacerbated by two other factors: the early arrival of the wet season monsoon in the northern tropics, and an expected increase in the number of cyclones.

During an average cyclone season (December–April), between nine and 11 cyclones impact Australian waters, with around four crossing the coast. Under the influence of La Niña, warmer than average ocean temperatures to the north of Australia and more intense convection are expected to generate a greater number of tropical cyclones, with the first arriving earlier in the season and more of them making landfall. Even tropical lows that do not intensify into cyclones, or lows that are the remnants of older cyclones, may present an increased risk of major damage through strong winds, heavy seas and tidal surges.

The north-west can usually expect six to seven tropical cyclones each year, but La Niña could generate at least this many, if not more, with some affecting the south-west coast of Western Australia.

Wetter conditions, increased cloud cover and higher humidity during La Niña are expected to result in cooler daytime temperatures and fewer extreme highs across most of the mainland south of the tropics, but warmer than average overnight minimums across northern and eastern Australia. Summer heat waves in south-east coastal regions are likely to be less intense but will last longer because weather systems will tend to move more slowly.

The shift in temperature extremes and above-average rainfall may reduce the bushfire risk somewhat, but are not likely to eliminate it. The Bureau of Meteorology suggests that the fires may be smaller and shorter than the long-campaign outbreaks that have occurred in recent years.

Increased rainfall, greater cloud cover and generally cooler weather along the coast will not make great conditions for a summer beach getaway. Stronger easterly trade winds could lift east coast swells, making it more dangerous for swimmers while delivering bigger waves for board riders. Cyclonic conditions and flood-borne debris will pose a danger to vessels in coastal waterways.

For the latest information on La Niña, visit the Climate Driver Update and ENSO Outlook, updated every fortnight at bom.gov.au/climate/enso. 

PREPARE YOUR VESSEL

As La Niña is tipped to deliver some heavy weather to coastal waters around Australia. Here are some things you can do to prepare your vessel and guard against storm damage before it occurs.

• Check the forecast — always check the marine weather forecast before taking your boat out.
• Storm Lines — ensure docking lines are of an adequate size, and double-up with a second, stronger set.
• Tying off — never depend on the same tie-up point. Tie-off the second set of lines to a different cleat or anchor point.
• Chafe guards — locate the chafe points on your boat and protect your lines with chafe guards.
• Swing Moorings — if you’re on a swing mooring, make sure it’s adequate for your boat. 
• Anchoring — choose anchorages with sandy or soft mud bottoms and ensure that the size of your anchor is adequate for your boat. If possible, use a second anchor.
• Eliminate drag — reduce sails and rigging as far as possible to reduce drag from windage and strain on mooring lines and anchors.
• Loose gear — remove, stow or tie down any loose gear.
• Gather information — three vital sources are the Bureau of Meteorology (bom.gov.au), your local maritime authority or council (for local cyclone procedures or protocols), and fellow boat owners or marina staff in the area.