The Bureau of Meteorology has reported that a La Niña has developed in the Pacific Ocean, leading to an increased chance of above-average rainfall across much of Australia estimated to continue into early autumn 2022. For many areas in Australia this is a sharp contrast to the past years of drought and floods are wreaking havoc throughout central Australia, New South Wales, and Victoria.
In Tasmania we are a long way from the tropics in the Pacific Ocean, so how does a tropical weather event influence growing conditions here?
First what is La Niña?
La Niña and her brother El Niño are events known as the El Niño — Southern Oscillation (ENSO). ENSO is a major change in ocean temperature, air pressure, and rainfall patterns between the Australian/ Indonesian region and the eastern Pacific Ocean.
El Niño was first recognized by fishermen off the coast of South America in the 1600s when unusually warm water appeared at the surface in the eastern tropical Pacific Ocean.
The name El Niño means The Little Boy, or Christ Child in Spanish, and was chosen based on the time of year (around December) when these warm-water events often reached their peak.
La Niña is El Niño's opposite phase when the eastern tropical Pacific Ocean is much cooler than normal.
La Niña means The Little Girl in Spanish.
La Niña events occur about every three to seven years. They are caused by stronger trade winds along the equator that change ocean surface currents and draw cooler deep water up from below. This results in a cooling of the central and eastern tropical Pacific Ocean. The stronger trade winds also help to pile up warm surface waters in the western Pacific and to the north of Australia.
The warming of ocean temperatures in the western Pacific means the area becomes more favourable for rising air, cloud development and rainfall. As a result, heavy rainfall can occur to the north of Australia.
So, heavier rainfall in the north… what about Tasmania?
Cloudiness and heavy rainfall to the north of Australia typically leads to higher-than-average rainfall in northern, eastern and central parts of the county with below-average daytime temperatures south of the tropics. In past La Niña events Tasmania has been no exception, experiencing higher than average rainfall in the northeast of the state. However, data from past years shows that La Niña’s effect in Tasmania is strong in spring but drops off during summer.
The above map displays this. All of Tasmania experiences higher than average rainfall for spring, however during summer only minor rainfall increases are seen in the states north.
La Niña also impacts the oceans around Tasmania. As the warmer water builds up in the western Pacific Ocean warm water is pushed between the islands of Indonesia and down the Western Australian coast. Several months later this warm water reaches Tasmania increasing ocean temperatures.
This years La Niña event is forecasted to bring higher than average rainfall to Tasmania particularly to the east of the state causing wetter soils and higher rivers therefore increased flood risk. Increase fire risk is also predicted due to more vegetation growth. And on the temperature front, heat waves are likely to be less intense but longer with higher humidity.
La Niña’s in the past
La Niña events have occurred in Australia 15 times since 1910, the most recent of these events was in the 2020-21 season. This makes this season the 2 consecutive season a La Niña has been declared. This however is not unusual and has occurred frequently in the past 100 years.
Significant La Niña events in the past include the longest sustained period of La Niña on record from 1973 to 1976. Many records where made during this period with 1974 still standing as Australia’s wettest year on record. Cyclone Wanda that caused flooding through Brisbane was also January of 1974.
In more recent history 2010-12 were two big La Niña events in a row. 2010 and 2011 combined are the wettest 24 months on record characterised by floods up and down the east coast including flash flooding in Tasmania’s east and through the Midlands.
However, the 2021-22 season is thankfully not expected to be as intense as these past events.
Is wet good?
Around Tasmania, the grass is high and green thanks to La Niña. The wet mild days that we have had recently means that there is a lot of vegetation growth. Is all the wet good for our farmers?
Many livestock farmers would say it has been a good season. Commodity prices are high especially for red meat and dairy. And there is plenty of feed to go around. As the rest of the country looks to restock after the drought, many Tasmanian producers are in a good financial position.
However, other Tasmanian farmers are finding the rain a challenge. It is currently potato planting season, and the continuing wet conditions are delaying planting for many growers causing concern for supply from the 2022 harvest.
The wet conditions from La Niña are a mixed bag for Tasmanian producers.
Planning my irrigation around La Niña
During a La Niña spring talking about irrigation seems almost ludicrous. Many farmers are struggling to manage waterlogged soils, runoff and flooding, adding water is not high on their list of considerations. Therefore, the focus is on pumping and capturing water so damage to crops and farm infrastructure can be managed.
However, with the end of spring comes drier days and long heat waves. Although heat waves are likely to be less extreme, they are predicted to last longer. Therefore, during a La Niña season, the best preparation is to capture and store as much water as possible throughout the spring while being aware that long heatwaves where that water is needed are likely to come during summer. Many farmers will also be watching their soil moisture closely aware that waterlogged soils can drain and dry out quickly after a few clear, windy days.
Past seasons have shown that on La Niña years irrigation is likely to start later and be required longer into autumn as the rainfall from La Niña subsides.
Information from the Australian Government Bureau of Meteorology http://www.bom.gov.au/, Australian Government Grains Research and Development Corporation https://grdc.com.au/news-and-media/, Australian Broadcast Corporation https://www.abc.net.au/