La Nina: What does it mean?

On Tuesday, 29 September 2020, the Australian Bureau of Meteorology (BoM) announced that a La Niña event is now underway.
So, what defines a La Na?
A La Niña is the positive phase of the El Niño Southern Oscillation (ENSO) which is one of several climate drivers that influences Australia’s climate. In addition to positive, the ENSO can be neutral as well as negative (El Niño).
For a La Niña event to occur certain changes in our atmosphere and the Pacific Ocean need to happen. These include:

  • Stronger than usual trade winds
  • Cooler sea surface temperatures in the central and eastern Pacific Ocean
  • Warmer than normal water over the western tropical Pacific Ocean
  • Increased convection or cloudiness in the central tropical Pacific Ocean
  • The southern oscillation index (SOI), a measure of pressure differences between Tahiti and Darwin, needs to remain positive for several consecutive months

Figure 1 provides a visual summary of the how the ocean temperatures and atmospheric conditions interact to result in the formation of a La Niña event.

What have we seen in the past during a La Niña?
During a La Niña, wetter than average conditions are likely over northern and eastern Australia during winter, spring and early summer.

However, every La Niña event is different, they don’t always follow the classic pattern of showing signs in autumn and strengthening in over winter.

Some La Niña events last just a few months whereas others can run across multiple seasons. It should also be noted that in the regions of Australia where a La Niña typically brings wetter than average conditions, there is still a chance of rainfall being average or below average.

This is evident when looking at how climate drivers such as the ENSO have influenced seasonal rainfall in the past. Using the Forecasts for Profit Local Climate Tool developed by Agriculture Victoria, Federation University & SARDI with funding from the GRDC, historical records from Birchip have demonstrated that since 1990 there have been 30 years when a La Niña event has taken place and of those approximately half were wetter than average for the month of October and for October to December (Fig 2).

In more recent times (since 1960) there have been 14 La Niña years. Half of those years were wetter than average for October, however for October to December nine out of the 14 La Niña years were wetter than average (Fig. 3).

Note due to changes in our climate, particularly in more recent years the classic investment fund disclaimer of “past performance may not always be a reliable indicator of future performance” should be considered when looking over historical records.

What does this mean for Wimmera and Mallee farmers?

There is an increased likelihood that we will experience a wetter finish for the year with a small chance that it could be drier. If one of Australia’s other climate drivers, the Indian Ocean Dipole (IOD) shifts from its current neutral state to weakly positive during October, and several climate models are predicting this to be the case, then this would strengthen the wetter finish for 2020.

Unfortunately, if this outlook delivers it maybe too late for some winter crops, particularly the earlier maturing varieties and it could also become a nuisance around harvest time. However, there may be opportunities to capitalise on this outlook by exploring summer cropping options and maximising the storage of any rainfall during late spring and summer into the soil for next year’s cropping season.

If you would like to see how past La Niña events have performed in your region check out the Local Climate Tool.

To learn more about the different climate drivers check out the Climatedog videos on the Climate Kelpie website.

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