The Australian Bureau of Meteorology (BOM) has officially put La Nina on a watch status for the second year running. This means there is around a 50% chance (double the normal chance) of the phenomenon re-forming in 2021.
This is seconded by the US National Oceanic and Atmospheric Administration (NOAA) climate prediction centre forecasting a 70% chance of La Nina forming for the second consecutive year.
The Summer and Autumn of 2020 – 2021 were the wettest and coolest experienced in Australia for the past 5 years, and this was driven by the La Nina effect. Although not as extreme as the 2011 La Nina event, which was one of the strongest la Nina’s seen on record, we would expect that if it does form again this year it would most likely follow last year’s pattern.
There are other drivers looking to encourage a wet summer as well.
The lesser known Indian Ocean Dipole (IOD) is the temperature difference between the Eastern and Western Indian oceans. A positive phase of the IOD would lead to less rainfall and a negative IOD leads to more rainfall in the South and East of Australia throughout Spring.
In 1974 a negative IOD occurred with a strong La Nina and Australia recorded the wettest year on record. Although the two can coincide it is a complicated relationship and a significant area of research within the Climate community.
This year the latest BOM outlook has 3 of the 5 international climate models moving the IOD negative from October. Although not sustained into 2022 and a weaker index than some previous years, a negative IOD would still be expected to increase the chance of a wet Spring. The weakening of the index into the Summer is a standard trajectory for the IOD.
On the other side of the continent, the Southern Annular Mode (SAM) (related to the Antarctic Oscillation (AAO)) is showing a slight positive indicator. This correlates with a strengthening La Nina index as the two tend to be more correlated than other indices. A positive SAM will lead to more rainfall in the East and reduces the chance of a hot Spring and Summer. Although as per the La Nina index these are not strong signals and could therefore change going into the later Summer and are mainly Spring indicators at the moment.
A final influence could come from the Madden-Julian Oscillation; however, these can only reliably be forecasted 14 days in advance as they pass around the planet every 30 to 60 days. If one forms you can expect North-Eastern Australia to experience significant rainfall as the system passed into the Pacific. They can bring monsoons and tropical Cyclones if occurring in the Spring and Summer and if this forms at a period of heavy rainfall we can expect it will significantly increase the flooding risks from the ground which is already saturated.
So, what does that do to our Summer? Well for us on the Eastern Coast it means don’t put away the gumboots and rain mac just yet. It is likely to be a rather wet Spring and depending on the development of some systems it could go all the way through our Summer. It also means we can expect the Spring to have cooler days across Eastern and Southern Australia and increased cloud cover leading to milder evenings.
From a generation point of view, increased cloud cover could lead to reduced Solar capacity for both large scale and roof-top systems and this will likely be coupled with below average winds in the Southern NEM. There is a winner though, the Hydro plants in NSW and Victoria will be happy campers, if not a little sodden, with a higher than average rainfall expected in those catchments this Spring.
Article Written by Kate Turner Senior Manager Markets, Analytics & Sustainability