Well what a year 2021 is turning out to be! After a very dry start to sowing, an ok to good winter, spring was rather disappointing until the welcome rain last week.
So, what on earth has been going on?
Back in autumn when the models have their lowest, but not no skill, all the influential climate drivers for north-west Victoria were neutral, so anything was possible. Unfortunately for many, when we threw all the possible combinations up in the air it was ‘dry conditions’ that fell on our lap for autumn.
As we entered winter the Indian and Southern Oceans began to emerge as regions to pay attention to. Sea surface temperatures in the eastern parts of the Indian Ocean were warming up, conditions which are typically conducive for moisture to be generated. At the time however we were lacking the vehicles (wind) and hitting too many road blockages (high pressure over central Australia) for this moisture to travel over to us.
In the Southern Ocean, the Southern Annular Mode also known as SAM spent some time in the negative phase. SAM is an index used to monitor the position of a belt of westerly winds that flow from west to east between Australia and Antarctica. When SAM is in the negative phases during winter, these winds and the cold frosts and low-pressure systems they carry, are located further north than usual for that time of year meaning they can often pass over mainland Australia instead of slipping south. It was the combination of the negative SAM, the sub-tropical ridge being located further north than its normal winter position and some rare pressure patterns that allowed our season to get back on track.
By mid-winter action in the Indian Ocean began to heat up further with some somewhat encouraging signs of a negative Indian Ocean Dipole (-IOD) forming. The IOD measures the difference in the sea surface temperatures between the tropical west (Horn of Africa) and the eastern Indian Ocean (coast of Sumatra). When the IOD is in the negative phase the sea surface temperatures are typically warmer off the coast of Sumatra and cooler off the Horn of Africa resulting in conditions conducive for moisture to be generated to the north-west of Australia. However for this moisture to move, the pressure and cloud patterns need to be favourable as well as strong trade winds to help move this moisture across Australia.
During late winter and into spring a -IOD was officially declared however it continued to tease us as not all ingredients that make up a successful -IOD were present (namely the pressure patterns) and so this event ended up as a bit of a ‘flop’ in terms of delivering ‘above average’ rainfall to our region.
That was until the very last week of September when a significant front developed bringing rain from across the Southern and Indian Oceans to eastern Australia. The front was trapped in this region by a ‘jellybean’ shaped high-pressure system off the south-east coast of Australia. This saw some significant rainfall totals in our region.
Will the rain continue?
Well the -IOD event is weakening and likely to breakdown over the coming weeks. However the sea surface temperatures in the Indian Ocean as a whole continue to remain warmer than normal, which is favourable for generating moisture. SAM is now also in a positive phase which, contrary to winter, in this position helps generate and bring moisture generated in the Tasman and Coral Seas across to Eastern Australia. The BoM’s ENSO Outlook status has also recently been updated to La Niña WATCH and in the past this has resulted in a La Niña forming around 50% of the time (double the normal likelihood). If a La Niña does develop then the odds of a milder and wetter finish to the season is much more likely, although there is still a chance that we get dealt the ‘below average’ rainfall card again.