Earlier this week, Perth had its coldest day in nine months, with the maximum temperature of 15 degrees, falling 7 degrees below the May average. A cold Antarctic air mass moved up from the Southern Ocean, bringing the cool air and freezing winds to southwestern WA. The cool blast has many people in the Wholesale Electricity Market (WEM) wondering, just how cool is this winter going to be?
While the models are predicting a near average winter, the days are certainly going to feel a lot cooler than the previous few winters, with more cool days forecast (figure 1). Overall, Weatherzone’s model OptiCast is forecasting eleven very cool days (maximum temperature below 16C), which is slightly warmer than the average of twelve days.
Figure 1: Perth winter days below 16C, showing observations for the past three winters, the average and the forecast for this winter.
Overnight temperatures are expected to be less chilly overall, thanks to the potential for increased cloud cover across Western Australia. Cloud cover effectively acts as a ‘blanket’ overnight trapping the heat in.
So, what is driving the temperature forecasts this winter?
As we approach winter and our descent into the cooler months, there are currently no strong climate drivers in play near Australia.
The La Niña pattern that influenced Australia’s weather during the last six months officially ended in late-March. This means the Pacific Ocean is currently in a neutral phase and it is likely to remain this way until at least the end of winter.
On the other side of the country, the Indian Ocean Dipole (IOD) has also been in a neutral pattern all year. As a result, the Indian Ocean has not played an abnormal role in determining Australia’s weather in recent months.
There are early signs that a negative phase of the IOD may emerge as we head into winter, with 3 out of the 5 climate models predicting it. While this climate driver is notoriously difficult to predict at this time of year, this should not be ignored.
A negative IOD would increase the likelihood of normal to below normal daytime temperatures due to increased cloud cover (figure 2), although the nights may be slightly warmer for this reason.
Figure 2: Bureau of Meteorology’s Negative IOD June to November maximum temperature deciles composite map.
In addition, a negative IOD can bring normal to above normal precipitation to Western Australia (figure 3), which is in line with the European model’s forecast this winter. However, the Australian model is predicting drier conditions across southwestern Western Australia, which is against the negative IOD scenario. In the following months, we will be watching the IOD closely and providing seasonal updates on the consequence for temperatures and rainfall this winter.
Figure 3: Bureau of Meteorology’s Negative IOD June to November rainfall deciles composite map.
The other main climate driver that influences Australian temperature, wind power and rainfall is the Southern Annular Mode (SAM). The SAM is an index that measures the north-south displacement of the westerly winds that flow between Australia and Antarctica.
When the SAM is in a negative phase, westerly winds and cold fronts are located further north than usual, which typically causes more cold outbreaks and increased wind power and rainfall for southern Australia. On the other hand, positive SAM phases can limit cool outbreaks, wind power and rainfall in Australia during winter. If a phase of negative SAM and negative IOD combines this winter, WA could see an increased number of cool outbreaks and rainfall events as well as windy periods.
Unfortunately, the SAM is much harder to predict at the seasonal scale compared to the other two climate drivers mentioned above. The SAM has just shifted into a negative phase which is expected to continue through the next fortnight. The SAM can only be predicted up to two weeks out, particularly when there is no major climate driver in the mix.
As the season unfolds, we will be keeping a close eye on the horizon here at Weatherzone, with our fourteen-day temperature, rainfall, and wind power forecasts. For more information, please contact us at firstname.lastname@example.org.