A Strange November of Restricted Rainfall
Australia and Europe have an unusual parallel this month: Both have experienced shortages of rainfall across most of their land area.
Below-left, I’ve saved a snapshot of the Australian Bureau of Meteorology’s analysis to 23rd November. It’s quite something to see rainfall at 0-40% of the average across most of such a vast country.
What’s more, this is happening during a moderate strength La Niña event (as confirmed by the BOM). This event typically sees anomalously warm water in the tropical waters just northeast and east of Australia.
That additional ocean warmth means more water evaporating into the atmosphere, which can mean heavier rainfall in the north and east of the country. You can see this pattern in a composite of all La Niña Novembers since 1970, shown below-right.
With that comes an increased risk of event cancellation due to excessive rainfall (either directly or via flooding).
This year, though, things are a little different. The water to the northeast of Australia has a mix of warm and cool anomalies (see below). Meanwhile, the water to the southeast is much warmer than average. It’s also unusually warm to the west of Australia.
Sea surface temperature (SST) anomalies observed 22nd Nov 2020. Adapted from the global coverage map sourced at https://psl.noaa.gov/map/clim/sst.shtml.
For the north and east, what I believe to be key here is the lack of a standout area of anomalous warmth associated with the La Niña event. It’s been ‘masked’ by above-normal sea temperatures occurring widely in the Pacific and Indian Oceans.
Elsewhere in the country, an exceptionally strong polar vortex is likely the main culprit. This has caused low pressure systems in the Southern Ocean to circulate Antarctica in a tighter ‘orbit’, further south than usual. That’s allowed high pressure to dominate the weather across most of Australia.
Will the ‘Masked La Niña Warmth’ Mess with the Summer Rainfall Response?
The polar vortex will inevitably weaken in the coming months, as the Southern Hemisphere transitions into summer.
That will make the sea temperature significant for what happens across most of the country.
The presence of a (gradually weakening) La Niña is a strong factor in the MetSwift analogues for this season. It has been considered alongside other key drivers of weather patterns, such as the Indian Ocean Dipole (IOD), in order to identify which historical years are likely to provide the best guidance for summer 2020-21.
The resulting signals for the weather tendency each month are shown below.
Please note that the MetSwift analogues are designed as guidance for gauging the risk of unusual weather conditions. What’s shown is the statistically most likely outcome.
Short Answer: Yes
According to the BOM, La Niña is typically associated with widely above-normal rainfall during winter, spring and early summer, with a wet start to the northern wet season (monsoon).
Yet the MetSwift analogues suggest an unusually dry December for central and northern parts of the country. This implies a weak or delayed atmospheric response, likely at least partly driven by the masking of the La Niña sea temperature pattern. A raised risk of rain-forced event cancellation is only found in the southwest.
By contrast, January sees a widespread increased likelihood of excessive rainfall and flooding. This includes some highly populated areas of the Victoria and New South Wales. This higher risk then becomes restricted to Western Australia in February.
A key measure of this is the Southern Oscillation Index (SOI). Very positive (+7 or above) values indicate weather patterns typical of a La Niña event.
In Oct-Nov, the SOI has been very varied, only intermittently reaching +7 or higher. The model behind the analogues predicts that this ‘undecided’ state of play will continue through Dec 2021. So, the atmosphere doesn’t display the usual La Niña response that month.
“Come January… plenty of unstable conditions supporting rain-bearing clouds”
Come January, though, above-normal rainfall is widespread. This tells us that when the atmospheric response has been weak in Oct-Dec, it’s usually part of a delayed response, rather than a complete failure. There’s below-normal sea level pressure across most of the country (not shown), which means plenty of unstable conditions supporting rain-bearing clouds. If the seas are will widely warmer than usual, that will contribute additional moisture – a very wet month is plausible.
February sees the influence of La Niña subsiding in typical fashion, with more varied anomalies across the country. It looks like there could be a weakly negative IOD driving above-normal rainfall in the west. A negative IOD often develops at some point during La Niña event.
Watch Out in the West
Bringing all three months together, only one region has persistent anomalies across all three months – western Western Australia (I’ve always wanted to write that…).
Here, above-normal rainfall is suggested for all three months. If that manifests, it will bring a risk of flooding, perhaps widespread among the lakes in the southwest.
It will be worth keeping a close eye on developments there this summer.
James Peacock MSc
Head Meteorologist at MetSwift