An unusual weather pattern has been affecting southern Oceania for a few weeks now. We’re here to answer two key questions: Why is this happening, and how likely is it to continue through spring and summer?
Wild Weather in Southern Australia
Scanning the news articles displayed on the weatherzone website, there was a clear theme to the Australian weather so far this August: Impressive storms and abundant rainfall in both the southeast and far-southwest.
In some areas, the abundance of rainfall was initially welcomed, but has since proven ‘too much of a good thing’ (e.g. exceptional rain gouging out a new ‘canyon’ in Western Australia). On the other hand, numerous droughts have been ended. For example, a third of New South Wales was declared drought-free on 12th August.
9th August saw emergency evacuations for residents of Moruya CBD and Sussex Inlet, as a strong windstorm brought severe flooding alongside damaging wind gusts.
In some southern parts of Australia, rare snow events have brought delight to some but blight for others. On 7th August, snowfall reached as far north as the upper Flinders Ranges. There’s also been plentiful snow in Tasmania.
Maps showing total rainfall (left) and mean air temperature (right) compared to the long-term average (1981-2010), for 1st-16th August 2020. The rainfall pattern is a big contrast to July, which was predominantly drier than normal with just a few exceptions (BOM summary).
In the coming week, drier weather is forecast to slowly spread across the southern half of the country, from the west.
A Different Story for New Zealand
New Zealand’s recent weather could hardly be more different. It’s been an impressively warm and dry month so far, albeit often windy, especially in the far north.
At the time of publication, the weather pattern is changing to a much wetter one. However, there are signs that this may only last about a week. Drier than usual conditions may soon return for most.
What’s Behind All This?
The weather patterns of Australia and New Zealand are significantly influenced by a Pacific ocean-atmosphere phenomenon. This is known as the El Niño Southern Oscillation (ENSO).
It oscillates between two phases, warm (El Niño) and cold (La Niña), with neutral spells in between.
Currently, a weak La Niña is developing in the tropical waters of the Pacific Ocean. Broadly, this event is characterised by below-normal sea surface temperatures (SSTs) to the east of the Dateline.
This is causing shifts in atmospheric circulation that reach far beyond the tropics.
One measure of this is called the Southern Oscillation Index (SOI). Its behaviour in the past fortnight has indicated a ‘La Niña-style’ weather pattern.
Studying the most relevant past examples, there’s a clear ‘typical response’ to a weak La Niña. Rainfall increases over the far southeast of South Australia, Victoria, most of New South Wales, and the northern half of New Zealand. Meanwhile, the southern half of New Zealand turns drier. With the extra rain, it also tends to be colder than average.
This is all associated with a northward shift in the path (storm track) of non-tropical lows traversing the Southern Ocean.
This past fortnight, however, the storm track has been even further north than is typical of a La Niña event.
What could explain that?
Well, the surface waters of the West Pacific are currently much warmer than average. This is typical during La Niña events, except that this year, the strength and coverage of warm anomalies is especially large.
This could be enabling the weak La Niña to behave like a stronger one, shifting the weather pattern more dramatically.
Will This Last Through Spring?
In an August 2016 article on La Niña events, the Australian Bureau of Meteorology (BOM) stated that, “Unlike El Niño years, the impacts of La Niña often continue into the warm months”.
Delving into historical data for weak La Niña months, we can apply further detail to this, as shown below.
Maps showing total rainfall compared to the long-term average (1981-2010), for historical Septembers, Octobers and Novembers (within 1950-2019) during which there was a weak La Niña events were occurring.
“Wetter toward the west, drier east.”
While rain-bearing weather systems continue travelling north of usual, there is a large westward shift in the affected area. It looks like most of Victoria, New South Wales and Queensland should see a much drier pattern establishing, compared to recent weeks.
It should be noted that these maps show an average, around which some variation can be expected. This is largely down to a phenomenon known as the Indian Ocean Dipole (IOD). It can either strengthen the La Nina-style pattern (negative IOD event) or weaken it (positive IOD).
At the time of writing, forecast models disagree over how this will behave during Sep-Nov 2020. So, it’s best not to focus on the details in the maps above.
The broad message to take home is ‘wetter toward the west, drier east’.
A Look Ahead to Summer with the MetSwift Analogues
Taking the historical years from 1950-2019 that fit best to the recent behaviour of 2020 in key regions of the planet, MetSwift produces skilful predictions based on how those years behaved during the following months and even years.
Below, those analogue years have been used to examine the chance of anomalous rainfall or temperatures in summer 2020-21.
Maps showing the percentage chance, based on the MetSwift analogue years, that (top row) rainfall is 25% or more away from average and (bottom row) mean temperature is 0.5°C or more away from average. Key contributing factors in the analogue’s prediction are a weak La Niña event, near-neutral Indian Ocean Dipole and continued much warmer than average western Pacific.
What stands out most is a signal for an unusually warm and dry summer for most of South Australia, Queensland, and New South Wales. This implies above-normal risks for both drought and wildfires. If this follows abundant springtime rainfall, as the weak La Nina event suggests for Northern Territory and South Australia, high vegetation coverage will further raise the wildfire risk.
A hotter than average summer has an above normal probability across a wide area. Broadly, this covers the southern two-thirds of Australia, Tasmania, and South Island, New Zealand. A cooler than average summer looks unlikely for all but the central area of Northern Territory, Australia, where this outcome is slightly favoured.
An unusually wet summer is suggested for a swathe of Western Australia, from the southeast to central north. The combination of this and above-average temperatures indicates a raised likelihood of large rainfall events, hence flash-flooding.
Overall, it appears it could be another eventful summer for Australia. Meanwhile, New Zealand may benefit from near-average conditions, which indicate a reduced likelihood of extreme temperature or rainfall events there.
James Peacock MSc
Head Meteorologist at MetSwift
Cover Photo by Unknown Author is licensed under CC BY-SA-NC