Welcome to our primary outlook for The Australian snow season. First however a few announcements are required:
- Unfortunately the long range outlooks during the season (in all regions) will be discontinued. COVID, time constraints and other factors have meant that it is no longer viable. However I will still be talking about long term drivers (during the snow season) on my twitter handle @longrangesnow, and on various snow/weather forums. And it does not mean the end of these seasonal forecasts on this blog.
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Okay now we are through that, please enjoy my outlook for the Australian snow season.
So for the sake of transparency and improvement, I am reviewing my past predictions. And they haven’t been great particularly last year. So one might ask what I am going to do to improve.
2017 “average season” > a very good season
2018 “slightly above average season” > a good snow season.
2019: “average season” > ended up being a pretty good season.
2020: “moderately above average season” > A pretty poor season overall.
This year I am going to stop doing specific Spencer Creek snow depth predictions on a plus and minus basis as I have done previously. It is clear that this is taken into account too many biased factors and makes for poor analysis.
Pacific Decadal Oscillation
We have a classic negative PDO pattern forecast by EC:
This slightly increases snow favourability for the Australian Alps.
Madden-Julian Oscillation (MJO)
The MJO is notoriously hard to predict on a long-term basis, but there is some tools we can use to help predict the rough location of the tropical forcing on a seasonal scale. The warmest SSTAs in the tropical region stretch from the Indian Ocean to Australia, so it is theoretically less likely we will see a strong Phase 6-7-8 MJO signal over winter.
The best period according to the CFS tropical forcing prediction would be in June with a Phase 5-6 MJO. July features weak tropical forcing too far out into the Pacific. And August and September predictions are too far to the west in the Indian Ocean. So the tropical forcing is probably going to provide a neutral impact on us, with an average amount of passes and strength in our region. When they do come, it provides good opportunity for snowfall in the Australian Alps.
Indian Ocean Dipole
We are looking at a cool neutral IOD this season, with only the Meteo-France and Chinese models forecasting a negative IOD pattern during the winter months from last month’s update.
The latest models from this month (less guidance until the other models update) show the Australian, Canadian and ECMWF models more inclined towards a negative IOD.
Below, we see pretty warm sea-surface temperature anomalies in the Eastern Indian Ocean and North of Australia.
These are likely to increase precipitation at least a little bit, in SE Australia. This can bring more snowfall, but it can also be less cold unless it is combined with strong polar cold fronts. A negative IOD is certainly possible in this environment.
El Nino Southern Oscillation (ENSO)
We are seeing the model consensus of the ENSO models in the cool neutral region of the ENSO index. This is arguably quite favourable for us going forward.
The key impact for us is that the Western Pacific is still warmer than normal and is likely to remain so (bar that cool patch off Brisbane), which is likely to increase precipitation. This may assist snowfall in the Australian Alps, but it can also bring down warmth that can bring rain to the mountains during winter. So it can be a double edged sword, but it usually is helpful combined with proper polar nodes.
One of the atmospheric indicators of ENSO, the Southern Oscillation Index (compares sea level pressure between Darwin and Tahiti) is forecast by EC as being Nina-esque (positive values align with a La Nina):
What does this mean for Australian snowfall? Probably a cold neutral/mild Nina effect combining oceanic and atmospheric elements of ENSO, which is pretty good for us.
If a large stratospheric volcano explosion occurs, Australian snowfall substantially increases for that winter. None have occurred in the last year, so this season won’t be affected.
There is a weak correlation between a higher amount of sunspots and more snow, as found by Francisco Sánchez-Bayo and Ken Green. We are still around the solar minimum, but we are heading out of it. It may have a minor detrimental effect on our snowfall.
Southern Australia/Tasman SSTs
If we look at the two key regions for us, they are basically flipped in our two models here. NMME forecasts a cool neutral SST in the Great Australian Bight, and a warm Tasman, which cancel each other out. EC has a slightly warm Bight, and a slightly warm Tasman, warmer than NMME in the Bight but cooler in the Tasman.
A warm Tasman brings the potential for warmer maritime air to deliver moisture in rain or heavy snow forms. I’d say we will see a neutral Bight, that may get slightly warmer than normal, so a fairly neutral effect on cold fronts. The Tasman will remain relatively warm, but perhaps not as warm as it has been in past seasons.
Southern Annular Mode/Stratosphere
There is a fairly strong correlation between negative SAM and more snowfall for the Alps. I talked about the seasonal prediction of SAM here. El Niño Modoki (a seasonal prediction factor of SAM) is at a neutral phase, and is likely to get slightly cooler towards winter.
Climate Change as a general trend is causing a more positive SAM. Furthermore on a seasonal scale, the general trend if it continues to hold over this winter is positive SAM-like.
The graphic to the left shows some above average strengthening of the Stratospheric Polar Vortex. The more numerous fluctuations and changes there are, the more interesting it could be for Australian snowfall.
The graphic to the right shows upper stratosphere temperatures above the Southern Hemisphere subtropics. It being colder than average means that more ozone can pass from the NH, and increases the chances for a sudden warming of the stratospheric vortex. This may help troughing and therefore snowfall in our region.
Let’s run an experimental analog like my ones done for the Northern Hemisphere. It is common in winter seasonal forecasts in the Northern Hemisphere (especially North America), but relatively rare among winter forecasts in Australia.
The years chosen in the first one are primarily due to their similarity with the warm Maritime Continent and Western Pacific waters, similarity to the Indian Ocean forecast and generally a largely neutral ENSO influence.
The most key feature here is the strong low between South America and NZ. The fact the Australian Alps in SE Australia are between two highs is hopeful, but certainly not definitive.
To try another set of analog years, we will look at just Neutral ENSO winters preceding La Ninas:
And interestingly we see similar patterns to the last one, despite only one year being the same. We see the strong low SW of South America with a corresponding ridge east of New Zealand, some high latitude blocking and some support for roughing in the SE Australia region. This is slightly increased on this.
But if we go even further and control for the Indian Ocean and get rid of +IOD seasons, we get:
We see a very decent troughing pattern for SE Australia under these underlying climatic conditions with these analog years. This may help to increase snowfall in The Australian Alps.
So let’s go through a roll-call of factors into this forecast in terms of impact on Australian snowfall:
- PDO: Mild benefit
- MJO: Neutral
- ENSO: Mild-moderate benefit
- IOD: Mild-moderate benefit
- SAM/Strat: Mild benefit
- Volcanoes: Neutral
- Sunspots: Negligible benefit
- Southern Australia/Tasman SSTs: Mild-moderate detriment.
- Analogs: Mildly-moderately positive for snowfall
So I think it’s fair to call for an average to slightly above average snow season. So around 175-200cm for the Spencer’s Creek snow depth prediction.
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