2019 snow depth prediction

TL;DR: The Spencers Creek¹ peak snow depth for 2019 will be 150 ± 44 cm.

 
I predict the season peak snow depth at Spencers Creek (midway between Perisher Valley and Thredbo, NSW, Australia) using a simple six-parameter multiple regression model based on well-known climatic modes and influences. After updating for the 2018 outcome, my mark-V prediction model goes as follows:

Spencers Creek peak depth (cm) = 266 – 15.5 x AAO + 2.41 x SOI – 52.1 x SST_Perth – 86.4 x SST_Tasman + 616 x aerosol + 0.19 x sunspots

 
The parameters are explained in the notes at the end. Click on the graphs for sources.

 

Antarctic oscillation (AAO)²

This parameter is hard to predict. AAO is persistent over months but can also change suddenly. There is some limited numerical guidance available from global climate prediction models like the Australian Bureau of Meteorology’s (BOM’s) new ACCESS-S model, but it is not routinely published. This is how AAO is looking (linked image will update):

Antarctic oscillation

My model uses the winter average — the “3-month running mean” to the end of August. As is common now due to the global warming uptrend in this parameter, the winter average AAO looks likely to be positive, which is bad for snow. I’m adopting +0.5.

 

Southern oscillation index (SOI)³

 
SOI reflects ENSO, which models have bordering on El Niño throughout this year (meaning SOI persistently near or below -10, or Nino3.4-area sea surface temperatures persistently near or above +0.8°C anomaly).

 
Internationally, the various model predictions look like this (remember we want the winter average — ‘JJA’; note that the models plotted here have widely varying skill):

 
I’m adopting a winter average SOI of -10, which is poor for snow.

 

Perth sea surface temperature (Perth_SST)⁴

Sea surface temperatures in the little box south-east of Perth correlate strongly with our snowpack depths. Here’s how they’ve been trending; the winter average over the snow depth record and the monthly temperatures in the last few years:

Perth SST winter trend

Perth SST monthly trend 2019

 
Here’s what NOAA’s CFSv2 model is predicting for winter (anomalies vs 1999-2010, add about 0.5°C for 1951-1980 in our region):

CFSv2 SST anomaly prediction for July-August-September 2019
(vs 1999-2010, add 0.5°C for 1951-1980)

 
Perth SSTs have taken quite a dive in the last couple of years, but have now nearly recovered to trend. I’m adopting +0.4°C, which is poor for snow (that’s anomaly vs 1951-1980, detrended about 2020 — i.e. as if the trend line were rotated upwards about 2020).

 

Tasman sea surface temperature (Tasman_SST)⁴

Sea surface temperatures in the western Tasman Sea also correlate strongly with our snowpack depths, even a little more so than those south of Perth. Here’s how the winter average has been trending, and how the monthly temperatures have been in the last few years:

Tasman SST winter trend 2019

Tasman SST winter trend 2019

Tasman SST monthly trend 2019

Tasman SST monthly trend 2019

 
The uptrend in Tasman sea surface temperatures is relentless. I’m adopting a winter average Tasman SST of +0.8°C, which is near trend and rather poor for our snow (vs 1951-1980, detrended about 2020).

 

Southern Hemisphere stratospheric aerosols⁵

Again there have been attempts at a proper explosive eruption (particularly Sinabung in Indonesia), but nothing much has eventuated. Here’s the Mauna Loa Observatory data, left (Hawaii — yes, not quite Southern Hemisphere, and not up to date), and at Tenerife in the Atlantic, right (also not SH…):

 
Optical thickness is the negative log of transmission, so both panels are inverted views. Given the Sinabung action, I’m adopting 0.005, which is a tiny mite positive for our snow.

 

Sunspots⁶

I’m leery of sunspot-weather correlations — I include them in my model for fun, and because the model statistics support that (really). The latest sunspot count looks like this:

Latest sunspot record — will update

 
We want the winter average — June, July and August. Obviously that’s going to be very low (bad for snow). I’m adopting 15.

 

Outcome

In the updated mark-V model, those six parameters give a 2019 best-estimate peak snow depth of 150 cm. The standard error (±1σ) is 44 cm, so the range 106 – 194 cm would be expected to include about two-thirds of likely outcomes, if the parameters were perfectly known (they’re not!). The estimated chance that the peak depth will exceed 2 metres is about 12%.

That looks like this:

Spencers Creek peak snow depth pre-season prediction for 2019

 

Notes

  1. Spencers Creek near Charlotte Pass, NSW, Australia, midway between Perisher Valley and Thredbo; data courtesy Snowy Hydro Limited.
  2. Antarctic oscillation (AAO), also called “southern annular mode” or SAM, is a measure of how tightly the circumpolar winds (“polar vortex” in one usage) blow around the pole. A loose pattern (negative AAO) leads to more polar storms reaching southern Australia, and more snow. The winter average AAO is used — the average of June, July and August.
  3. Southern oscillation index (SOI) is the difference between Tahiti and Darwin surface atmospheric pressure expressed as monthly standard deviations times ten. SOI is an indicator of the El Niño Southern Oscillation (ENSO), an east-west quasicycle in equatorial Pacific Ocean surface temperature and wind patterns which correlates with precipitation across much of Australia, including with alpine snow. A positive SOI is associated with more (and wetter) Australian snow. The winter average is used.
  4. My new model splits the local sea surface temperature influence into two zones: north-west Tasman Sea, and western Great Australian Bight:

    SST influence boxes

    Temperatures are HadISST anomalies from the 1951-1980 mean in degrees Celsius, detrended about 2020. The winter average is used. (I detrend the SSTs to make the factors in the model equation appear more sensible. Because of the model linearity, linear detrending doesn’t significantly alter the regression outcome.)

    The influence boxes are:

    • North-west Tasman Sea: latitude 27.5-40°S, longitude 147.5-160°E
    • Western Great Australian Bight: latitude 32.5-37.5°S, longitude 115-122.5°E

    Cool SSTs in both zones correlate strongly with more snow, but unfortunately local SSTs are rising rapidly with global warming, especially in the Tasman.

  5. Stratospheric aerosol optical thickness is the average for the Southern Hemisphere at 550 nm wavelength (green visible light). Large optical thicknesses from big volcanic eruptions correlate with big snow seasons. The winter average is used. More at my post here.
  6. Sunspots indicate a more active sun, more solar output and very slightly warmer conditions on Earth. But, for poorly understood reasons, they are also weakly correlated with increased precipitation in some places, including with increased snowfall. The net effect for us is a slight positive correlation between sunspots and snowpack depth. More here.

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