Closing out the 2015 snow depth prediction

Last year’s peak snow depth prediction was another triumph, right?  I predicted 141 ± 44 cm and the peak depth came in at 148.8 cm (at Spencers Creek¹, midway between Perisher Valley and Thredbo, NSW).  Well, maybe…

 
Regulars will know that my method uses regression with well-known climatic influences and modes, specifically:

  • Antarctic oscillation²
  • Southern oscillation index³
  • Indian ocean dipole⁴
  • Pacific decadal oscillation⁵
  • Local sea surface temperature⁶
  • Southern Hemisphere stratospheric aerosols (from volcanoes)⁷

 
The issue is that the model mostly uses the winter averages of those (June-July-August) but the prediction is made pre-season (March), when the winter parameters can really only be rough projections.

A better test of the model is how it went using the actual parameter outcomes. For 2015, that looked like this:

 

2015 parameter Pre-season Post-season Notes
Antarctic oscillation +1.0 +1.2 We want it negative, guys
Southern oscillation index -10 -15.5 Worse than expected
Indian ocean dipole -0.1 -0.1 Had to get one of them right…
Pacific decadal oscillation +1.0 +1.1 For 2 years to the end of August
Sea surface temperature +0.65 °C +0.45 °C Better than expected
Southern Hemisphere aerosols 0.003 0.003 Estimate — awaiting data
Snow depth ‘prediction’: 141 cm 156 cm Outcome:  148.8 cm

 
 
So in the end the model did a touch better than the pre-season guess, giving 156 cm vs the measured 148.8 cm peak. Its long term performance looks like this:

Peak snow depth hindcasts (i.e. using post-season data)

Peak snow depth hindcasts (i.e. using post-season data)

 
 
As usual my 2016 prediction will be out in March. Right now it’s looking up a bit on 2015.

 
 

Notes

  1. Spencers Creek near Charlotte Pass, NSW, Australia; 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. Indian ocean dipole (IOD) is an ENSO-like variation in the smaller Indian Ocean, which correlates with winter precipitation across southern Australia, including with alpine snow. Negative IOD is associated with more snow (the sign is consistent with SOI, but we’re on the opposite side of the Indian Ocean). The winter average is used.
     
  5. Pacific decadal oscillation (PDO) is a long-cycle, largely north-south variation in the Pacific Ocean, which interacts with (and perhaps partially mediates) ENSO. Negative long-average PDO is weakly correlated with more Australian snow. The 2-year average to August is used.
     
  6. Sea surface temperature (SST) is that in the Great Australian Bight and northwest Tasman Sea, averaged over the box: latitude 30-37°S, longitude 115-160°E, and expressed as degrees Celsius anomalies from the 1951-1980 mean, detrended about 2015. Cool SSTs correlate strongly with more snow, but unfortunately local SSTs are rising rapidly with global warming. The winter average is used. More at my post here.
     
  7. Stratospheric aerosol optical thickness is the average for the Southern Hemisphere at 550 nm wavelength (greenish visible light). Large optical thicknesses from big volcanic eruptions correlate with big snow seasons. The winter average is used. More at my post here.

3 comments to Closing out the 2015 snow depth prediction

  • Richard Jones

    Is it just my imagination? Seems to be less variation in the snow depth in say the last five years or so and the minimums seem to be not declining as fast as the maximums.

    • Gerg

      It’s probably real. As I tried to argue here (down a bit), you’d expect a reduction in absolute variability as the depth declines. Care is need though; some of those big depth peaks in the ‘distant’ past are clearly volcanic (Pinatubo — 1991 & 1992; Angung — 1964). You might say we’re due for another.

      The trend in Spencers Creek peak depth variability looks like this (sorry, needs updating):

  • richardoz45

    Glen, thanks, I appreciate your work.

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