From: david@mullara.met.unimelb.edu.au (David Jones)
Newsgroups: aus.snow
Subject: BigDudd
Date: 21 Aug 92 03:55:38 GMT
Distribution: aus
Organization: University of Melbourne Dept of Meteorology


(Dave Harsant) writes:
> A few years ago, one of the ski mags had charts of the cumulative
> snow depths for the Aus Alps (from gauges near Falls Creek) for each
> year, going back about 40 years.  The result was that there is no
> discernible pattern, no five year cycles, no consistent big year
> followed by little year.
> Dave Harsant,


I also saw these figures and would claim they neither refute nor
support the "five year cycle" or big year followed by small year
theory. From memory "Australia Skiing" had these in during a 1987
issue. These were of a relatively poor form with variable sampling
from season to season and a number of apparent error.





Two features are at play in the determination of a ski season,
these being rainfall and temperature. At the higher levels 1800+
precipitation dominates while below about 1400m temperature 
is frequently the limiting factor. This means that resorts such
as Lake Mt. and Donna Buang are influenced by temperature a feature
that is random in its variations. The higher melt rates at these
resorts also means snow cover is strongly influenced by individual
events because much of the cover is often lost between these
leading to a very jagged snow depth signal. This is evidenced
by the Vic Roads data (See the Alpine Resorts Commission) which
show seemingly random snow depth signals both in a season and
from season to season. At higher altitude, the signal is
random to a degree, but would also seem to posses a cyclic signal
most probably due to the El Nino - Southern Oscillation (ENSO)
phenomena. Although I still haven't seen any studies on this
the ENSO cycle which has a period of typically 3 - 6 years, with
typical return period of ~5 years, seems to play a strong role in
the determination of the snow season. This variation is
tied to the broad circulation over the Pacific Ocean and can and 
often influences the weather over wide areas of the Globe.

El Nino years, are typically dry over large portions of Eastern
Australia, while anti El Nino (La Nino) years are wet.
A good example is to be found in the 1981/82 cycle. Although no
episode follows the average, commonly the atmosphere is in a
non decisive state for typically 3 years, swings into an anti-El Nino 
phase for typically a year then abruptly goes into the El Nino phase.
This dry phase may last anything from months to a year or more.

If the dry phase coincides with your winter, snowfalls are usually
reduced and hence a bad season.

(This is a problem with Big Al's idea. The El Nino signal responsible
 for the dry conditions over most of Australia during the last 1/4
 of last year and the first 1/3 of this year has been in a decline this 
 Winter. This means that conditions were expected to return to normality
 this winter with close to average rainfall. In this context the
 degree of dryness implies nothing about the coming season.)

It would appear this cycle, which frequently results in a wet
year followed by a dry is responsible for much of the
good/bad season swings I made comment on in an earlier posting.

For those who would question this such as Dave (nothing personal),
do so with lower altitude data, data which is influenced to a great 
extent by temperature. This is not always
the case through with 1981/82 being such a strong signal that all
snow data I have seen show a dramatic swing. 

I rest my case with the data from the Snowy Mountains Authority
snow course from Spencers Creek (alt. 1800 m half way between Perisher
and Charlottes pass). If one takes the four seasons to have a snow
depth exceeding 300 cm, (what I would call a bumper season (~ 1 in 10 years))
one finds:

    1956:   318 cm.
    1957:   151 cm.


    1964:   356 cm.
    1965:   146 cm.


    1968:   307 cm.
    1969:   115 cm.


    1981:   360 cm.
    1982:   91  cm.

The average max snow depth at these resorts is 203 cm. As can be seen
every bumper season in the period 1954-1989 has been followed by a dudd.
Pretty strong evidence. Each of these has been associated with strong swings in 
the ENSO cycle from wet to dry.

The 1990/1991 is the only exception (I have not included it because I have no official 
depths on hand). This is because the 1990 season was
not characterized by a strong anti El Nino phase. The major reason for this
season being a bumper was a series of very strong systems in late June early
July which dropped 100--200+ cm of snow. During the rest of the season, the 
depth only increased by 100 cm a fairly typical value for July and 
September. 1991 well that was a freak (the snowiest for 50+ years).
This shows the limitation of any one theory in explaining the very
strong variations in the snow season and the random behavior of the 
individual systems which can and do occasionally clutter for no obvious
reason and makes you look a fool. Of note a dry phase of the El Nino was
developing during 1991 (c.f. the drought in Queensland and NSW), but this
did not extend into South East Australia until October, with little consequence
for the 1991 ski season.

For this season, with a mixed ENSO signal, no large temperature anomalies in
Autumn or the like, the season on the basis of these arguments would
be expected to be average. So far I would say it is, it certainly is not in 
the league of 1990 but at the other extreme far better than the 1982 
non event. 

Of course It could snow for the next month and blow all this apart but 
who would complain?? I just hope there is good snow for my next ski,
in a weeks time...


David.