Many of you powder hounds out there have heard the
terms "El Niño" and "La Niña" thrown around quite a bit in
the context of seasonal prediction, but these are not the only long-term
atmospheric oscillations that are potentially useful for looking into an upcoming winter.
Ever heard of the Arctic Oscillation?
In the 2nd installment of the series of posts
giving the people what they want (speculation about the upcoming winter), we
will take a brief look the Arctic Oscillation Index and its potential impact on
monthly snowfall in the Wasatch.
First of all, what is the Arctic Oscillation? The
AO takes into account the difference in sea-level pressure between the Arctic
and the mid latitudes (i.e. the continental U.S.). When the pressure is
relatively low in the Arctic and relatively high in the mid latitudes, this
promotes a predominately east-west jet stream and a "bottling-up" of
colder air in the high latitudes. This is a positive AO phase. When the
opposite occurs (pressure is relatively high in the Arctic and relatively low
in the mid latitudes), the jet stream may exhibit more "buckling" and
north-south flow, allowing cold air to spill southward more readily. This is a
negative AO phase, and the Central and Eastern U.S. often see significant
outbreaks of cold air in this scenario.
So where does Utah fit into this? Let's take a look at the correlation
of the monthly mean AO Index and monthly snowfall at the Snowbird SNOTEL site.
Below is a figure plotting the AO Index versus total monthly SWE at Snowbird
for the period 1991-2014. The red line is the best fit line.
The results are…well…not encouraging. Here are the R-squared values (a
value of 1 indicates a perfect fit of the best fit line) for each month:
November: 0.03
December: 0.16
January: 0.01
February: 0.12
March: 0.00
April: 0.16
In other words, the AO Index has no correlation to snowfall at Snowbird
in November, January, and March, and a very slight correlation the rest of the
winter. It is interesting that December is the only month with a trend of
greater snowfall for a lower AO Index…this makes me even more wary of drawing
any conclusions from the slight “trends” observed. So, the AO is overall not of
much use for predicting snowfall in the Wasatch on the monthly scale.
While medium-range weather forecasting (out to 10 days) has become
increasingly skilled in the computer age, seasonal prediction is still in its
infancy. Also, the Arctic Oscillation is also calculated over the entire Northern
Hemisphere, and there are other climatic oscillations such as the Pacific
Decadal Oscillation (PDO) that capture more of the conditions that impact
western North America. So perhaps some of these other indices have a more
useful correlation to Wasatch snowfall…we’ll explore this some other time.
Until then, stick to the 7-day forecast and keep doing those snow dances.
No comments:
Post a Comment