Climate change has been implicated in the decline of our postcard glaciers – but it can't be blamed for recent bad snow years at New Zealand's busiest spot for skiing.
A new study suggested it was instead inter-annual variability that explained why snow cover had been either poor and plentiful in the lower South Island's Clutha catchment over the past two decades.
Along with supporting the majority of our country's major commercial skifields - among them the Remarkables, Cardona and Treble Cone - snow in the 21,000sq km catchment also provided a water reservoir for hydroelectric generation, irrigation and urban water supplies.
The study, led by Otago University PhD student Todd Redpath, drew upon daily satellite imagery to analyse detailed daily snow cover and snow line elevation levels across the catchment between 2000 and 2016.
Over this period the results showed huge variability in snow cover and elevation between years highlighting the complexity of processes involved in maintaining snow cover for any given day or season.
Redpath said there is nothing in the research findings that suggests New Zealanders should avoid investing in winter ski-passes in the near future.
"Kiwis tend to remember the really good and bad snow years, and generally forget about all the others in between," he said.
"But it's the variability of those in-between years that actually represent what is normal, and it's really difficult to predict what snow will do to from one year to the next."
"The recent late snow for example, is similar to 2011 when it didn't arrive till mid-July, so even though there's a perception the snow is very late this year, the data shows it has happened previously."
Covering a wide range of topography from the Main Divide in the Southern Alps to the east coast, there are many different processes and topographical controls influencing snow cover across the Clutha catchment.
"What the research has highlighted is the complexity of processes that can influence snow from big features like the ENSO (El Nino Southern Oscillation), wind direction and solar radiation, to smaller scale factors such as topographical features that can impact on local air flow for example; it's hugely complex."
One of the significant findings of the study is the effect and role of wind direction for snow cover over the catchment.
"When wind direction changes from the predominant westerly flow to an easterly orientation, even one big single snow event which can occur when accompanied by winds from this direction can have a marked impact on snow cover."
Another observation showing consistency in the records is a drop in snow cover during the month of July as this was generally associated with the presence of blocking anticyclones that curb snow events.
While New Zealand's annual mean temperatures had increased by nearly 1C over the past century, the analysis found that short-term variability had had a much larger influence than any longer-term signal driven by climate change.
The paper noted, however, that climate change was expected to gradually result in a reduction in both the duration of snow cover and peak snow depth in the South Island.
Redpath also pointed out that the public often incorrectly thought trends observed in retreating glaciers would also translate to observations of snow cover.
"Glaciers are heavily impacted by what is happening in the summer, whereas snow cover is a winter phenomenon, so they are being driven by some different factors and processes."