First rivers in the sky, now whiplash

Postgraduate students from the University of Otago carry out snow density and depth observations...
Postgraduate students from the University of Otago carry out snow density and depth observations and obtain data from the School of Geography automatic weather station on the Pisa Range.
Winter weather whiplash is playing havoc with our seasonal snow.

It didn’t take New Zealanders very long to embrace the term "atmospheric rivers" once our media began using it to describe the extreme weather events leading to exceptional flooding, such as the recent event that hit Nelson and other northern parts of the South Island. Atmospheric rivers or "rivers in the sky" are highly concentrated corridors of water vapour being transported in the lower atmosphere and while they are not new phenomena they are having widespread and significant impacts.

Interestingly, the term atmospheric rivers was introduced by researchers to the scientific literature in the early 1990s, demonstrating that it often takes some time for new climatological concepts to be accepted and more widely used by the media and public. With this in mind, I would like to introduce readers to another important concept, "winter weather whiplash", which has yet to gain much traction by our media but goes hand in hand with some of the extreme weather events we have observed in recent winters.

Whiplash, in a climatological context, refers broadly to wild and rapid changes in weather conditions. In particular, the shift from one extreme weather condition to another has been behind its usage, with the intensification of the water cycle a key focus of current whiplash research. For example, despite only modest changes to the average amount of precipitation in the rainy season in California, researchers have demonstrated that anthropogenic warming will bring about large increases in the frequency of precipitation extremes similar or greater in magnitude to those that have caused widespread disruption in the past.

In New Zealand, our efforts to show that precipitation extremes are becoming more frequent and increasing in magnitude are still in their infancy, but recent events this winter highlight the need to build our knowledge about current and future changes in precipitation. What we already know is that July of this year was the wettest month on record nationally while it was also the fourth-warmest. The outcomes of this for Aotearoa New Zealand were record-breaking rain, air temperatures and wind, with significant snowfall in some areas and very little elsewhere.

The huts at Craigieburn Valley Ski Club in mid-August when winter had its 
The huts at Craigieburn Valley Ski Club in mid-August when winter had its hold on the eastern parts of the South Island.
This unpredictability in our winter weather and the see-sawing or whiplash from one weather condition to another is not only a challenge to manage, but it is also forcing us to change the way we think about winter. For example, ski areas in the eastern areas of the South Island were big winners this year, with significant snowfall leading to some of the best early snow conditions seen in years, while the North Island suffered through some of its worst due to uncharacteristically warm weather conditions.

We know that the seasonal snow that builds up every winter in the Southern Alps is under threat due to global warming. This is important for our ski areas, of course, as well as playing a crucial role in how we manage our water resources from our largest mountain catchments. Our ski areas are well aware that their snowlines, or the lowest altitude at which their snowpacks naturally form, are likely to move up hill in the future. In many instances, this will be something they can manage by gradually shifting infrastructure to higher ground and by being clever with their snow making.

What we do not know is whether winter weather whiplash will bring more snow or not. Will extreme precipitation events counter the threat of global warming by making big snowfalls in winter even bigger? While atmospheric rivers are often responsible for our biggest snowfalls, they are also typically warmer than other snow-bearing storms. What we have seen this winter is that there is a fine margin between big snow storms and flood-producing rain-on-snow events that significantly reduce snow cover. A good example of the latter is the early closure of Mt Lyford Ski Area, which was enjoying one of its best winters in some time before the atmospheric river that hit Nelson destroyed its snowpack.

To fully understand the sensitivity of seasonal snow to climate change, we must account not only for the gradual warming of our climate system but also its sensitivity to weather extremes. In the near future, winter weather whiplash is most likely going to have the final say in determining how much seasonal snow falls on our mountains and how good or bad our ski seasons end up being. Our new normal is likely going to be the unpredictable so we should probably start rethinking how we approach our winter activities. Finding some creative and more sustainable ways to enjoy our natural snow resources might be a logical pathway to begin this new journey together.

Associate professor Nicolas Cullen is a climatologist in the School of Geography at the University of Otago and a member of the Mountain Research Centre and He Kaupapa Hononga: Otago’s Climate Change Research Network. Each week in this column, one of a panel of writers addresses issues of sustainability.