Climate variability and climate change researcher Dr Kevin Trenberth from the University of Auckland says part of the cause is our hills. Storm-force winds blow across the terrain and are rotated by contact with the rise and fall of the ground.
Further, two main ingredients for tornadoes are instability and wind shear, explains 1News weather expert Dan Corbett. Instability is caused when clouds are lifted from the surface to great height as warmer air rises through much colder air above. Shear occurs when there’s a change in wind direction at this height that can cause twisting in the clouds.
Wind changing in direction and being exposed to different temperatures rotates the air like candyfloss on a stick. Weather systems active 5km above us suck these spinning wind shifts skywards, giving the tornadoes their signature spout formation and away they go.
There may be a perception that tornadoes are a relatively new phenomenon to New Zealand and more of a historical issue for the Midwest of the United States, but that may be because of media coverage. In fact, the deadliest tornado killed 1300 people in a small area of Bangladesh and East India in 1989.
Niwa reports New Zealand has seven to 10 tornadoes most years, although we can go some periods without them, for instance there were none reported between a March 1992 event in Auckland and one the same month five years later in Hawke’s Bay in 1997. The first recorded tornado-like event was a waterspout seen at the Maitai Valley in February 1868 that reached an estimated height of 30m. Māori knew of the phenomenon as āwhiowhio.
What we now have is record-high sea temperatures, causing more evaporation. The same elevated sea temperature fed and prolonged the intensity of Cyclone Gabrielle. Warmer sea temperatures are anticipated to continue and this means more chaotic events such as those experienced by Ramil Casas, Kenny Cripps and Tristan Deihl.
Current Environment Ministry estimates predict an increase of mean air temperature of +1.0C (in the 0.60 to 1.32C range) compared to 1995-2014 is projected by mid-century for the New Zealand region (over land and sea), and an increase of +1.6C (1.03 to 2.26C) by the end of the century.
The good news is, we’re getting better at being warned and getting prepared.
As forecasters know the conditions leading to the likelihood of twisters, more alerts can be issued. Most of our tornadoes are associated with bands of thunderstorms embedded in a strong, unstable pre-frontal northwesterly flow. Throw in a sudden rise in humidity and the tornado clock may be counting down.
Do not be lulled if repeated warnings fail to eventuate in the worst, however. New Zealand is a narrow string of islands between two very different bodies of water. Expect the words “changeable conditions” to prevail in the long term. The ocean around New Zealand also generates so many small storms that they can cancel each other out, restricting how big one can get.
Three within a week is a lot. There may be more and we should be ready.
