Tornadoes don't indicate extreme weather is increasing

I popped out for a meeting this afternoon and came back to the office to discover a tornado had torn through the street I grew up in, Waimarie Rd, in Whenuapai, just a stone’s throw from the airforce base perimeter.

My parents were out shopping at the time thankfully, but still can’t get back into Whenuapai where roads are closed until trees and fallen power lines can be removed. The power pole at the entrance to their section is down and their car port has taken a hammering. But they got off lightly compared to the three who died in West Auckland this afternoon, in an extreme weather event that echoes that of last year’s fatal tornado that touched down on Auckland’s North Shore.

My old stomping ground, Waimarie Road in Whenuapai, this afternoon credit: NZ Herald

An obvious question that has sprung up on social networks this afternoon, perhaps with the Sandy disaster still fresh in people’s minds, is whether tornadoes are increasingly frequent in New Zealand and a manifestation of the changing climate.

The answer, according to climate expert, Victoria University’s Associate Professor Jim Renwick, is “no”.

He told the SMC this afternoon:

The occurrence of damaging tornado events is associated with localised severe thunderstorm activity. Analysis of weather records does not show a pattern, nor are there trends obvious in tornado occurrences. These events strike at random from time to time, but they are very localised and sporadic and are not obviously tied to trends in the large-scale climate. At this stage, we have no indication that tornado occurrences will become more or less frequent in future.

Aucklanders will relate to the comments about the tornadoes being “very localised”, but other parts of the country are from time to time ravaged by tornadoes. Just up the coast from me at Waikanae, a tornado tore through houses last year, almost killing members of one family.

University of Canterbury lecturer in meteorology, Dr Marwan Katurji, said the Taranaki region is the record holder for tornadoes, with the Auckland region coming in second.

“The North Island, especially the west coast, is more vulnerable to westerly and northerly winds that are associated with weather fronts. Warm moist air from the warmer Tasman Sea carries within it embedded thunderstorms. When the air hits land it interacts with the topography to create convergence zones and the wind speeds are higher in these areas and the storms get more severe in this case.”

And some comfort for Cantabrians – who have become expert at dealing with disaster:

“In Canterbury we are blessed with the Southern Alps that do shield off the severe westerly storms. But occasionally we do get the odd small waterspout off the Banks Peninsula coast.’’

While environmentalists were gratified to see climate change put squarely on the agenda days before the US election when Sandy caused mayhem along the east coast of the US, scientists there have also warned about overstating the link between extreme weather events and climate change. Writes environmental scientist Amy Luers:

The link between today’s extreme weather and greenhouse gas policy is weak. Policy decisions made today are not going to eliminate or even significantly alter the patterns of these extreme weather events in the next few decades. This is due to the long lifetime of the heat-trapping gases in the atmosphere coupled with the time it takes to change our societal infrastructure.

However, some scientists, such as climate expert and kiwi expat Dr Kevin Trenberth, suggest global warming is exacerbating the effects of extreme weather events. He said this of Sandy:

“Global climate change has contributed to the higher sea surface and ocean temperatures, and a warmer and moister atmosphere, and its effects are in the range of 5 to 10%. Natural variability and weather has provided the perhaps optimal conditions of a hurricane running into extra-tropical conditions to make for a huge intense storm, enhanced by global warming influences”.

Back in New Zealand last year, Dr Trenberth, who is based in Colorado, participated in a Science Media Centre briefing on extreme weather, which you can listen to here.

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