Study looks at potential climate change influence on Northland 2014 storm

A yacht beached at Opua. Northland storm - July 10, 2014.

The potential hand of climate change in one of the worst storms ever to hit Northland, causing tens of millions of dollars worth of damage, has been revealed in a new study.

Over five days in July 2014, Northland was swamped by a low pressure system that brought extreme rainfall, flooding and high winds, knocking out power to 16,000 homes and putting about 5000ha of dairy land underwater for up to 10 days.

In a report published today by the Bulletin of the American Meteorological Society, National Institute of Water and Atmosphere climate scientist Dr Suzanne Rosier picks apart the potential influence of human-driven climate change on the event.

She drew upon data collected with help from the weather@home project, in which people donated their computers' spare processing power to crunch a vast amount of weather data from climate models.

With the extra computing power, making it possible to run thousands of simulations, she used the state-of-the-art Australia New Zealand regional climate model to run two sets of experiments.

The first looked at the five-day weather data under present climate conditions, while the second analysed the weather data under climate conditions as they might have been had there been no human influence.

While the rainfall that led to the 2014 Northland floods could have occurred in a world without climate change, Dr Rosier found that the chances of such heavy rain had increased as a result of human influence.

The study's best estimate was that the risk of such an event had approximately doubled because of human interference with the climate system.

"Since weather patterns are chaotic, we need such a large number of simulations to help us estimate the range of possible weather outcomes under any particular set of driving conditions," she said.

"By having this very large number of estimates both with and without the human influence on climate we are able to draw more confident conclusions about the human impact on extreme weather events than would otherwise be the case."

It followed similar NIWA studies by NIWA analysing the effects of climate change on two previous events - the extreme rainfall in Golden Bay and Nelson in 2011 that flooded more than 300 homes and properties, and severe drought across the North Island in early 2013.

The Golden Bay study concluded that an increase in greenhouse gases helped increase atmospheric moisture levels, contributing to extreme rainfall in the area.

In the case of the 2013 drought, scientists found that human influence had made conditions more favourable for drought.

The Northland storm of July 8-11, 2014

• Kaikohe recorded a July rainfall total more than 300% of normal
• Total insurance claims from the deluge topped $18 million.
• 16,000 homes were left without power at the height of the storm
• The Fire Service carried out 15 rescues, mostly of motorists trapped in floodwaters.